Thursday, February 26, 2009

Alternate Light Source

Inspection with an alternate light source (ALS) is the second step of every processing technique in the FBI processing guide. It can enhance the visibility of a wide range of evidence. While inherent luminescence is secondary to visual inspection, there are certain forensic techniques that absolutely depend upon ALS.

There are several types of forensic light sources on the market. Many of the newest ALS units are designed around light emitting diodes (LED) and resemble flashlights. Older models look like a box with a flexible tube protruding from the side. In any case, the method of use is the same. An item is illuminated using a very specific wavelength of light while it is viewed through a filter. Inherent or chemical fluorescence often creates an intense change in contrast which is usually the desired result. Since there are several wavelengths of light that can be used in combination with several different filters, finding the best combination can be a meddlesome task.

In order to make life easier and encourage the use of a very effective tool, I have compiled the following information. Click on the chart to the right for a larger view. The left column contains the type of chemical process used on an item. The second column has the preferred wavelength for illumination with the background color defining the color of the filter. The third column lists the combination that produces the second best result. The last two columns show the filter colors that work best when blacklight or ultraviolet light is used as the light source.

These values do not reflect the only possibilities. They do, however, typically produce the best results without having to hunt through numerous combinations when the source of fluorescence is controlled.