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X-Ray Baggage Screening Image Interpretation

Short Course Modules:


Module 1: Introduction to X-Ray Baggage Screening

1.1. Understanding the Basics of X-Ray Baggage Screening

1.2. Importance of Baggage Screening in Aviation Security

1.3. Role of Security Personnel in Baggage Screening

1.4. Regulatory Framework and Compliance in Aviation Security

Module 2: X-Ray Imaging Technology

2.1. Principles of X-Ray Imaging in Baggage Screening

2.2. Different Types of X-Ray Scanners

2.3. X-Ray Machine Components and Functions

2.4. Radiation Safety and Protection Measures

Module 3: Image Interpretation Fundamentals

3.1. Image Enhancement Techniques in X-Ray Screening

3.2. Recognizing Common Objects and Materials

3.3. Anomalies and Threat Indicators in X-Ray Images

3.4. Limitations of X-Ray Baggage Screening

Module 4: Legal and Ethical Considerations

4.1. Legal Framework for Baggage Screening

4.2. Privacy and Passenger Rights

4.3. Ethical Decision-Making in Baggage Screening

4.4. Handling Sensitive or Restricted Items

Module 5: Threat Detection Techniques

5.1. Identifying Explosive Threats in Baggage

5.2. Recognizing Weapons and Firearms

5.3. Detection of Prohibited Items (e.g., Sharp Objects)

5.4. Screening for Concealed Liquids and Gels

Module 6: Behavior Analysis and Passenger Profiling

6.1. Behavioral Indicators in Passenger Baggage

6.2. Passenger Profiling Techniques

6.3. Recognizing Suspicious Behavioral Patterns

6.4. Passenger Interaction and Communication

Module 7: Threat Response and Crisis Management

7.1. Responding to Suspected Threats

7.2. Evacuation Procedures and Emergency Protocols

7.3. Coordination with Law Enforcement Agencies

7.4. Crisis Communication and Public Safety

Module 8: Technology Integration and Automation

8.1. Automated Threat Recognition Systems

8.2. Machine Learning and AI in Baggage Screening

8.3. Integration with Security Information Systems

8.4. Advancements in Baggage Screening Technology

Module 9: Operational Challenges and Best Practices

9.1. Overcoming Challenges in High-Traffic Airports

9.2. Training and Skill Development for Screeners

9.3. Baggage Screening Quality Control

9.4. International and National Standards Compliance

Module 10: Real-World Scenarios and Case Studies

10.1. Case Studies of Successful Threat Detection

10.2. Lessons Learned from Past Security Incidents

10.3. Interactive Simulation and Practical Exercises

10.4. Continuous Learning and Professional Development in Baggage Screening


An Overview:



X-ray baggage screening image interpretation is a critical component of aviation security. It involves analyzing X-ray images of checked and carry-on luggage to identify any potential threats, prohibited items, or suspicious objects. The goal is to ensure the safety and security of passengers, crew, and aircraft by preventing dangerous items from being brought on board.


Here’s a comprehensive explanation covering all aspects of X-ray baggage screening image interpretation in aviation security:

  1. Principles of X-Ray Baggage Screening:


X-Ray Technology: Baggage screening utilizes X-ray technology, which involves projecting X-rays through luggage and capturing the transmitted radiation to create images.


Dual-Energy X-Ray: Some systems use dual-energy X-ray, which allows for better differentiation between organic and inorganic materials.


  1. Role of Security Personnel:


Screening Officers: Trained security officers are responsible for operating the X-ray machines and interpreting the images.

Human Element: Image interpretation involves both automated system alerts and the human eye, as security personnel make final judgments.

  1. Components of X-Ray Scanners:


X-Ray Source: The machine emits X-rays that pass through the baggage.


Detector Array: Detects the X-rays after they pass through the luggage.


Display Monitor: The image is displayed on a monitor for interpretation.


Computer Software: Specialized software assists in threat detection and anomaly identification.


  1. Image Enhancement Techniques:


Contrast Enhancement: Adjusting the contrast and brightness to make objects more visible.


Zoom and Rotate: Tools for closer inspection of specific areas.

Colorization: Adding color to images to highlight different materials.


  1. Recognizing Common Objects and Materials:


Shapes and Outlines: Understanding the shapes and outlines of common items like clothing, toiletries, and electronics.


Material Density: Recognizing the density of materials, such as metals, plastics, and organic substances.

6 Colour Imaging Technology uses imaging analysis to assign colours based on the effective atomic number.


This technique provides better differentiation of items which may have similar composition; each screened object appears in one of the 6 colours based upon a specific range of atomic numbers.

This imaging feature uses imaging analysis to assign colours based on the effective atomic number. 6 Colour imaging provides better differentiation of items which may have similar composition; each screened object appears in one of the 6 colours based upon a specific range of atomic numbers.

Competitor’s 3 Colour imaging systems do not provide the operator with as much information as 6 Colour Imaging.

3 Colour vs 6 Colour imaging  

This imaging feature uses imaging analysis to assign colours based on the effective atomic number. 6 Colour imaging provides better differentiation of items which may have similar composition; each screened object appears in one of the 6 colours based upon a specific range of atomic numbers.

Competitor’s 3 Colour imaging systems do not provide the operator with as much information as 6 Colour Imaging.

6 Colour imaging provides material discrimination

6 Colour imaging provides material discrimination, allowing the operator to determine between threat and non-threat items. The operator is able to interpret the X-Ray image more quickly and isolate threats with better precision, increasing the accuracy of detection and throughput.

6 Colour is most effective when used in combination with the Screener Assist Optional Feature.

In the right-hand image above you can see how the explosive appears as a light brown via 6 Colour Imaging – however in the image to its left, it remains orange in 3 Colour Imaging.

Another example is the glass cup appears yellow while utilising 6 Colour, but remains green in 3 Colour. The additional colours shown in the screened images are a few examples of how 6 Colour Imaging adds a unique image differentiation. Advantages of 6 Colour Imaging versus 3 Colour are numerous and may vary by application. With minimal time to identify the various threats common to the field or area, the utilisation of 6 Colour Imaging enables high security threats to be more easily identified by operators.

  1. Threat Indicators and Anomalies:


Prohibited Items: Identifying items on the list of prohibited items, such as weapons or liquids over the limit.

shapes that do not match expected objects or materials.


Unusual Arrangements: Paying attention to irregular or unexpected arrangements of items.


  1. Limitations of X-Ray Baggage Screening:


Image Clutter: Over-packed luggage can obscure the view of items.

Dense Materials: Dense or overlapping materials can make it challenging to identify objects.


Operator Fatigue: Extended periods of image interpretation can lead to fatigue and reduced effectiveness.


  1. Legal and Ethical Considerations:


Privacy Laws: Ensuring that screening respects passenger privacy, including laws governing the use of body scanners.


Passenger Rights: Recognizing and respecting the rights of passengers during the screening process.


  1. Training and Skill Development:


Security Officer Training: Security personnel undergo specialized training in X-ray baggage screening.


Simulated Exercises: Regular practice with simulated scenarios helps maintain and improve skills.


Continual Education: Ongoing education ensures that security personnel stay updated on the latest threats and technologies.

  1. Machine Learning and Automation:


Automated Threat Recognition: Some systems use machine learning to automatically flag potential threats, reducing the workload on human screeners.


Human-in-the-Loop: While automation can assist, human screeners still play a crucial role in final decision-making.



In conclusion, X-ray baggage screening image interpretation is a complex process that requires a combination of technology, training, and human judgment. Security personnel are tasked with identifying potential threats and prohibited items, respecting legal and ethical considerations, and continually adapting to evolving security challenges. Automation and machine learning are becoming increasingly important tools in this field, helping to improve the efficiency and accuracy of baggage screening.



“X-Ray Baggage Screening

Image Interpretation”


Course Modules:

10 modules for a short course on “X-Ray Baggage Screening Image Interpretation” in aviation security, with at least four relevant sub-topics for each module:

Module 1: Introduction to X-Ray Baggage Screening

1.1. Understanding the Basics of X-Ray Baggage Screening

1.2. Importance of Baggage Screening in Aviation Security

1.3. Role of Security Personnel in Baggage Screening

1.4. Regulatory Framework and Compliance in Aviation Security

1: Understanding the Basics of X-Ray Baggage Screening

1.1 Definition and Description:

  • X-Ray baggage screening is a security process used to inspect the contents of baggage, packages, and cargo for prohibited items or potential threats using X-ray technology. It involves the transmission of X-rays through objects, creating images that can be interpreted by security personnel.
  • X-ray machines used in baggage screening produce two types of images: transmission and backscatter. Transmission X-rays are the most common and provide a clear view of the object’s internal structure.

1.2 Importance:

  • X-ray baggage screening is essential for identifying concealed weapons, explosives, and other prohibited items in passenger baggage and cargo.
  • It plays a crucial role in enhancing aviation security by preventing potential threats from being transported onto aircraft.

1.2: Importance of Baggage Screening in Aviation Security

2.1 Definition and Description:

  • Baggage screening is a fundamental component of aviation security, ensuring that passengers and cargo do not pose risks to flight safety. Screening processes can include X-ray imaging, physical inspection, and explosive detection systems (EDS).
  • The primary objective of baggage screening is to prevent prohibited items, such as weapons, explosives, or hazardous materials, from being brought onto aircraft.

