watch your step! a frustrated total internal reflection approach to forensic footwear imaging - black acrylic sheet
Forensic image retrieval and processing is an important tool to combat crime. g.
During fingerprint collection
However, despite recent advances in machine vision technology and image processing technology (
Contrary to popular fiction)
Forensic image retrieval is still widely used with outdated practices involving ink pads and paper.
Continued changes in government policies, an increase in crime rates and a decrease in the budget for forensic services increasingly require faster and more efficient collection and processing of evidence.
One consequence of this is that new, low
In order to improve the quality and throughput of evidence processing at the same time, cost imaging technology is required.
This is particularly true in emerging areas of forensic footwear analysis, where shoe print images are used to link individuals to crime scenes.
Here, we describe a method based on frustration-based all-internal reflection imaging that can be used to obtain images of areas where the shoe touches the rigid surface.
The wave tube optical element for infrared lighting is made of a piece of glass (or acrylic)
This is 600mm x 600mm and 30mm thick.
As shown, lighting of the optical wave duct element is achieved by wrapping an ultra-extreme right white LED light bulb around the rough edge of the glass/acrylic sheet.
The LED light strip is then secured in place using black tape.
This helps protect the LED light strip and prevents ambient light from entering the waveguide at the edge.
Then use black tape to mask the edges of the two largest faces of the paper (see ).
The way this is done is that the light from the led will suppress the angle below the critical angle required for the total internal reflection, thus preventing the light from escaping from the wave tube sheet.
Therefore, only light with an angle greater than is irradiated on the surface and is limited to the waveguide due to full internal reflection.
However, when the shoe is in contact with the waveguide element, the condition of the total internal reflection is changed and the strongly scattered light in the contact area is observed.
Shoes can then be imaged from below using a webcam ().
To ensure that images printed across the sole are captured, each webcam is used to capture short films that last only a few seconds, during which the person standing is asked to swing back and forth, to imitate the action of walking (
See provided in).
The Python Software then looks at each pixel in turn and extracts the maximum intensity value measured by that pixel in a short movie sequence.
In this way, an image similar to In can be obtained for the entire sole of the shoe ().
Then convert these images to black and white images (binarised, )
Prepare a comparison with the contact images stored in the national footwear database.
This process is by setting all pixels whose intensity is greater than the selected threshold to 0 and the value whose intensity is less than this value to 255 (8 bit image).
In this way, the contact area is displayed as a black object on a white background.
Traditional lighting is achieved using a second LED mounted below the waveguide (see ).
Collect images of shoes using traditional lighting so that it is also possible to identify wear marks, scratches and other traces that are not in contact with the surface ().
The Python Software then alternately uses between conventional lighting and infrared lighting through a series of simple relays that turn on/off the power supply of the two lighting sources.