Franziska Hofer

Measuring visual abilities and visual knowledge of aviation security screeners

A central aspect of airport security is reliable detection of forbidden objects in passenger bags using X-ray screening equipment. Human recognition involves visual processing of the X-ray image and matching items with object representations stored in visual memory. Thus, without knowing which objects are forbidden and what they look like, prohibited items are difficult to recognize (aspect of visual knowledge). In order to measure whether a screener has acquired the necessary visual knowledge, we have applied the prohibited items test (PIT). This test contains different forbidden items according to international prohibited items lists. The items are placed in X-ray images of passenger bags so that the object shapes can be seen relatively well. Since all images can be inspected for 10 seconds, failing to recognize a threat item can be mainly attributed to a lack of visual knowledge. The object recognition test (ORT) is more related to visual processing and encoding. Three image-based factors can be distinguished that challenge different visual processing abilities. First, depending on the rotation within a bag, an object can be more or less difficult to recognize (effect of viewpoint). Second, prohibited items can be more or less superimposed by other objects, which can impair detection performance (effect of superposition). Third, the number and type of other objects in a bag can challenge visual search and processing capacity (effect of bag complexity). The ORT has been developed to measure how well screeners can cope with these image-based factors. This test contains only guns and knives, placed into bags in different views with different superposition and complexity levels. Detection performance is determined by the ability of a screener to detect threat items despite rotation, superposition and bag complexity. Since the shapes of guns and knives are usually known well even by novices, the aspect of visual threat object knowledge is of minor importance in this test.

Using threat image projection data for assessing individual screener performance.

Threat image projection (TIP) is a technology of current x-ray machines that allows exposing screeners to artificial but realistic x-ray images during the routine baggage x-ray screening operation. If a screener does not detect a TIP within a specified amount of time, a feedback message appears indicating that a projected image was missed. Feedback messages are also shown when a TIP image is detected or in the case of a non-TIP alarm, i.e. when the screener indicated that there was threat but in fact no TIP was shown. TIP data is an interesting source for quality control, risk analysis and assessment of individual screener performance. In two studies we examined the conditions for using TIP data for the latter purpose. Our results strongly suggest using aggregated data in order to have a large enough data sample as the basis for statistical analysis. Second, an appropriate TIP library containing a large number of threat items, which are representative for the prohibited items to be detected is recommended. Furthermore, consideration should be given to image-based factors such as general threat item difficulty, viewpoint difficulty, superposition and bag complexity. Different methods to cope with these issues are discussed in order to achieve reliable, valid and standardized measurements of individual screener performance using TIP.

Aviation Security Screeners Visual Abilities & Visual Knowledge Measurement

A central aspect of airport security is reliable detection of forbidden objects in passenger’s bags using X-ray screening equipment. Human recognition involves visual processing of the X-ray image and matching items with object representations stored in visual memory. Thus, without knowing which objects are forbidden and what they look like, prohibited items are difficult to recognize (aspect of visual knowledge). In order to measure whether a screener has acquired the necessary visual knowledge, we have applied the prohibited items test (PIT). This test contains different forbidden items according to international prohibited items lists. The items are placed in X-ray images of passenger bags so that the object shapes can be seen relatively well. Since all images can be inspected for 10 seconds, failing to recognize a threat item can be mainly attributed to a lack of visual knowledge. The object recognition test (ORT) is more related to visual processing and encoding. Three image-based factors can be distinguished that challenge different visual processing abilities. First, depending on the rotation within a bag, an object can be more or less difficult to recognize (effect of viewpoint). Second, prohibited items can be more or less superimposed by other objects, which can impair detection performance (effect of superposition). Third, the number and type of other objects in a bag can challenge visual search and processing capacity (effect of bag complexity). The ORT has been developed to measure how well screeners cope with these image-based factors. This test contains only guns and knives, placed into bags in different views with different superposition and complexity levels. Detection performance is determined by the ability of a screener to detect threat items despite rotation, superposition and bag complexity. Since the shapes of guns and knives are usually well-known even by novices, the aspect of visual threat object knowledge is of minor importance in this test. A total of 134 aviation security screeners and 134 novices participated in this study. Detection performance was measured using A’. The three image-based factors of the ORT were validated. The effect of view, superposition, and bag complexity were highly significant. The validity of the PIT was examined by comparing the two participant groups. Large differences were found in detection performance between screeners and novices for the PIT. This result is consistent with the assumption that the PIT measures aspects related to visual knowledge. Although screeners were also better than novices in the ORT, the relative difference was much smaller. This result is consistent with the assumption that the ORT measures image-based factors that are related to visual processing abilities; whereas the PIT is more related to visual knowledge. For both tests, large inter-individual differences were found. Reliability was high for both participant groups and tests, indicating that they can be used for measuring performance on an individual basis. The application of the ORT and PIT for screener certification and competency assessment are discussed.

The X-ray object recognition test (X-ray ORT) - a reliable and valid instrument for measuring visual abilities needed in X-ray screening

Aviation security screening has become very important in recent years. It was shown by Schwaninger et al. (2004) that certain image-based factors influence detection when visually inspecting X-ray images of passenger bags. Threat items are more difficult to recognize when placed in close-packed bags (effect of bag complexity), when superimposed by other objects (effect of superposition), and when rotated (effect of viewpoint). The X-ray object recognition rest (X-ray ORT) was developed to measure the abilities needed to cope with these factors. In this study, we examined the reliability and validity of the X-ray ORT based on a sample of 453 aviation security screeners and 453 novices. Cronbach Alpha and split-half analysis revealed high reliability. Validity was examined using internal, convergent, discriminant and criterion-related validity estimates. The results show that the X-ray ORT is a reliable and valid instrument for measuring visual abilities needed in X-ray screening. This makes the X-ray ORT an interesting tool for competency and pre-employment assessment purposes.