2.2 Importance:

  • Baggage screening enhances aviation security by mitigating potential threats to passengers, crew, and aircraft.
  • It is critical for compliance with international aviation security regulations and standards.

 1.3: Role of Security Personnel in Baggage Screening

3.1 Definition and Description:

  • Security personnel, including screeners and operators, play a pivotal role in the baggage screening process. They are responsible for operating X-ray machines, monitoring images, and identifying potential threats.
  • Personnel are trained to recognize prohibited items and take appropriate actions in response to suspicious findings.

3.2 Importance:

  • The efficiency and effectiveness of baggage screening heavily depend on well-trained and vigilant security personnel.
  • Trained personnel are essential in preventing security breaches and ensuring the safety of air travel.

1.4: Regulatory Framework and Compliance in Aviation Security

4.1 Definition and Description:

  • Aviation security is subject to a robust regulatory framework, including guidelines and standards set by international organizations like the International Civil Aviation Organization (ICAO) and national aviation authorities.
  • Compliance with these regulations is crucial for ensuring the highest level of aviation security.

4.2 Importance:

  • Regulatory compliance provides a common standard for aviation security across the globe.
  • It ensures that airports and airlines adhere to best practices and consistently apply security measures, including baggage screening.


X-ray baggage screening is a critical component of aviation security that helps identify prohibited items and potential threats. It relies on X-ray technology to create images that are interpreted by security personnel. The role of trained security personnel in this process is vital, as they are responsible for recognizing and responding to suspicious findings. Compliance with international and national aviation security regulations is essential to maintaining a high level of security and ensuring that best practices are followed consistently across the aviation industry. Baggage screening plays a significant role in safeguarding air travel and enhancing passenger safety.

Module 2: X-Ray Imaging Technology

2.1. Principles of X-Ray Imaging in Baggage Screening

2.2. Different Types of X-Ray Scanners

2.3. X-Ray Machine Components and Functions

2.4. Radiation Safety and Protection Measures

2.1. Principles of X-Ray Imaging in Baggage Screening

2.1. Definition and Description:

  • X-ray imaging in baggage screening is based on the principle of using X-rays to create images of the contents of baggage and cargo. X-rays are high-energy electromagnetic waves that can penetrate various materials to different degrees.
  • When X-rays pass through an object, they are attenuated or absorbed, depending on the object’s density and composition. This attenuation is captured by detectors, creating a grayscale X-ray image.

2.2 Importance:

  • Understanding the principles of X-ray imaging is essential for interpreting baggage screening images effectively.
  • It helps security personnel identify objects, their shapes, and their density variances, which can indicate the presence of prohibited items.

2.2: Different Types of X-Ray Scanners

2.3 Definition and Description:

There are different types of X-ray scanners used in baggage screening, including:

  1. Single-View X-Ray Scanners: These provide a single X-ray view of the baggage and are commonly used for carry-on baggage screening.
  2. Dual-View X-Ray Scanners: These scanners offer two perpendicular views of the baggage, enhancing the ability to detect concealed threats.
  3. Multi-View and 3D X-Ray Scanners: These advanced scanners provide multiple views and three-dimensional reconstructions of the baggage, improving detection capabilities.

2.4 Importance:

  • The type of X-ray scanner used can significantly impact the effectiveness of baggage screening.
  • Different scanners have varying capabilities in terms of threat detection and image quality.

2.3: X-Ray Machine Components and Functions

2.5 Definition and Description:

An X-ray baggage screening machine consists of several key components, including:

  1. X-ray Tube: This component generates X-rays.
  2. Detector Arrays: These capture the attenuated X-rays to create the image.
  3. Conveyor Belt: Baggage is transported through the machine on a conveyor belt.
  4. Lead Curtains and Baffles: These shield the surrounding area from X-ray exposure.
  5. Control Console: Operators use this to monitor and interpret the X-ray images.

2.6 Importance:

  • Understanding the components and functions of an X-ray machine is essential for operating and maintaining it effectively.
  • Proper knowledge ensures the safety and accuracy of the baggage screening process.

2.4: Radiation Safety and Protection Measures

2.7 Definition and Description:

  • X-ray baggage screening involves ionizing radiation, which can pose health risks to personnel if not managed properly.
  • Radiation safety measures include lead shielding, safety interlocks, and dose monitoring to minimize radiation exposure.

2.8 Importance:

  • Radiation safety is crucial to protect the health and well-being of personnel involved in baggage screening.
  • Compliance with radiation safety measures is essential to ensure the safety of both operators and passengers.


Understanding the principles of X-ray imaging, the types of X-ray scanners, the components and functions of X-ray machines, and radiation safety measures is critical for effective baggage screening image interpretation. This knowledge empowers security personnel to recognize potential threats and ensure the safety and security of air travel. Baggage screening is a vital aspect of aviation security, and these principles and measures play a significant role in its success.

Module 3: Image Interpretation Fundamentals

3.1. Image Enhancement Techniques in X-Ray Screening

3.2. Recognizing Common Objects and Materials

3.3. Anomalies and Threat Indicators in X-Ray Images

3.4. Limitations of X-Ray Baggage Screening

3.1 Image Enhancement Techniques in X-Ray Screening

3.1.1 Definition and Overview:

  • Image enhancement techniques in X-ray screening involve the application of various digital and software-based processes to improve the quality and clarity of X-ray images. These techniques aid in making concealed objects or materials more visible and identifiable.
  • Image enhancement plays a crucial role in baggage screening, ensuring that security personnel can interpret X-ray images accurately and identify potential threats effectively.

3.1.2 Key Image Enhancement Techniques:

Brightness and Contrast Adjustment: This technique involves fine-tuning the brightness and contrast of the X-ray image to improve the visibility of different materials.

Edge Enhancement: It emphasizes edges and boundaries of objects within the image, making it easier to distinguish between items.

Pseudo-Coloring: Assigning different colors to materials with varying densities can enhance the differentiation of objects.

Zoom and Pan: These functions allow security personnel to zoom in on specific areas of the image or pan across the image for a more detailed examination.

Inversion: Flipping the colors or inverting the image can highlight hidden or hard-to-see objects.

Filtering: Various filters, such as edge-detection filters, can be applied to emphasize specific features.

3.1.3 Importance:

  • Image enhancement techniques are crucial for improving the interpretability of X-ray images and reducing the likelihood of missed threats.
  • Enhanced images can help security personnel recognize concealed objects, materials, and anomalies more effectively.

3.2 Recognizing Common Objects and Materials

3.2.1 Definition and Overview:

  • Recognizing common objects and materials is a fundamental skill in X-ray baggage screening. Security personnel must be trained to identify everyday items to distinguish them from potential threats.
  • This aspect of image interpretation ensures that benign items do not result in unnecessary alerts or delays.

3.2.2 Key Elements in Object Recognition:

Familiarization with Common Items: Training includes familiarizing security personnel with the appearance of everyday items commonly found in baggage.

Shape, Density, and Position: Observing the shape, density, and position of objects in the X-ray image can help determine their nature.

Consistency with Passenger’s Profile: Recognizing objects that are consistent with the passenger’s profile and travel purpose is essential for efficient screening.

3.2.3 Importance:

  • Efficient object recognition reduces the number of false alarms, streamlining the screening process and minimizing passenger inconvenience.
  • It allows security personnel to focus on identifying true threats effectively.

3.3 Anomalies and Threat Indicators in X-Ray Images

3.3.1 Definition and Overview:

  • Identifying anomalies and threat indicators involves recognizing deviations from the expected appearance of objects in X-ray images. These deviations may indicate the presence of prohibited items or potential threats.
  • Security personnel must be trained to pinpoint these anomalies.

3.3.2 Key Threat Indicators:

Hidden Objects: Objects concealed within other items or obscured by packing materials.

Unusual Shapes: Objects with shapes inconsistent with benign items.

Uncommon Density: Items with density variations that don’t match typical contents.

Wire Configurations: Wire patterns that deviate from standard wiring in electronic devices.

3.3.3 Importance:

  • Recognizing anomalies and threat indicators is essential for detecting concealed weapons, explosives, or other prohibited items.
  • Effective identification of such anomalies is critical for enhancing aviation security.

3.4 Limitations of X-Ray Baggage Screening

3.4.1 Definition and Overview:

  • X-ray baggage screening has inherent limitations, including difficulties in distinguishing certain materials and the potential for human error.
  • These limitations emphasize the importance of complementary security measures and the ongoing development of screening technologies.

3.4.2 Key Limitations:

Difficulty with Organic Materials: X-rays have limitations in distinguishing between various organic materials, making it challenging to differentiate harmless and hazardous substances.

Operator Fatigue: Security personnel may experience fatigue, reducing their ability to maintain consistent vigilance.

Evolving Threats: As threats evolve, X-ray screening technology may become less effective in detecting new or unconventional threats.

3.4.3 Importance:

  • Understanding the limitations of X-ray baggage screening is essential for optimizing aviation security.
  • It underscores the necessity of integrating multiple security measures and advancing screening technology.


Image interpretation in X-ray baggage screening involves image enhancement, object recognition, anomaly identification, and acknowledging the limitations of the screening process. Security personnel must be trained to recognize potential threats and differentiate them from benign items. Effective interpretation ensures aviation security and minimizes the likelihood of security breaches. It’s essential to maintain vigilance, improve screening technologies, and consider complementary security measures to address the ever-evolving landscape of aviation security.

