Measuring visual abilities and visual knowledge of aviation security screeners

Abstract

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. Abstract -A central aspect of airport security is reliable detection of forbidden objects in passenger bags using Xray 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 he 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 d...

Reliable and valid measures of threat detection performance in x-ray screening

“…More specifically, the measures that are most often reported in the detection literature, A' and d', correlated in all four test conditions with r >= .75. And in a recent study using computer-based tests with different types of threat items even higher correlations between A' and d' were found (r > .90, [4]). …”

“…[26], [5], [1]), several studies have used "nonparametric" A' because its computation does not require a priori assumptions about the underlying distributions (e.g. [27], [28], [4]). ROC analysis can be used to test whether the assumptions of SDT are fulfilled.…”

“…While CBT can increase detection performance substantially [1], recent results suggest that training effects are smaller for increasing the ability to cope with image-based factors such as effects of view, superposition and bag complexity [4]. However, this conclusion relies on the availability of reliable and valid measures of detection performance.…”

“…If a certain type of forbidden item has never been seen before, there exists no representation in visual memory and the object becomes very difficult to recognize if it is not similar to stored views of another object. Besides the aspect of memory representation, image-based factors can also affect recognition substantially (for a detailed discussion see [4]). When objects are rotated, they become more difficult to recognize (effect of view).…”

Evaluating the two-component inspection model in a simplified luggage search task

“…In these models, the search stage is largely bottom-up, rapid, and global; it is serially linked to the decision stage, which is more considered and involves both bottom-up and top-down processes that are mediated by existing knowledge and expertise (Schwaninger, Hardmeier, & Hofer, 2004). The decision component has its complement as the criterion acquisition time in SSMs (Smith, 2000), which is a function of rate of information with a 5-min break between sessions.…”