Evaluation of CBT for increasing threat detection performance in X-ray screening

The relevance of aviation security has increased dramatically in recent years. Airport security technology has evolved remarkably over the last decade, which is especially evident for state-of-the-art X-ray screening systems. However, such systems will be only as effective as the people who operate them. Recognizing all kinds of prohibited items in X-ray images of passenger bags is a challenging object recognition task. In this article we present a method to measure screener detection performance based on signal detection theory. This method is applied to measure training effects resulting from individually adaptive computer based training (CBT). We have found large increases of detection performance and substantial reductions in response time suggesting that CBT is a very effective tool for increasing effectiveness and efficiency in aviation security screening.

Adaptive Computer-Based Training Increases on the Job Performance of X-Ray Screeners

Due to severe terrorist attacks in recent years, aviation security issues have moved into the focus of politicians as well as the general public. Effective screening of passenger bags using state-of-the-art X-ray screening systems is essential to prevent terrorist attacks. The performance of the screening process depends critically on the security personnel, because they decide whether bags are OK or whether they might contain a prohibited item. Screening X-ray images of passenger bags for dangerous and prohibited items effectively and efficiently is a demanding object recognition task. Effectiveness of computer-based training (CBT) on X-ray detection performance was assessed using computer-based tests and on the job performance measures using threat image projection (TIP). It was found that adaptive CBT is a powerful tool to increase detection performance and efficiency of screeners in X-ray image interpretation. Moreover, the results of training could be generalized to the real life situation as shown in the increased detection performance in TIP not only for trained items, but also for new (untrained) items. These results illustrate that CBT is a very useful tool to increase airport security from a human factors perspective.

Configural information is processed differently in perception and recognition of faces

Several previous studies have stressed the importance of processing configural information in face recognition. In this study the perception of configural information was investigated. Large overestimations were found when the eye-mouth distance and the inter-eye distance had to be estimated. Whereas configural processing is disrupted when inverted faces have to be recognized the perceptual overestimations persisted when faces were inverted. These results suggest that processing configural information is different in perceptual as opposed to recognition tasks.

A psychophysically plausible model for typicality ranking of natural scenes

Natural scenes constitute a very heterogeneous stimulus class. Each semantic category contains exemplars of varying typicality. It is, therefore, an interesting question whether humans can categorize natural scenes consistently into a relatively small number of categories, such as, coasts, rivers/lakes, forests, plains, and mountains. This is particularly important for applications, such as, image retrieval systems. Only if typicality is consistently perceived across different individuals, a general image-retrieval system makes sense. In this study, we use psychophysics and computational modeling to gain a deeper understanding of scene typicality. In the first psychophysical experiment, we used a forced-choice categorization task in which each of 250 natural scenes had to be classified into one of the following five categories: coasts, rivers/lakes, forests, plains, and mountains. In the second experiment, the typicality of each scene had to be rated on a 50-point scale for each of the five categories. The psychophysical results show high consistency between participants not only in the categorization of natural scenes, but also in the typicality ratings. In order to model human perception, we then employ a computational approach that uses an intermediate semantic modeling step by extracting local semantic concepts, such as, rock, water, and sand. Based on the human typicality ratings, we learn a psychophysically plausible distance measure that leads to a high correlation between the computational and the human ranking of natural scenes. Interestingly, model comparisons without a semantic-modeling step correlated much less with human performance, suggesting that our model is psychophysically very plausible.

External and internal influences on the security control process at airports

In past years, a lot of research has been done in the field of airport security control. Mostly, researchers that focused on human factor issues looked very closely at the x-ray screening process or at covert testing. However, it has to be taken into account that security control is typically integrated in a competitive, market-oriented environment. Thus, facilitation aspects, such as the maximal capacity (also known as throughput) of a security control line, clearly reflect one important key performance index. In a series of studies, the impact of external factors, for instance seasonal variability of temperature or number of manual baggage inspections, as well as the impact of internal factors, such as the allocation of tasks within security control crew members, were measured. The scenarios with varied internal factors consisted of different manipulations in the work process without changing given security rules, protocols or changes in infrastructure. For example, the Security Officer helping passengers preparing their bags at the roller table has been taken away from this position and assigned as second person for baggage inspection. Implications of these scenarios on measures of efficiency, such as throughput, as well as on subjective measures, such as ratings of workload and stress, have been assessed. Results show that external factors correlate with throughput remarkably and that even slight changes in the process can have a significant impact on the mentioned domains. Experiences made are discussed in order to provide suggestions of useful and feasible optimizations of the security control process.

Covert testing at airports: Exploring methodology and results

Large efforts are made in order to enhance human factors in airport security, but one very important area, namely covert or infiltration tests, are often neglected. Covert tests are most important not only for quality control and risk analysis, but also for training purposes, such as for example the training of appropriate reactions in dangerous situations. Therefore, conducting covert tests at airports is a very useful approach for assessing the effectiveness of the security control and offers a possibility to uncover weaknesses in the security process by simulating the case of emergency. Its main benefit is the high ecological validity almost unparalleled by other methods. The present scientific exploratory applied study is one of the first of its kind worldwide, which analyses the results of covert tests collected at a large European airport over a period of fourteen calendar months. Statistical analyses were performed using Mann-Whitney u-tests, chi-square tests and binary logistic regression analysis. Results will focus on differences in detection performance between different test objects and different places of concealment. Additional factors included in the analyses will be time taken for pat-down search, sex, age, work experience, as well as the security officerpsilas reaction to weapon detection. Methodological problems will be discussed and suggestions made for further research in this area.