Forensic Techniques: Light sources

Blood, saliva, semen, and urine are among the most important biological evidences in crime scene investigation. But spotting traces of these substances with the naked eye can be troublesome, which is why forensic light sources will always have a special place in just about any investigation.

Biological evidences can be detected by light sources due to natural characteristics, such as light absorption (in the case of blood), or fluorescence (urine and semen, for example). Below we look at some of the most common of these evidences, and how they’re detected with various light sources.

Blood

Untreated dry blood is particularly characteristic: it shows high absorption in a broad region of light wavelength (usually measured in nanometre, or nm). Under UV light and infrared light dry blood will therefore appear as a dark stain, much like it does under visible light.

This doesn’t mean that a forensic light source is useless when detecting the presence of blood, however: these light sources are good at highlighting the contrast between biological evidence and the background surface. In this case a blood stain will appear bright against a dark surface, where it would otherwise be invisible to the naked eye.

Semen

Semen stains are somewhat easier to detect since semen possesses natural photoluminescence properties. (It’s interesting to note that in this case semen absorbs light of a certain wavelength, then re-emits said light at a longer wavelength). Semen detection using a forensic light source is usually accompanied by a filter and/or goggles.

Tests suggest the best fluorescence effect for semen is found in wavelengths between 320nm and 510nm using orange goggles.

Saliva

Dried saliva is nearly colourless, which makes detection using the naked eye difficult. Saliva typically exhibits a low intensity fluorescence. As such various methods for the detection of saliva is in use, depending on the environment.

When exposed to UV light, saliva will appear bluish-white. Seeing as other stains appear the same, saliva can be difficult to distinguish using the UV light source. As such forensic investigators will often use a combination of light sources and filters (and sometimes goggles), to accurately detect the presence of saliva.

Urine

Urine stains are slightly more complicated to detect than saliva stains; abnormal substances may result in different colouring when using certain light sources. It should also be noted that urine becomes more easily diluted on fabric surfaces, thereby affecting its intensity further.

Forensic light sources to detect urine:

• UV light (colour may vary depending on chemical composition of urine)
• 415nm wavelength with yellow goggles
• 450nm wavelength with orange goggles
• 505nm wavelength with red goggles

Why use forensic light sources?

Various tests exist to identify biological evidence. They can be divided into categories: presumptive tests and confirmatory tests. Presumptive tests are by and large screening tests which are used to indicate the likely nature of a substance. Confirmatory tests, on the other hand, are used to conclusively determine the substance and any other relevant details, such as DNA tests.

Most confirmatory tests are, however, destructive in nature – the substance is likely to be destroyed during the testing process. As such, leading with presumptive tests during a forensic investigation can preserve valuable evidence, which can later be analysed in a lab.