USE OF ULTRAVIOLET LIGHT IN ARSON DETECTION

Ultraviolet light is a simple, reliable, cost effective method of arson detection that is too frequently over-looked in arson investigation. UV light not only assists in rapidly locating accelerant residues, it also assists in locating the point of origin of the fire. The color which accelerants glow is affected by heat exposure; the longer an accelerant is exposed to heat (i.e. the origin), the more differentiated its fluorescence color will be from other less exposed areas.Evidence of accelerants is almost always invisible to the naked eye after they have been absorbed in a fire. However, the area tarnished by accelerants are easily discernible under UV light.

Volatile hydrocarbons such as gasoline, kerosene and other petroleum fractions, benzene, acetone, grease, lard, vegetable oils, paints, etc fluoresce when exposed to UV light. In addition, UV light can be helpful in locating fragments incendiary devices since explosive wrappings are frequently fluorescent. The use of UV light enables the investigator to accurately identify locations where samples should be collected for further laboratory analysis. Samples collected in cans and plastic evidence bags can then be heated in warm water to form condensation. Latent accelerant residue may then be brought to the surface and seen under UV light.

In order to successfully use UV light at a fire scene, ambient light must be reduced to a minimum. This can be done by covering widows and other exposed areas with an opaque sheeting such as black plastic.

It is important to put collected samples in a sealed airtight container and properly label the evidence (date, where found and the mark of the investigator) before dispatching and storing.

Ultraviolet light in comparison to "Sniffers"

UV lamps and sniffers are tools in arson detection, each of which aids the investigator. While it is not recommended that one of these tools be used in place of the other, there are advantages that UV light lends to an investigation:

• Use of UV light is not contingent upon wind conditions.

• The sniffer does not detect specific infected areas, but rather indicates a general area of saturation, making pour pattern sampling difficult.

• The sniffer is effective only two  to three days after saturation, while UV lamps have been proven to effectively fluoresce samples up to two months after an incident.

• After a hot lengthy fire, the odors of accelerants are usually gone.

• A sniffer will often pick up false positives, such as human saliva as a positive identification of an accelerant

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