Explaining the Alternate Light Source (ALS)

Examining evidence with an ALS

The Alternate Light Source (ALS), also referred to as a Forensic light Source is a device that transmits light through Ultra-violet (UV) range barrier filters from 190-580 nanometers. These devices are used to locate latent, i.e. invisible, evidence not seen in the visible light spectrum. They work by shining a particular wavelength of light over certain evidence which will fluoresce at that wavelength (see photo). Obviously, they are best used in darkened environments. Body fluids such as semen, urine, and saliva are one type of fluorescent evidence. Bone and teeth will also fluoresce which can be helpful during excavations. Other items like synthetic fabrics and cleaners may also fluoresce.

Blood however, will not fluoresce in absence of a reagent. In its natural state (neat) blood will absorb UV light and appear black in color. Criminalists also use an array of reagents (fingerprint powders, liquid chemical) that will fluoresce under UV light as well. Primarily these are used to develop fingerprints, footwear, or blood evidence. These reagents are designed to make it easier for the analyst to locate and document the evidence, especially on distracting multi-colored surfaces.

ALS devices can be small portable hand-held units that are easily carried around the crime scene. These units typically are of lower wattage (less bright than their large counterpart) and have fewer barrier filters to choose from. Other units like the SPEX CS-16-500 are much larger and need a 110 power source. While they can be used at crime scenes these larger units are often found in the laboratory. They have a very bright 500 watt bulb and have a range of filters throughout the UV and infra-red spectrum. These units have even been used to detect blood under four coats of paint!

Obviously, this is a device that can turn your story on a dime. For example, you may find semen on a girl’s dress or in a politician’s office.  Imagine how a character might react to seeing this device being used. Because urine will fluoresce bathrooms are a real light show. Criminalists also adhere to one of the cardinal rules of professional conferences; do not check your hotel room bed sheets with a light source! Yuck. I made that mistake once and regretted it all night.

Staining on women's panties