Alternate Light Source Analysis

A forensic examination that is conducted only using the visible light wavelengths that are emitted from conventional bulbs may not reveal all the evidence present. Depending on the composition of the evidence or the material it is in contact with, its presence may be poorly revealed, if at all, by conventional light. The use of what has been termed alternate light sources can reveal otherwise hidden evidence.

The use of alternate light sources in forensic investigations was pioneered by the Royal Canadian

Mounted Police in the 1970s. Then, the units were typically water-cooled argon-ion lasers. These large and expensive machines could not be transported to the scene. Rather, samples needed to be brought to a laboratory equipped with the laser.

Portable lasers capable of delivering a single wavelength of light were developed in the late 1980s. In the 1990s, the development of a high-intensity incandescent bulb allowed a wider range of wavelengths to be generated, from the ultraviolet range of the light spectrum to the near infrared. Modern alternate light sources can be less than 20 pounds (9 kg) and are easy to operate.

In general, an alternate light source consists of the light itself (such as a laser or incandescent bulb), a filter or combination of filters that enable all but the selected wavelengths of light to be screened out, a device to deliver the light to the area being examined and appropriate viewing accessories (such as protective goggles, if the wavelengths of light being used are potentially harmful, or goggles equipped with a filter to further screen the incoming light).

Since alternative light source examinations are typically done at the scene of the accident or crime, the light-delivering device needs to be rugged and portable. Typically the light will be equipped with a wheel containing a half dozen or so filters, which can be rotated to bring a different filter into the light path. Another alternate light source design utilizes a flexible fiber-optic cable, which is advantageous in examining confined spaces.

The key to the use of alternate light sources is fluorescence; the absorption of light at one wavelength and the emission of light at a longer wavelength. The emitted light can be detected by use of the screening filters, which block out the other wavelengths of light.

Most organic materials can be made to fluoresce. As an example, a fingerprint can be invisible to the naked eye. But, when illuminated using an intense blue-green light from a laser or incandescent source, the organic materials in the fingerprint will fluoresce yellow. The fluorescence is visible without the addition or powders or dyes.

The same principle applies to other organic samples including semen, saliva, fibers from materials, and ink. Furthermore, different organic materials will absorb light and fluoresce at different wavelengths. This means that evidence such as a fingerprint or a bite mark can be detected on materials as diverse as skin, paper, rubber, and cloth fabric.

This sort of differentiation requires a skilled operator, since the color of the illuminating light is critical to elicit the maximum fluorescence from the evidence while minimizing the background fluorescence from the supporting material. As well, the selection of the filter(s) is important, since it will block all but the desired wavelengths of the illuminating light while not blocking the wavelengths of the emitted fluorescent light (which is generally much less intense).

One use for alternate light sources is the examination of a weapon such as a gun or knife. Finger and palm prints are nearly invisible on such metal surfaces when examined under room light. But, when viewed under a green light and observed through an orange filter, the prints can easily be seen. Similar lighting conditions and special film can be used to photograph prints.

Body fluids can be detected using alternate light sources. For example, the use of near-ultraviolet light can readily reveal the presence of semen stains. Other body fluids that can be detected include saliva, blood, urine, and vaginal secretions.

Gunshot residue can also be readily observed using the appropriate illumination. Small bits of paint, fiber, hair, glass, crystals, and other “trace evidence,” which can be physically difficult to find at a crime or accident scene, can be located using alternate light. Moreover, the differing chemistries and shapes of the objects can produce different patterns of fluorescence, allowing different objects to be coincidentally detected.

As a final example, document forgery can be detected using alternate forensic light sources, since differences in ink composition and the differing periods of time that the ink was in contact with the paper will produce detectable changes.

See also Bloodstain evidence; Crime scene investigation; Forensic science.

Brian Hoyle