Module 4: Legal and Ethical Considerations

4.1. Legal Framework for Baggage Screening

4.2. Privacy and Passenger Rights

4.3. Ethical Decision-Making in Baggage Screening

4.4. Handling Sensitive or Restricted Items

4.1 Legal Framework for Baggage Screening

4.1.1 Overview and Definition:

  • The legal framework for baggage screening encompasses the laws, regulations, and international standards that govern the screening of baggage and cargo in aviation security.
  • It provides the legal basis for conducting screening operations, including passenger rights and responsibilities.

4.1.2 Key Components:

International Aviation Regulations: Organizations like the International Civil Aviation Organization (ICAO) set international standards for aviation security, which member states are required to implement.

National Legislation: Each country has its own laws governing aviation security, often aligned with international standards.

Passenger Rights: Legal frameworks include provisions for passenger rights, such as the right to privacy and protection from unlawful searches.

4.1.3 Importance:

  • The legal framework establishes the authority for conducting baggage screening and ensures that the process aligns with international and national legal requirements.
  • It safeguards passenger rights and security by providing a legal basis for screening operations.

4.2 Privacy and Passenger Rights

4.2.1 Definition and Overview:

  • Privacy and passenger rights are critical considerations in baggage screening. Passengers have the right to privacy and protection from intrusive or unwarranted searches.
  • Balancing security needs with privacy and passenger rights is a fundamental aspect of baggage screening.

4.2.2 Key Privacy and Passenger Rights Considerations:

Protection from Unlawful Searches: Passengers have the right to protection from invasive or unlawful searches during the screening process.

Informed Consent: Passengers should be informed of the screening process, their rights, and any potential impact on their privacy.

Data Handling: The collection and storage of passenger data should comply with privacy regulations.

4.2.3 Importance:

  • Respecting privacy and passenger rights is essential to maintaining the trust and cooperation of travelers.
  • It ensures that baggage screening remains within legal and ethical boundaries.

4.3 Ethical Decision-Making in Baggage Screening

4.3.1 Definition and Overview:

  • Ethical decision-making in baggage screening involves making choices that align with moral principles, values, and professional ethics.
  • Security personnel must navigate ethical dilemmas and challenges that can arise during screening.

4.3.2 Key Ethical Considerations:

Balancing Security and Privacy: Security personnel must weigh the importance of security against the rights and privacy of passengers.

Non-Discrimination: Ethical screening practices should not discriminate based on factors such as race, religion, or nationality.

Transparency: Open communication and transparency are essential in maintaining ethical baggage screening.

4.3.3 Importance:

  • Ethical decision-making ensures that baggage screening is conducted with integrity and fairness.
  • It upholds the public’s confidence in the screening process and the aviation security system.

4.4 Handling Sensitive or Restricted Items

4.4.1 Definition and Overview:

  • Baggage screening may reveal sensitive, restricted, or hazardous items that require special handling and procedures.
  • Security personnel must be trained to address these items while adhering to legal and safety requirements.

4.4.2 Key Aspects of Handling Sensitive Items:

Hazardous Materials: Identification and handling of hazardous materials in compliance with safety regulations.

Restricted Items: Recognizing restricted items, such as firearms, and following established protocols for their handling.

Emergency Response: Knowing how to respond to incidents involving sensitive or hazardous items.

4.4.3 Importance:

  • Handling sensitive or restricted items in a safe and compliant manner is essential for aviation security and passenger safety.
  • Proper training and procedures ensure that security personnel respond effectively to potential threats.


The legal framework, privacy and passenger rights, ethical decision-making, and the handling of sensitive or restricted items are integral components of baggage screening. These considerations ensure that screening operations are conducted within legal and ethical boundaries while safeguarding passenger rights. Maintaining a balance between security and privacy is critical in baggage screening, and proper handling of sensitive items is essential for aviation security and passenger safety. It is imperative that security personnel are trained to navigate these complex aspects of the screening process with integrity and professionalism.

Module 5: Threat Detection Techniques

5.1. Identifying Explosive Threats in Baggage

5.2. Recognizing Weapons and Firearms

5.3. Detection of Prohibited Items (e.g., Sharp Objects)

5.4. Screening for Concealed Liquids and Gels

5.1 Identifying Explosive Threats in Baggage

5.1.1 Overview and Definition:

  • Identifying explosive threats in baggage is a critical aspect of X-ray baggage screening. It involves recognizing items or materials that may pose an explosive risk to aviation security.
  • Explosive threats can vary in form, including conventional explosives, improvised explosive devices (IEDs), and components used for explosive devices.

5.1.2 Key Techniques:

Recognizing Suspicious Shapes: Security personnel must be trained to identify shapes or patterns that may indicate the presence of explosives.

Understanding Density and Composition: The density and composition of objects are crucial indicators of potential threats.

Trace Detection: Trace detection equipment may be used to identify microscopic traces of explosives on baggage.

5.1.3 Importance:

  • Identifying explosive threats is a primary objective of baggage screening to prevent acts of terrorism.
  • Effective identification contributes significantly to aviation security.

5.2 Recognizing Weapons and Firearms

5.2.1 Definition and Overview:

  • Recognizing weapons and firearms is essential for aviation security. Security personnel must be trained to identify these items, which may be concealed in baggage.
  • Weapons can include firearms, knives, and other objects that can pose a threat to passengers and crew.

5.2.2 Key Elements in Weapon Recognition:

Firearms: Distinguishing firearms and their components in X-ray images.

Bladed Weapons: Identifying knives and sharp objects that may be concealed.

Weapon Components: Recognizing parts or components that could be assembled into functioning weapons.

5.2.3 Importance:

  • The recognition of weapons and firearms is vital for passenger safety and preventing potential acts of violence onboard aircraft.
  • Proper identification contributes to the overall effectiveness of aviation security.

5.3 Detection of Prohibited Items (e.g., Sharp Objects)

5.3.1 Definition and Overview:

  • Detection of prohibited items, such as sharp objects, is a key aspect of baggage screening. Security personnel must identify items that could be used as weapons or pose a security risk.
  • Prohibited items can include scissors, razor blades, and other sharp objects.

5.3.2 Key Considerations:

  • Shape and Size: Recognizing the shape and size of prohibited items is essential.
  • Risk Assessment: Security personnel must assess the potential threat posed by each item.

5.3.3 Importance:

  • Detecting prohibited items, particularly sharp objects, helps prevent incidents of violence or hijacking.
  • It contributes to ensuring the overall safety of aviation operations.

5.4 Screening for Concealed Liquids and Gels

5.4.1 Definition and Overview:

  • Screening for concealed liquids and gels is a specific focus in baggage screening. This is a response to the threat of liquid explosives or other concealed threats.
  • Passengers are typically restricted from carrying large quantities of liquids or gels in their carry-on baggage.

5.4.2 Key Screening Techniques:

  • Recognizing Liquid Containers: Identifying containers that may hold liquids or gels.
  • Volumetric Assessment: Assessing the volume of liquids or gels to ensure compliance with restrictions.

5.4.3 Importance:

  • Screening for concealed liquids and gels is essential to prevent potential threats involving liquid explosives or other dangerous substances.
  • Compliance with these screening procedures enhances aviation security.


Identifying explosive threats, recognizing weapons and firearms, detecting prohibited items, and screening for concealed liquids and gels are fundamental aspects of baggage screening. These skills and techniques are essential for aviation security, ensuring that potential threats are identified and mitigated effectively. Training security personnel to navigate these challenges contributes to the overall safety and security of air travel.

Module 6: Behavior Analysis and Passenger Profiling

6.1. Behavioral Indicators in Passenger Baggage

6.2. Passenger Profiling Techniques

6.3. Recognizing Suspicious Behavioral Patterns

6.4. Passenger Interaction and Communication

6.1 Behavioral Indicators in Passenger Baggage

6.1.1 Overview and Definition:

  • Behavioral indicators in passenger baggage refer to signs or cues within the baggage itself that may suggest suspicious or abnormal behavior by the traveler.
  • These indicators include irregular packing, unusual item combinations, or patterns that raise concerns regarding the passenger’s intent.

6.1.2 Key Behavioral Indicators:

Discrepancies in Itinerary: Baggage that does not align with the passenger’s stated travel plans.

Nervousness or Anxious Items: Unusual materials or items that may indicate the passenger’s anxiety.

Illogical Item Pairing: Items that seem unrelated or mismatched within the baggage.

6.1.3 Importance:

  • Recognizing behavioral indicators in passenger baggage is essential for identifying potential threats or illegal activities.
  • It enhances aviation security by focusing on passenger behavior beyond traditional screening methods.

6.2 Passenger Profiling Techniques

6.2.1 Definition and Overview:

  • Passenger profiling techniques involve the assessment of passengers based on various factors, including their travel history, behavior, and risk assessment.
  • Profiling helps security personnel identify passengers who may require additional scrutiny or monitoring.

6.2.2 Key Profiling Factors:

Travel History: Analyzing a passenger’s travel history, including recent destinations and frequency of travel.

Behavior Analysis: Observing and analyzing behavioral cues, including nervousness or avoidance of eye contact.

Risk Assessment: Determining a passenger’s risk level based on various factors.

6.2.3 Importance:

  • Passenger profiling is a proactive approach that assists in identifying potential threats before they reach the screening checkpoint.
  • It complements traditional screening methods and enhances overall aviation security.

