11.3: Alternate Light Source or Forensic Light Source

An Alternate Light Source (ALS) narrows the bandwidth of light to a very specific spectrum wavelength. True ALS units tend to be very large, expensive, and are used primarily in the laboratory but are capable of being tuned to a wide range of bandwidths. A forensic light source achieves these results through the use of colored filters or colored LED lights. They do not have the range of bandwidths of the ALS, but they are smaller, less expensive, and can be easily brought into the crime scene. Both forms of light sources will be referenced in this chapter as ALS, although it is important to understand the difference between the two.

Certain items react better to different light wavelengths. Blood is best visible under 415nm light. Body fluids like sweat, saliva, and semen are best visualized under 455nm to 470nm. Hairs and fibers may be best visualized between 455nm and 485nm. It is important to have a wide range of forensic lights available either in the laboratory or in the crime scene. It is also important that forensic technicians use at least two different types of ALS when examining evidence, as sometimes one light source may be tuned slightly closer to the desired wavelength of light and therefore will detect what the other may not.

The ALS is an unobtrusive method of locating, visualizing, and documenting evidence that might otherwise be overlooked. They are easy to use and convenient to bring into the crime scene. They also will not alter or destroy DNA, with one exception that will be detailed later in this chapter.

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