Processing crime scenes usually involves finding fingerprints to identify the perpetrator. There are many different types of processes used to collect latent fingerprints from evidence. The most common method is fingerprint powder which has the ability to be lifted with tape and placed on a card. Other methods make prints visible on the surface of the evidence, but need to be photographed since they cannot be lifted. Occasionally, the developed print may not contrast well enough with the surface to photograph easily. An example of this is the use of Amido Black which appears purplish-black and often blends in with the surface color of the evidence. Additionally, textured surfaces can further inhibit photography of the print.
I have tried several solutions to this problem ranging from side-lighting, ring-lighting, and alternate light sources. At one point, I was investigating a homicide that occurred inside of an automobile. I was particularly interested in the back side of the rear-view mirror. After I had finished all other methods of processing, with no results, I applied Amido Black in an effort to stain blood traces on the mirror. When the Amido was applied, fingerprint ridge detail appeared immediately on the wet surface. As soon as the surface dried, however, it was no longer visible. Subsequent processing with Amido did not make the print visible again. I knew the print was there, but I could not see it after the initial application. Even the alternate light source did not improve the visibility.
The solution was incredibly low-tech and I discovered while working with two of my mentors who were certified photography instructors. We took a Styrofoam coffee cup and cut the bottom off to make a hole for the camera lens. We then placed the coffee cup over the area of interest on the mirror and held it in place with articulating arms. The camera was placed on a tripod with the lens positioned just inside the hole in the bottom of the cup. All of the lights in the room were turned off and the alternate light source set at 455nm was placed against the side of the coffee cup. As we looked through the camera, we moved the light source to different angles. The coffee cup acted as a combination diffuser/reflector enabling the specific wavelengths of light to illuminate the surface. Since Amido Black is a slightly different color and texture than the surface of the rear-view mirror, we were eventually able to find a combination of wavelength and angle that made the print visible and gave us the ability to photograph it for comparison and preservation.
I have since used the coffee cup technique on numerous other processes including fluorescent applications. Even though fluorescent photography is not particularly difficult and does not require this type of enhancement, the increased contrast and reduction of hotspots yields a much higher quality photograph with better detail. This technique takes practice, but costs very little to use. It is definitely a tool that every forensic investigator needs to have in their arsenal.