6.3 Recognizing Suspicious Behavioral Patterns

6.3.1 Definition and Overview:

  • Recognizing suspicious behavioral patterns involves identifying consistent or repeated actions, reactions, or behaviors that may indicate a passenger’s intent to commit a security breach or unlawful act.
  • Security personnel must be trained to identify such patterns for early threat detection.

6.3.2 Key Behavioral Patterns:

Repetitive Movements: Consistent actions, such as repeatedly checking or adjusting baggage.

Avoidance Behaviors: Avoiding eye contact with security personnel or displaying avoidance behaviors.

Abnormal Reactions: Unusual or extreme reactions to routine screening procedures.

6.3.3 Importance:

  • Recognizing suspicious behavioral patterns is a valuable skill in aviation security for early threat detection.
  • It enables security personnel to respond swiftly to potential threats and maintain the safety of air travel.

6.4 Passenger Interaction and Communication

6.4.1 Overview and Definition:

  • Passenger interaction and communication are essential elements of baggage screening, focusing on the engagement between security personnel and travelers.
  • Effective communication and interaction with passengers play a crucial role in aviation security, ensuring cooperation and compliance.

6.4.2 Key Components of Passenger Interaction and Communication:

Clear and Courteous Communication: Security personnel must communicate screening procedures, expectations, and any necessary actions in a clear and respectful manner.

Information Sharing: Passengers should be informed of what to expect during the screening process, promoting a sense of transparency and understanding.

Addressing Concerns: Security personnel must be prepared to address passengers’ questions or concerns, providing reassurance and guidance.

6.4.3 Importance:

  • Effective passenger interaction and communication foster cooperation, reduce stress, and maintain a positive screening experience.
  • Building trust between security personnel and passengers is vital for ensuring the smooth flow of baggage screening operations.


Behavioral analysis in baggage screening is a multifaceted approach that encompasses identifying behavioral indicators in passenger baggage, utilizing passenger profiling techniques, and recognizing suspicious behavioral patterns. These aspects of baggage screening are instrumental in enhancing aviation security by focusing on passenger behavior and intent. By incorporating these techniques, security personnel can proactively identify potential threats and maintain the safety and security of air travel.

Module 7: Threat Response and Crisis Management

7.1. Responding to Suspected Threats

7.2. Evacuation Procedures and Emergency Protocols

7.3. Coordination with Law Enforcement Agencies

7.4. Crisis Communication and Public Safety

7.1 Responding to Suspected Threats

7.1.1 Overview and Definition:

  • Responding to suspected threats involves the procedures and actions taken by security personnel when a potential threat is identified during baggage screening.
  • It includes assessing the situation, making informed decisions, and taking appropriate steps to address the threat.

7.1.2 Key Components of Responding to Suspected Threats:

  • Threat Assessment: Evaluating the nature and severity of the suspected threat.
  • Alert and Notification: Promptly notifying relevant authorities and agencies about the threat.
  • Mitigation and Resolution: Implementing measures to address and resolve the threat, which may include further inspection, quarantine, or evacuation.

7.1.3 Importance:

  • Responding effectively to suspected threats is critical for aviation security and the safety of passengers and crew.
  • Proper procedures help prevent the escalation of threats and ensure swift action to mitigate potential risks.

7.2 Evacuation Procedures and Emergency Protocols

7.2.1 Overview and Definition:

  • Evacuation procedures and emergency protocols are established processes to guide the safe evacuation of passengers and personnel in the event of a security breach, threat, or other emergencies.
  • These protocols include steps for safely evacuating an area or facility.

7.2.2 Key Elements of Evacuation Procedures and Emergency Protocols:

Evacuation Routes: Designated paths and exits for evacuating a facility.

Communication: Clear and effective communication with passengers and personnel during an emergency.

Emergency Equipment: Access to necessary equipment, such as fire extinguishers or medical kits.

7.2.3 Importance:

  • Evacuation procedures and emergency protocols are crucial for the safety and well-being of individuals in the event of security breaches or emergency situations.
  • Ensuring that security personnel are well-trained in these procedures is essential for aviation security.

7.3 Coordination with Law Enforcement Agencies

7.3.1 Overview and Definition:

  • Coordination with law enforcement agencies involves working closely with local, state, and federal law enforcement authorities to address suspected threats, security breaches, or emergencies.
  • Security personnel must be aware of and adhere to established procedures for collaborating with law enforcement.


7.3.2 Key Aspects of Coordination with Law Enforcement Agencies:

Reporting Incidents: Promptly reporting incidents or suspected threats to law enforcement.

Sharing Information: Collaborating with law enforcement agencies to share information and intelligence.

Providing Assistance: Assisting law enforcement authorities in their response efforts, as necessary.

7.3.3 Importance:

  • Effective coordination with law enforcement agencies is essential for a unified and rapid response to security threats or emergencies.
  • It ensures a comprehensive and coordinated approach to addressing security breaches or crises.

7.4 Crisis Communication and Public Safety

7.4.1 Overview and Definition:

  • Crisis communication and public safety pertain to the dissemination of information to passengers, personnel, and the public in the event of a security incident, threat, or emergency.
  • Timely and accurate communication is critical for managing the situation and ensuring public safety.

7.4.2 Key Elements of Crisis Communication and Public Safety:

Information Dissemination: Communicating essential information regarding the situation, including risks and safety measures.

Media Relations: Managing interactions with the media and providing accurate updates to the public.

Reassurance and Support: Offering support to affected individuals and assuring them of their safety.

7.4.3 Importance:

  • Crisis communication and public safety are fundamental in maintaining calm, managing public perception, and ensuring a coordinated response to security incidents or emergencies.
  • Timely and accurate communication helps maintain public trust and confidence.


Responding to suspected threats and emergencies is a critical aspect of baggage screening and aviation security. Effective threat response, evacuation procedures, coordination with law enforcement agencies, and crisis communication are vital components of ensuring the safety and security of passengers and aviation operations. Proper training and adherence to established procedures are essential for security personnel to respond swiftly and effectively in the face of potential threats and emergencies.

Module 8: Technology Integration and Automation

8.1. Automated Threat Recognition Systems

8.2. Machine Learning and AI in Baggage Screening

8.3. Integration with Security Information Systems

8.4. Advancements in Baggage Screening Technology

8.1 Automated Threat Recognition Systems

8.1.1 Overview and Definition:

  • Automated Threat Recognition Systems are advanced technologies that use artificial intelligence (AI) and machine learning to analyze X-ray images of baggage and identify potential threats.
  • These systems can automatically flag suspicious items or areas within the baggage image for further inspection by security personnel.

8.1.2 Key Features of Automated Threat Recognition Systems:

AI Algorithms: Machine learning algorithms that can identify patterns and anomalies in X-ray images.

Real-time Analysis: The ability to analyze images in real-time, allowing for quick threat detection.

Improved Efficiency: Reduces the workload on human screeners by automating the initial threat recognition process.

8.1.3 Importance:

  • Automated Threat Recognition Systems significantly enhance the efficiency and accuracy of baggage screening.
  • They help reduce the risk of human error and improve aviation security.

8.2 Machine Learning and AI in Baggage Screening

8.2.1 Overview and Definition:

  • Machine learning and AI are technologies that empower baggage screening systems to learn from data and adapt to evolving threats.
  • These technologies enable the development of intelligent, self-learning systems for baggage screening.

8.2.2 Key Aspects of Machine Learning and AI:

Training Models: Developing AI models to recognize threats based on historical data and examples.

Adaptability: The ability to update and improve threat recognition capabilities as new threats emerge.

Integration with Existing Systems: Seamlessly integrating AI and machine learning with current baggage screening infrastructure.

8.2.3 Importance:

  • Machine learning and AI in baggage screening offer the potential for continuous improvement in threat recognition and adaptation to emerging security risks.
  • They enhance the effectiveness and reliability of baggage screening.

8.3 Integration with Security Information Systems

8.3.1 Overview and Definition:

  • Integration with security information systems involves connecting baggage screening systems with broader security infrastructure.
  • This allows for the sharing of threat data, real-time information, and coordination with other security measures.

8.3.2 Key Aspects of Integration with Security Information Systems:

Data Sharing: Exchanging information with other security components, such as biometric systems or access control.

Real-time Alerts: Receiving immediate alerts and notifications about potential threats or security incidents.

Streamlined Coordination: Ensuring that baggage screening is part of a holistic security approach.

8.3.3 Importance:

  • Integration with security information systems creates a more comprehensive and coordinated security environment.
  • It enhances the ability to respond effectively to potential threats and ensures a seamless security experience for passengers.

8.4 Advancements in Baggage Screening Technology

8.4.1 Overview and Definition:

  • Advancements in baggage screening technology encompass the development and deployment of innovative tools and systems to improve screening accuracy, speed, and safety.
  • These advancements may include new X-ray systems, detection methods, and user interfaces.

8.4.2 Key Advancements:

Improved X-ray Resolution: Higher resolution images for better threat detection.

Reduced False Alarms: Enhanced algorithms to reduce false positive rates.

User-Friendly Interfaces: Intuitive interfaces for security personnel.

8.4.3 Importance:

  • Advancements in baggage screening technology contribute to the ongoing improvement of aviation security.
  • They enhance threat detection, reduce operational errors, and improve the passenger experience.


