Watch your step! A frustrated total internal reflection approach to forensic footwear imaging

Abstract

Forensic image retrieval and processing are vital tools in the fight against crime e.g. during fingerprint capture. However, despite recent advances in machine vision technology and image processing techniques (and contrary to the claims of popular fiction) forensic image retrieval is still widely being performed using outdated practices involving inkpads and paper. Ongoing changes in government policy, increasing crime rates and the reduction of forensic service budgets increasingly require that evidence be gathered and processed more rapidly and efficiently. A consequence of this is that new, low-cost imaging technologies are required to simultaneously increase the quality and throughput of the processing of evidence. This is particularly true in the burgeoning field of forensic footwear analysis, where images of shoe prints are being used to link individuals to crime scenes. Here we describe one such approach based upon frustrated total internal reflection imaging that can be used to acquire images of regions where shoes contact rigid surfaces.

Figure 1

(a) Schematic diagram of the dual wave guide and conventional LED illumination apparatus for imaging shoe prints. (b) The waveguide is formed by wrapping ulltrabright LEDs around the perimeter of a thick glass (or perspex) sheet. The waveguide is masked so that only light incident on its surface at angles greater than θ_c is allowed to propagate. This light is confined inside the waveguide. (c) When a shoe contacts the waveguide surface, light is scattered strongly and can be imaged. (d) A gentle rocking motion of the shoes allows the wearer to mimic the walking action and a more complete image of the contact regions can be obtained. (e) The images in (d) are binarised to obtain a black and white representation of the contact regions of the shoes (black denotes the contact regions). (f) Conventional illumination can also be used to identify any defining features that are not in intimate contact with the waveguide surface.

Figure 2. Examples of conventional (1^st column) and waveguide (2^nd column) images as well as the corresponding binarised image (3^rd column) obtained for different shoe types.

The last two columns show the software determined threshold (I_th) and the user optimised threshold (I_opt) values used to produce the binarised images for each shoe (left and right).

Figure 3. Thresholding the waveguide images in panel (a) involves converting the images to greyscale (panel (b)).

The greyscale images are then binarised to form a black and white image of the contact regions (panel (c)). Panel (d) shows an example of an intensity histogram (red area) collected from the greyscale image of the left shoe shown in panel (b). Several peaks are present at different intensities corresponding to the background pixels (I_l), the contact regions (I_h) and regions of the shoe that are close to, but not in contact with, the waveguide surface (two peaks in the vicinity of I_m). The solid black line in panel (d) is the result of a fit to the data using a sum of 4 Lorenztian peaks. The coloured rectangles and associated images show which peaks are associated with each of the features in the greyscale images. The threshold determined by the software is set to be at an intensity, I_th, that is 60% of the way between I_m and I_h (see text).

Figure 4. Pressure analysis of contact shoe print.

Panel (a) shows a contact image obtained using waveguide illumination and panel (b) shows the corresponding pressure distribution that was obtained from this image. The conversion between panel (a) and panel (b) was performed using the calibration curve shown in panel (c), where measurements of applied pressure are related to the average pixel intensity in the contact regions. The solid line in this figure is the result of a 3^rd order polynomial fit to the data. The insets show waveguide contact images of the small rubber probe used in the calibration experiments (see text). The dashed circles mark the edge of the circular probe and show that only a small proportion of the probe is in contact.