Do "Image Enhancement" Functions Really Enhance X-Ray Image Interpretation?

Do Image Enhancement Functions Really Enhance X-Ray Image Interpretation? Stefan Michel (s.michel@psychologie.unizh.ch) University of Zurich, Department of Psychology, Binzmuhlestrasse 14, 8050 Zurich, Switzerland, and Max Planck Institute for Biological Cybernetics, Dept. Bulthoff, Spemannstrase 38, 72076 Tubingen, Germany Saskia M. Koller (s.koller@psychologie.unizh.ch) University of Zurich, Department of Psychology, Binzmuhlestrasse 14, 8050 Zurich, Switzerland Markus Ruh (m.ruh@psychologie.unizh.ch) University of Zurich, Department of Psychology, Binzmuhlestrasse 14, 8050 Zurich, Switzerland Adrian Schwaninger (a.schwaninger@psychologie.unizh.ch) University of Zurich, Department of Psychology, Binzmuhlestrasse 14, 8050 Zurich, Switzerland, and Max Planck Institute for Biological Cybernetics, Dept. Bulthoff, Spemannstrase 38, 72076 Tubingen, Germany Abstract State-of-the art x-ray screening systems offer a variety of so- called “image enhancement” functions (IEFs). Examples are color inversion, edge-enhancement, organic only, metal only etc. IEFs are often promoted because they would bring out detail that is obscured or highlight certain features, such as for example organic content. In this study, we investigated the usefulness of IEFs for cabin baggage screening (CBS) and hold baggage screening (HBS) in airport security. The results showed that the standard image provided the best detection performance. Some IEFs impaired detection performance substantially, which was also dependent on threat type (guns, knives, improvised explosive devices, other threat items). Together with previous work (Klock, 2005), these results highlight the importance of systematically studying the usefulness of IEFs in order to optimize human-computer interaction in x-ray screening. Keywords: Aviation security; image enhancement filters; computer based training; image display technology; object recognition. Introduction In recent years, the importance of baggage x-ray screening at airports has increased dramatically. The image quality of older x-ray screening equipment was sometimes in need of improvement. For example an early version of a coloring algorithm as enhancement function did not serve the purpose of increasing detection performance of threat objects, actually it impaired it. This was due to the occlusion of object parts by the opaque coloring algorithm (Schwaninger, 2005a; Schwaninger, 2005b). But there has been much technological progress in the last years, especially regarding x-ray screening machines, which nowadays provide high image quality and various image enhancement functions (IEFs). The main objective of such functions is to process an image so that the result is more suitable than the original image for a specific application as for example x-ray screening at airports (Gonzalez & Woods, 2002). In x-ray images, the image enhancements might increase the visibility of objects within the bag and remove background noise. State-of-the-art x-ray machines provide many IEFs. The aim of this study is to investigate whether IEFs actually help human operators (screeners) to better detect threat items in x-ray images of passenger bags. Interestingly, reports regarding an evaluation of IEFs have not been publicly available except two recent publications (Klock, 2005; Schwaninger, 2005b). Klock (2005) examined whether IEFs increase screeners’ threat detection performance when visually inspecting carry-on bags using a Rapiscan emulator. She found that high penetration, organic stripping and inorganic stripping functions resulted in decreased probability of detection (see below for more information on different IEFs). Crystal clear, black and white, and low penetration resulted in the best performance, while it should be noted that the original color image was not included in the analysis. Klock (2005) also found that these effects are dependent on threat type, i.e. whether guns, knives, improvised explosive devices (IEDs) or other prohibited items had to be detected. Schwaninger (2005b) reported a study on the effects of IEFs for the detection of IEDs in hold baggage. He found that the original image resulted in the best performance, whereas the organic stripping, organic only and luminance negative functions substantially impaired detection of IEDs. The purpose of this study is to extend previous research in order to evaluate the value of different IEFs. In addition, a comparison between IEFs used in cabin baggage screening (CBS) and hold baggage screening (HBS) was of interest. The nine IEFs examined in this study can be applied to the x-ray images online when working at an aviation security checkpoint. Each pixel in the image format used in these x-ray machines has a material and a luminance value. To show the images on a screen, the pixel values are color coded using red for organic, blue for metallic and green for mixed organic/metallic material. The luminance value defines the luminance of the pixel. Table 1 gives an overview and description of all IEFs used in Experiment 1.