Technological advancements in baggage screening, including automated threat recognition systems, machine learning and AI, integration with security information systems, and innovations in baggage screening technology, play a pivotal role in enhancing aviation security. These technologies offer more efficient and accurate threat detection, adaptability to evolving security risks, streamlined coordination, and overall improvements in baggage screening operations. Staying current with these advancements is essential for maintaining the safety and security of air travel.

Module 9: Operational Challenges and Best Practices

9.1. Overcoming Challenges in High-Traffic Airports

9.2. Training and Skill Development for Screeners

9.3. Baggage Screening Quality Control

9.4. International and National Standards Compliance

9.1 Overcoming Challenges in High-Traffic Airports

9.1.1 Overview and Definition:

  • Overcoming challenges in high-traffic airports involves addressing the unique difficulties associated with processing a large volume of baggage and passengers.
  • High-traffic airports often face time constraints, resource limitations, and increased security risks.

9.1.2 Key Challenges in High-Traffic Airports:

Time Sensitivity: The need for quick and efficient screening to minimize passenger delays.

Resource Allocation: Ensuring adequate personnel and equipment for screening.

Security and Efficiency Balance: Maintaining both robust security and a smooth passenger experience.

9.1.3 Importance:

  • Overcoming challenges in high-traffic airports is essential for maintaining both security and operational efficiency.
  • It requires effective planning, resource allocation, and innovative solutions.

9.2 Training and Skill Development for Screeners

9.2.1 Overview and Definition:

  • Training and skill development for screeners are critical for ensuring that security personnel are proficient in interpreting X-ray images and identifying potential threats.
  • This training includes both initial instruction and ongoing skill enhancement.

9.2.2 Key Aspects of Training and Skill Development:

Initial Training: Providing foundational knowledge and skills for baggage screening.

Ongoing Education: Continuing training to stay current with evolving security threats and technology.

Proficiency Assessment: Evaluating the performance and skills of security screeners.

9.2.3 Importance:

  • Training and skill development are fundamental for maintaining a high level of threat detection accuracy.
  • Well-trained screeners are crucial for aviation security.

9.3 Baggage Screening Quality Control

9.3.1 Overview and Definition:

  • Baggage screening quality control encompasses processes and measures to ensure the accuracy and effectiveness of baggage screening operations.
  • It involves checks, audits, and reviews to maintain the highest standards of security.

9.3.2 Key Components of Quality Control:

Random Inspections: Conducting periodic audits and spot checks to assess screener performance.

Feedback Mechanisms: Providing feedback to screeners and addressing any issues or discrepancies.

Data Analysis: Reviewing data to identify trends, areas for improvement, and areas of excellence.

9.3.3 Importance:

  • Quality control is essential for identifying and rectifying screening deficiencies, ensuring a high level of threat detection.
  • It contributes to maintaining the safety and security of air travel.

9.4 International and National Standards Compliance

9.4.1 Overview and Definition:

  • Compliance with international and national standards is vital to ensure that baggage screening operations meet regulatory requirements.
  • These standards are set by aviation authorities and government agencies to maintain consistent security practices.

9.4.2 Key Aspects of Standards Compliance:

Regulations and Guidelines: Adhering to international and national regulations and guidelines for baggage screening.

Security Protocols: Implementing security protocols that align with regulatory requirements.

Audit and Assessment: Participating in audits and assessments to verify compliance.

9.4.3 Importance:

  • Compliance with international and national standards is critical for demonstrating a commitment to security and ensuring alignment with best practices.
  • It contributes to the overall safety and security of air travel.


Operational excellence in baggage screening involves addressing the unique challenges of high-traffic airports, providing comprehensive training and skill development for screeners, implementing effective baggage screening quality control measures, and ensuring compliance with international and national standards. These aspects are integral to maintaining a high level of security in baggage screening operations, thereby ensuring the safety and security of air travel.

Module 10: Real-World Scenarios and Case Studies

10.1. Case Studies of Successful Threat Detection

10.2. Lessons Learned from Past Security Incidents

10.3. Interactive Simulation and Practical Exercises

10.4. Continuous Learning and Professional Development in Baggage Screening

10.1 Case Studies of Successful Threat Detection

10.1.1 Overview and Definition:

  • Case studies of successful threat detection involve in-depth examinations of real-world incidents where security personnel successfully identified and intercepted potential threats.
  • These studies provide valuable insights into effective threat detection practices.

10.1.2 Key Components of Case Studies:

Incident Description: A detailed account of the incident, including the threat type and circumstances.

Screening Process: An analysis of the baggage screening process that led to threat identification.

Lessons Learned: Identifying key takeaways and best practices for threat detection.

10.1.3 Importance:

  • Case studies of successful threat detection offer practical examples of effective screening procedures.
  • They serve as educational tools for improving baggage screening practices and enhancing security.

10.2 Lessons Learned from Past Security Incidents

10.2.1 Overview and Definition:

  • Lessons learned from past security incidents involve the analysis and review of previous security breaches, incidents, or failures.
  • These lessons provide valuable information for preventing similar incidents and enhancing security measures.

10.2.2 Key Aspects of Lessons Learned:

Incident Review: A comprehensive review of the security incident, including its causes and consequences.

Root Cause Analysis: Identifying the underlying factors that led to the incident.

Recommendations: Offering actionable recommendations for preventing similar incidents.

10.2.3 Importance:

  • Learning from past security incidents is essential for identifying vulnerabilities and implementing proactive security measures.
  • It contributes to continuous improvement in baggage screening practices.

10.3 Interactive Simulation and Practical Exercises

10.3.1 Overview and Definition:

  • Interactive simulation and practical exercises involve hands-on, scenario-based training for security personnel.
  • These exercises simulate real-world threat scenarios and require participants to apply their knowledge and skills.

10.3.2 Key Elements of Simulation and Exercises:

Scenario Creation: Designing realistic threat scenarios for training.

Participant Engagement: Involving security personnel in simulated exercises.

Debriefing and Feedback: Providing feedback and insights to participants after the exercise.

10.3.3 Importance:

  • Interactive simulation and practical exercises offer a practical and immersive way to enhance threat detection skills.
  • They help security personnel prepare for real-world security incidents and improve their response capabilities.

10.4 Continuous Learning and Professional Development in Baggage Screening

10.4.1 Overview and Definition:

  • Continuous learning and professional development encompass ongoing training and skill enhancement for security personnel in baggage screening.
  • This involves staying current with evolving security threats, technology, and best practices.

10.4.2 Key Components of Continuous Learning:

Continuing Education: Participating in regular training and education programs.

Knowledge Sharing: Collaborating with peers and sharing experiences and insights.

Certification and Qualification: Pursuing certifications and qualifications in baggage screening.

10.4.3 Importance:

  • Continuous learning and professional development are essential for maintaining a high level of expertise in baggage screening.
  • They ensure that security personnel are well-prepared to address emerging security challenges.


Learning from experience in baggage screening involves studying successful threat detection case studies, extracting valuable lessons from past security incidents, engaging in interactive simulation and practical exercises, and committing to continuous learning and professional development. These aspects are crucial for enhancing the effectiveness and readiness of security personnel in baggage screening, contributing to the overall safety and security of air travel.

Course Conclusion

Each module addresses essential aspects of X-Ray baggage screening image interpretation, ranging from technology and threat detection to legal and ethical considerations, providing a comprehensive foundation for aviation security personnel.


Using Eye Movements to Understand how Security Screeners Search for Threats in X-Ray Baggage.



There has been an increasing drive to understand failures in searches for weapons and explosives in X-ray baggage screening. Tracking eye movements during the search has produced new insights into the guidance of attention during the search, and the identification of targets once they are fixated. Here, we review the eye-movement literature that has emerged on this front over the last fifteen years, including a discussion of the problems that real-world searchers face when trying to detect targets that could do serious harm to people and infrastructure.

  1. Introduction

The job of an airport screener is to stop a wide range of prohibited items being taken on board aircraft. Some of these prohibited items are threats (e.g., guns, knives, explosives) and some are restricted for other reasons (e.g., liquids, narcotics, etc.). Airport screeners search for the presence of these prohibited items in two or three-dimensional images derived from X-raying screening trays. Their job is, therefore, one that requires the inspection of visual images for the presence of prohibited items.

While the ability to detect prohibited items in X-ray images of baggage has been explored for some decades, it came on to the radar of experimental psychologists following the terrorist attacks on the World Trade Center in New York in 2001. From this time onwards, researchers sought to better understand the perceptual and cognitive challenges associated with airport baggage screening. In this review, we report some key data from experimental studies in which eye movements have been recorded in order to better understand how, when, and why errors are made during the search. The results have been informative from both a theoretical and practical perspective.

  1. What Are the Images Used in Baggage Screening?

Compared to most real-world search stimuli and medical images, baggage images have less structure. The contents and arrangement of items in bags and of bags on screening trays are difficult to predict. This leaves the screener with few expectations about where these objects should appear within the image, and thus makes decision making more uncertain than in most other search tasks.

Baggage images are typically, but not always, colored (or more correctly pseudo-colored) using a standard color mapping (see Figure 1). The artificial color mapping depicts the density of the objects in the image, and also provides some information about object materials. Interpreting density is relatively straightforward: areas with no material or with very low density appear as white, with higher density regions appearing as darker and more saturated colors. If a region is so dense that no X-rays pass through it, it is depicted as black. Interpreting X-ray images is tricky because these arbitrary color mappings produce a visual world that is very different from what we are accustomed to.


Figure 1. Example X-ray images of bags, two contain threats (top left—knife; bottom right–pistol); images from the CaSePIX image library. Reproduced with permission from the copyright holder Dr Greg Davis.

The depiction of information about the materials making up the objects is more complex. The X-rays that have passed through the objects can be used to infer an atomic number, which is then used to put each part of the image into one of three categories. Those regions with the lowest atomic numbers will include most explosives, but will of course include many non-threatening organic materials as well, such as foods and fabrics. These regions of the image are assigned an orange hue in the standard color mapping. Regions with the highest atomic numbers include metal weapons such as guns and knives, but also many harmless objects, and appear as blue. Atomic numbers in between these ranges are often shown as green, and can generally be regarded as less threatening because they are not metal and are not explosives.

The challenge to the interpretation of X-ray images is increased because all but the densest objects have a certain amount of transparency. Once X-rays have been emitted from the X-ray sources and passed through one object, they may pass through other objects before striking the collector and being registered. Thus, one part of the image often depicts information about multiple objects. This overlap introduces the additional problem of segmenting the different objects from one another.

There is another aspect of object overlap in X-ray images that is very different from our usual experience. When an orange, green, or blue hue is assigned to each part of the image, it is based on an average of the atomic numbers of the different materials that the X-ray beam has passed through. Thus, if an organic object with atomic number in the low range overlaps with a metal object with atomic number in the high range, the region of overlap in the image may be depicted as belonging to the middle range.

Clearly, interpreting overlapping objects in X-ray images requires careful consideration. Adding to the challenge is that these searches are often conducted in a busy work environment with time pressure from long lines of passengers who are concerned about missing a flight. Thus, it is important for us to find ways to make the task easier and to increase accuracy and efficiency. This is particularly important in light of the fact that, although search times (and limits) vary from airport to airport and from country to country, screeners often will take around only six seconds to complete their searches 

Recent innovations to security screening include dual view where screeners make their decisions from interpreting two orthogonal views (and sometimes a photographic image of the tray as well) . Computed tomography (CT) scanners have also become available as an alternative to more traditional X-ray machines. An apparent advantage of CT scanners is that they can generate a 3D image of a tray which can be manipulated on-screen, including advancing through 2D slices of the full 3D volume . The challenge to threat detection in baggage search that these conditions create is altered but not fundamentally changed by recent innovations aimed at supporting image interpretation by providing richer imagery.

While these advanced imaging systems add information that should help in the detection of prohibited items, they may also create new challenges. We note two of these challenges here as we think they are important, despite being largely unexplored. First, search and interpretation from viewing two orthogonal views requires the ability to make eye movements to the same or related locations on the different images showing orthogonal views. As far as we are aware there is only a single unpublished study on the success and utility of making these eye movements with respect to baggage screening . Second, radiological imaging requires searching for targets in slices of a 3D volume, but what is being imaged in medical imaging is of a known structure and complexity, and what is being identified are variations from a norm. The risk to baggage screening of using this technology is that features consistent with an early emerging target may be ‘tracked’ through slices, leading to misses from targets appearing later in the sequence of slices. We know of only a single study that has explored an analogue of this situation .

  1. What Are the Decisions Made by Baggage Screeners?

While we refer to baggage search throughout this review, in line with the existing literature on this topic, in the real world of baggage screening, the images are more correctly defined as the contents of screening trays. Screening trays can contain bags, laptops, and other items that are routinely X-rayed such as keys, wallets and mobile telephones. The clarification is an important one to make. It is important to be clear that the visual search and threat identification challenge that these different situations deliver for screeners changes somewhat unpredictably on a tray-by-tray basis. The important point is that some decisions can be made without the need for extensive searching at all, but on the basis that the gist of the tray indicates the presence of very little or nothing that comes close to resembling a target. We define gist here in a very limited way as providing an estimation of the clutter of the objects within a tray. Virtually empty trays may allow rejection on the basis that the limited material present can easily be discounted as resembling a target. We do not mean to imply gist emerging from object co-occurrence, configurations or scene backgrounds.

A screener can either “clear” a bag and let it pass, or “reject” it. A bag may be rejected because a prohibited object(s) can be seen and readily identified in an image. However, a bag may also be “rejected” simply because the screener cannot be certain that prohibited objects(s) are not present in an image, probably because there are many objects overlapping one another. In either case, the decision to reject bags usually leads to further investigation through hand searching.

  1. The Security Search Task

Having considered the images from which ‘clear’ and ‘reject’ decisions are made and the nature of those decisions, we now turn to eye movements and the security search task. The color coding of atomic density allied to the uncertainties created by contents, the arrangement of contents, and viewpoint mean that color is often used to guide attention to possible targets. Fortunately, we know quite a lot about how people search for color and the eye movement behavior associated with this search. In addition, studies have explored the costs associated with simultaneously searching for more than one color, as is the case when searching for, for example, the orange indicating possible plastic explosive and the blue/black indicating possible wire, guns or knives.

There are a number of prominent models of visual search, and fewer such models of visual search and eye movement behavior. Despite their differences, the models of search share the common idea that, during the search, our limited resources need to be directed to given objects in order to determine whether those objects are targets. In the classic Guided Search model, which has been revised several times since its inception , attention is directed towards target-similar objects, one at a time, until a target is found, or until the searcher decides to quit. The decision to terminate search can depend on a number of factors, including the time spent searching and errors made on previous trials.  Models of eye movements and visual search share much the same overall architecture, though they do generally focus more on the mechanisms that govern when and where to move the eyes during a search.

The recording of eye movements during the search opened up new avenues for understanding the information processing that takes place during the search. Many early studies of eye movements and the search of complex images involved radiographic image screening. These studies adapted and developed the language of guidance in visual search to focus on the fractionation of what takes place in terms of information-processing during visual search. Put simply, by recording eye movement behavior, researchers were able to determine when, how and why targets were missed; this was, and still is, a question of vital importance when it comes to real-world search tasks in which missing a target can have severe consequences. These studies developed a new framework for understanding eye movements during the search by focusing on failures of guidance and failures of decision-making.

Failures of guidance were measured using three eye movement metrics: namely, the probability that a target was fixated during a search, the speed at which a target was fixated during the search, and the number of nontargets fixated. Connecting with classic models of search, such as Guided Search , under this view, an easier search task can be conceptualised as one wherein the target is directly and rapidly fixated. As a search task becomes more difficult, there is a longer delay before the target is fixated, as more and more nontargets are fixated, and the target is less likely to be fixated at all. As a direct consequence of this reduction in the efficacy of guidance, targets in more difficult search tasks can then be missed purely because of a failure in guidance. An early eye tracking study by McCarley and colleagues found that after practice, subjects fixated fewer nontargets, but were no more likely to fixate the target. We will discuss what factors make search tasks more difficult in the context of baggage screening in more detail below.

Even if a target is directly fixated, this is not a guarantee that it will be detected. Such failures of decision-making in search can also be measured using two eye movement metrics: the time between fixating the target and identifying it as a target, and the probability of detecting a target after fixating it. When a target is more complex or difficult to recognise, this increases the time required to identify it, and, worse still, reduces the probability that it will be detected even after being directly fixated. The study mentioned earlier by McCarley et al. found that search practice substantially improved target identification, although some of that improvement was tied to specific target images.

With this basic framework now set out, we will turn to outlining several ways in which exactly these forms of error have been studied extensively in the context of tasks inspired by airport X-ray baggage screening, beginning with the study of target templates in baggage search.

An understanding of the time taken to search baggage comes from studies of attentional control and guidance. A number of different theories of attentional control  are built on the assumption that early visual processing provides limited types of information that can be used to guide attentional selection. In difficult searches like baggage search, this attentional guidance is based on a mental representation with information about the target to be found, sometimes called the “target template”. Once attentional guidance has determined which portions of the input should be selected, those portions are then processed more fully to recognize objects and interpret their configurations relative to one another. In general, the information stored in the target template seems to be constrained by the rather severe limitations that apply to visual working memory. Despite those limits, visual search can be very flexible, as demonstrated by experiments by Wolfe and colleagues in which participants are asked to search for as many as 100 different targets. These searches probably do not benefit from the accurate guidance that is possible when searching for a single color or orientation.

In bags that are largely empty, the guidance of attention may be so precise that attention is quickly directed to the search target, or the absence of any target can be quickly determined. Guidance may instead be imprecise, perhaps because it has not been possible to develop an effective target template (although even target templates that inaccurately specify target features afford guidance proportional to their target similarity). In addition, it may also be the case that a bag is so densely packed with multiple metal and organic objects that focused attention is required to segment and identify and possible objects. Many baggage searches will fall somewhere in between these easy and difficult extremes. In general, attention should be guided toward the dark blue and black regions that might be metal weapons, and to the orange regions that might be explosives. By preventing attention to the light and green regions, guidance can limit the amount of time spent in attentional processing.

Attentional guidance is fairly accurate when a target is known to have a specific color. Information about the target color can be used to form a mental representation or template to guide attention toward items with similar colors, avoiding most of the other colors. Security search is somewhat more complicated than this simple single-color search, however: the threat targets can be either black, blue, or orange, and the regions that can be ruled out based on color can be either green or low-saturation colors close to white. Performance in searches for multiple targets is often worse in the search for two targets than in the search for a single target, although the nature of the cost varies depending on the task. Sometimes, accuracy is lower or response times are longer in dual-target search, as shown by Menneer and colleagues in searches for abstract target stimuli defined by color, orientation, or shape. Menneer et al also found dual-target costs in accuracy for searches among X-ray images of objects, as long as the two targets differed from one another in color. Eye-tracking studies, both with abstract shapes and with X-ray images have helped to illustrate one source of the dual-target cost. In those experiments, participants who were searching for two types of targets with different colors made a number of fixations to distractors that had colors different from either target.

The drop in search efficiency between 1-color search and 2-color search was explored with a set of very basic abstract stimuli by Stroud et al. The target was distinguished from distractors by a difficult shape discrimination, but it could nonetheless be found quickly if its color was known in advance. In 1-color search, fixation rates to colors that were very different from the target were low, demonstrating effective color guidance. However, when the target could appear in either of two very different colors, there were many more fixations to colors that were very different from the distractor. Dual-target search was much slower because participants were spending time fixating distractors that should have been excluded by color guidance. Search guidance could be degraded further by expanding the set of possible target colors to eight, so that the range of possible colors covered half of the possible hues The implication of this finding for baggage screening is clear: the simultaneous search for more than one target reduces guided eye movements to targets and increases unguided eye movements to things that are very unlikely to be targets. The time limited aspect of baggage screening makes the increased number of unguided fixations in multiple target versus single target search a significant problem when considered in the context of the job.

While color may be the best cue to guide attention in baggage search, it is not a reliable cue to the presence of prohibited items, as many non-prohibited items are also coded in orange and blue. Participants in these studies have the option to employ color guidance or not in these particular tasks because the target can often be identified by shape. The Stroud et al. study shows that if many of the items with the target color also have a shape that makes them a distractor, then participants are much less likely to guide their eye movements by color, but if most of the items with the target colors are actual targets, then there will be a stronger tendency to direct fixations to the target colors and away from colors that were never targets. This pattern suggests that participants have some high-level control over how guidance is used in search: if they expect color to lead them to a target on a high proportion of trials, then they will employ color guidance more often. For baggage screeners to make use of color to guide their eye movements, they may well need to resist this tendency, and instead decide to use color guidance, despite the low likelihood that what is fixated will be a prohibited item.

Other experiments have demonstrated flexibility in search guidance that may be informative to understanding how to manage searching for multiple targets. In an eye-movement study by Beck, Hollingworth, and Luck, participants could either search for two target colors simultaneously, or search first for one color and then switch to the other, depending on the instructions. When left to choose their search method on their own, participants seem to do something in between purely simultaneous and purely successive search: they often switch between fixating one target color and fixating the other, but at a given moment during the search they seem to favour one target over the other.

In summary, when a search task, like baggage search, requires holding two colors in working memory, participants are more likely to perform the task without guiding attention by color, even though guidance may make their search more efficient. This occurs whether both colors are search targets, or one is held for another task. When guidance is not used, there may be many fixations to distractors that could be avoided, and the search may take much longer to complete.

  1. Identification of Complex, Overlapping Transparent Objects

As noted above, one of the key problems associated with searching through baggage X-rays is the fact that they contain a wide array of varied overlapping and transparent objects. The manner in which these objects combine creates a uniquely difficult task wherein objects can be difficult to identify (see reference for an excellent illustration of this point).

Applying what is known regarding human search behavior to the basic properties of the images that screeners search through is a difficult task, mainly because the vast majority of visual search experiments have utilised displays wherein objects do not overlap with one another. Studies that have examined the deleterious effect of searching through cluttered, overlapping displays find that search is impaired for these difficult images.

More recently, we conducted a series of experiments that focused on the problem of overlap and transparency in search and also recorded the eye movement behavior of participants as they searched the displays. We found that, for opaque displays, target detection rates fell substantially and reaction times increased when object overlap increased. We also found that perceptual selection and perceptual identification errors increased substantially as overlap increased. Moving to transparent displays, the same basic pattern emerged, with the important difference that reaction times were even longer for higher levels of overlap than in the opaque displays. Crucially, this was likely to be the result of the fact that, in transparent images, the complexity of the displays obscures but does not remove information as it does in opaque displays. The cost associated with perceptual identification in transparent images led to long verification times because of the need to examine multiple different possibilities of grouping and object identity before a final decision could be reached.

Despite the apparent concerns regarding the effects of overlapping images in complex displays, there is reason to be confident that there may be ways to assist those searching these difficult images. In the same set of studies, we also found that presenting the objects on separate three-dimensional depth planes aids search and ameliorates the effects of overlap, suggesting that future research would benefit from further understanding the benefits of depth in aiding in the segmentation and identification of complex, overlapping objects during the search.

  1. The Problem of Low Prevalence

It is fortunate that targets in X-ray baggage are very rare indeed, but the relative scarcity of targets can influence the likelihood of detecting those targets once they finally do appear. One line of studies focuses on the prevalence of targets, which is defined as the proportion of trials on which a target is presented. These studies demonstrate that low target prevalence, e.g., 2%, results in a reduction in the target detection rates compared to higher prevalence levels, e.g., 50% .

Early studies of this prevalence effect used behavioral measures, including response times, response accuracy, and signal detection theory measures The general finding was that reductions in target prevalence resulted in a shift in the response criterion such that participant responses became more conservative and less likely to respond that a target was present. By this we mean that their hit rate and false alarm rate were reduced and target-absent responses were sped up relative to when target prevalence was higher.

Later and more recent studies of eye movement behavior in conditions of varied prevalence have helped to elaborate on how, when and why rare targets are missed when prevalence is low. For example, a reduction in target prevalence increases failures of perceptual selection, whereby searchers fixate fewer objects in each trial as prevalence is reduced. It is worth noting that efforts to overcome this shortcoming by providing feedback in relation to where fixations have and have not been made are unlikely to help .

Moreover, the failure of perceptual selection is added to by the fact that targets are fixated more slowly when target prevalence is low. Moreover, a reduction in target prevalence also increases the likelihood of perceptual identification errors, with participants not just being slower to identify targets after fixating them, but also less likely to detect targets after fixating them.

It is worth noting that some of the challenges caused by the low prevalence of actual threat items are mitigated in the real world of baggage screening by the inclusion of other prohibited items that must be searched for. Even without the inclusion of other prohibited items that must be searched for, the detection of threat items may be aided by the inclusion of Threat Image Projection (TiP) items amongst baggage images. TiP items are fictitious guns, knives and IEDs that are pseudo-randomly presented in the context of whole bags to screeners items are always of low prevalence – one published study reported on TiP detection rates using data from real screeners performing their jobs in situ  where the prevalence rate was set at 4%. That low prevalence rate is, however, higher than the actual incidence of these kinds of threats.

Although target prevalence remains an issue for baggage screeners, they are detecting prohibited items, including threat items, more often than might be supposed. Importantly, with the inclusion of prohibited items beyond threats, they are doing so at a higher rate than envisaged in some experimental analogues of baggage screening where target prevalence is manipulated.

  1. Differences in Individual Screeners

So far we have considered how studying eye movements has informed us of the challenges associated with finding and identifying prohibited items during baggage search. In doing so it seems to us that key eye movement metrics reflect different stages of processing and decision making associated with baggage search. In this final section, we consider how these metrics might be influenced by individual differences.

As a population, baggage screeners vary in their age, experience and training, in addition to their cognitive abilities and affective characteristics. Differences in screener performance are certainly contributed to by individual differences in basic perceptual processing (e.g., references). There is evidence for reliable individual differences over time in sensitivity and some evidence of modest age-related decline that has been attributed to reducing efficiency of perceptual and cognitive abilities that cannot be overcome by years of performing the screening task.

More importantly, and perhaps not surprisingly, differences in screener performance are contributed to by training focussed on developing robust templates of prohibited items . A question that emerges is whether training improves search guidance to possible prohibited items or verification for identification? While there is likely to be some improvement in both, there is good evidence that the effect of training is more striking on processes associated with verification than guidance . The improvement in target detection that comes with training is more striking with respect to IEDs than guns, knives and other kinds of prohibited items . It seems that training mostly (though not exclusively) improves the robustness of the templates that allow screeners to more readily match what is seen to what they know, and improvement is greatest when there is most to learn.

A pattern emerges of good screeners being defined as having (1) an ability to make a fast and accurate ‘clear’ decision based on the gist derived from the image; (2) excellent guidance of attention to potential prohibited items; (3) speed in verifying or rejecting item identity based on having robust target templates to match items against; while also ensuring that they are (4) sufficiently exhaustive in search to ensure all possible targets are investigated and all viable interpretations of items are tested against target templates.

While these are distinct issues that could be explored in future studies, the current literature allows only a coarser analysis, which we consider in terms of three broad categories. The first is how working memory capacity and attentional control might influence search and guidance. The second is how factors related to conservatism in decision-making influence the thoroughness of search and the time given to verify target presence and absence (and how this relates to the first category). Finally, we consider a third broad category of individual differences that may relate specifically to search through slices of a 3D data volume.

Our goal in exploring these issues is simple. Prior evidence clearly shows that baggage screening performance is associated with basic perceptual skills and task knowledge. Beyond that, is there any evidence of systematic relationships between cognitive and affective factors and eye movement behavior when searching complex images then we might be able to use to inform the selection of baggage screeners? We ask this question in light of the four characteristics of good screening that were outlined above and in the spirit of a hypothesis worthy of exploration. We do not have a view of the relative importance of the individual differences that we discuss for baggage screening, especially since they may jointly influence the performance of any given individual.

  1. Working Memory Capacity and Attentional Control

While working memory capacity (WMC) does not predict performance in very simple searches, it does in complex search tasks, including those that demand sustained high levels of attentional control . We previously discussed how target templates are held or processed in visual working memory (e.g. references. It follows from this that effective dual-target search will typically involve more WMC (even in searches which require long-term memory storage , WM is still needed for encoding, retrieving and maintaining templates).

It is also conceivable that holding two targets in WM, or one target in conjunction with an item from a simultaneous memory task, may require some sort of memory organization or segregation to minimise interference. Maintaining this WM segregation may require more resources than merely searching for a single target. This is consistent with both the study noted earlier which showed that adding extra WM load can interfere with search guidance and lead to more unguided fixations and with evidence that when search distractors match a color held in WM, saccades are slower and less accurate . Greater WMC must therefore be advantageous in dual-target search tasks that involve the guidance of eye movements to two targets.

Extra resources will also help sustain high levels of attentional control and avoid erroneous eye movements due to the conflicts that must be resolved as different targets try to pull attention in different directions . Furthermore, maintaining attentional control is likely to be particularly challenging when a dual-target search is coupled with low levels of target prevalence. It is beyond the scope of the current review to discuss the relationship between WM and attention at any length (see reference for more detail), but we can say that WMC predicts attentional control in an antisaccade task and, in the same study, high WMC participants exhibited less of a performance cost when switching from an anti- to a prosaccade task. Similar costs may be observed when two target templates are active successively and guidance shifts from one target to another over time; not only will extra resources be needed to coordinate the switch, but residual information from previous task sets, including target templates, will also increase the likelihood of eye movements to irrelevant items.

As individuals search, some of the resources that could have been allocated to dual-target guidance may instead be engaged in other cognitive tasks; they may be monitoring how well they are doing in the task, or speculating on the motivation for the experiment, or making plans for the rest of the day. Participants may also be motivated to hold some resources in reserve due to a built-in aversion to high levels of resource utilization that ensures resource availability for unanticipated future tasks. In summary, while it not been tested directly, it follows from these behavioral and eye movement laboratory studies that good baggage screening performance would be associated with high WMC and good attentional control.

  1. Setting Conservative Decision Thresholds

While low target prevalence makes search and target verification more challenging, good baggage screeners will be relatively resistant to these effects; they will tend to ensure all locations that might contain a target are searched and that those locations are processed until all possible interpretations of image features are considered before deciding to clear or reject. Doing so equates to good baggage screeners setting conservative decision thresholds for terminating their inspection of individual fixations during the search and for continuing fixations before ending search, including when no target is found. The ability to set conservative decision thresholds will be influenced by training and a number of studies have examined the performance of trained screeners. In these studies, screener performance is often characterized in terms of Signal Detection Theory measures that reflect the relationship between screeners’ hit rates and false alarm rates.

Apart from training, the setting of decision thresholds will also be influenced by the individual tendency to be ‘satisfied’ with search. ‘Satisfaction of search’ was a term used primarily in the context of radiographic screening to describe failures to detect subsequent targets following the detection of a first. Different accounts of this effect exist, but it is likely that it is not related to satisfaction at all, but rather a combination of a bias towards finding subsequent targets that are similar to an initial target (perceptual set bias) and the depletion of cognitive resources associated with finding an initial target . This phenomenon has since been renamed ‘subsequent search misses’ to reflect the contribution of these other factors. Individual tendencies linked to satisfaction are an important source of individual variance in the setting of quitting thresholds and response criteria during the search. Consistent with the notion of resource depletion, recent evidence links individual differences in WMC to errors of both perceptual selection (failure to fixate targets) and perceptual identification (failure of verification) during the search. In a follow-up study, the effect of WMC on perceptual selection errors was attributed to quitting thresholds that were not adequately conservative; that is to say, searchers quit before they had fixated targets.

There are a number of psychological measures that tap into individual tendencies for satisfaction during the search. One of these is the ‘Maximization Scale’, a 13-item personality scale that assesses the degree to which an individual is a ‘maximizer’, who will strive for the best outcome, or a ‘satisficer’, who will accept outcomes that are good enough. Maximization is also conceptually linked to perfectionism and it has been demonstrated that perfectionists who engage with X-ray search and object identification tasks perform these tasks more accurately and faster than other individuals. Attention to detail, as assessed by a subscale of the Autism Quotient, predicts accuracy in X-ray baggage search and has been the basis of a recent scale developed specifically to assess aptitude for X-ray baggage screening, the XRIndex.

One component of trait anxiety, Intolerance of Uncertainty (IU), may also lead to a tendency to settle for poorer performance in a search. IU represents the extent to which individuals experience (and can cope with) worry about uncertain future events  and can bias decision making towards minimizing uncertainty. IU positively predict false alarm rates in a complex search task where participants searched for low prevalence color targets. In this task the color targets appeared in arrays of colored squares whose appearance changed dynamically over time through an ordered color space. The increased false alarms reflect a decision to prematurely classify as targets items that are still distractors (and have yet to become targets).

Compared to simple laboratory search tasks, the complexities of X-ray search may increase the likelihood of individuals who tend to be easily satisfied, or those with greater IU, to terminate searches prematurely (i.e. lowering quitting thresholds). We suspect that there are some contingencies between the thresholds for terminating fixations both to possible targets and in target verification, WMC, maximisation and IU. At the very least, it is an area worthy of further research.

  1. Searching through Three-Dimensional Volumes

Earlier in this review we discussed how baggage search is merely altered, and not fundamentally changed, by advances in screening technology. One of the ways in which screening is altered by CT scanning technology is that screeners are able to search through sequential 2D slices of a 3D volume. Interacting with images in this way introduces a dynamic element into the search task, whereby the image to be searched changes dynamically as screeners move through the slices. Each 2D slice is also related to the slices either side of it in 3D space, such that moving through slices will cause objects to emerge over time in a predictable way. This technology is relatively new in baggage screening and the implications of it for the eye movements made during the search are unclear (though see reference for a discussion of related issues in medical imaging ).

A priori it seems to us that searching for prohibited items with a technology that allows search to have a predictive aspect across ‘slices’ brings a risk that search can become unduly focussed on these locations. From a different paradigm, there is evidence that individual differences in WMC and IU influence the number of fixations made when monitoring dynamically changing color displays for the onset of targets. For individuals with low WMC, those with high IU made fewer eye movements relative to those with low IU. This issue requires much further investigation. Nevertheless, this result emphasizes the importance of measuring eye movements as a tool to help develop our understanding some of the psychological challenges that new ways of the data visualizations bring.

  1. General Discussion and Summary

We have known for a long time that security searches of baggage can be very difficult. Security screeners start at a disadvantage when searching through X-ray images, because those images differ in fundamental ways from real-world images. The objects do not appear in the colors that we associate with them, and overlapping objects produce a very unnatural sort of transparency. After years of research, we now have a better understanding of some of the other factors can make searches especially difficult.

Some factors, such as the large numbers of objects in some bags and the degree to which those objects overlap one another in the image, may be addressable by developing new technology that can enhance images, perhaps by rotating them or presenting them in depth. Eye tracking has been a useful tool in assessing technologies and measuring how searchers use them, and as both the display technologies and the experimental methods advance, we can expect eye tracking studies to be even more valuable in assessing future technological advances.

Other factors that complicate X-ray security searches include limitations on cognitive mechanisms for attention and object recognition. Eye tracking data have allowed us to better understand some of these limitations, such as the prevalence effect and the dual-target cost. It may be possible to partly or fully overcome some of these limitations by developing new training methods. For instance, it may be possible to train subjects to search thoroughly even though targets rarely appear, or to guide their search effectively using multiple target templates. Once those methods are developed, additional eye tracking studies will allow us to measure their benefits.

If some of these limitations cannot be eliminated by training, it may still be possible to improve security searches by devising ways to identify those individuals with the most effective perceptual and attentional mechanisms for this type of search. The studies reviewed above suggest that testing for WMC, maximization and tolerance to uncertainty may help to find better screeners. Research on individual differences that are relevant to security search is just getting off the ground, so there is probably lots to learn here.

The eye tracking studies reviewed here have given us a better understanding of the many factors that make security searches difficult, and in some cases they are helping to find methods of improving performance. These are, of course, important contributions to the safety and well-being of large numbers of people, but there have been other benefits to emerge from this research as well. In addition to the practical improvements in detecting threats, this research has contributed to our understanding of visual search and the mental processes that control attention and object identification. For instance, one set of studies shows that decisions to stop search are not driven solely by the stimuli but are influenced by internal factors that have their origins outside of the attentional system. Another set of studies shows that subjects do not effectively guide their attention when searching for two colors if they can instead identify the target with a single shape discrimination, even though this method makes the search much less efficient. These findings open up new questions about the basic nature of attentional control that go beyond security search. We can expect that future research in this area will lead to further advances in our theoretical understanding of visual cognition, while also enhancing our abilities to detect threats.