Forensics Tutorial

Forensic light sources were first used by the Royal Canadian Mounted Police in the late 1970s. These early units, based on water-cooled argon-ion lasers, were very large, very expensive, and could only be used in the laboratory, not in the field. In the late 1980s, Omnichrome (now a part of CVI Melles Griot) developed portable systems based on compact air-cooled lasers that were somewhat less expensive and more reliable, but only provided a single wavelength. Today’s Omnichrome units are based on high-intensity incandescent bulb technology that can provide a wide range of wavelengths from the ultraviolet to the near-infrared. Field units weigh as little as 18 lbs, are extremely easy to use, and can be taken virtually anywhere.

The Science Behind It

To a greater or lesser degree, all organic materials can be made to fluoresce (absorb light at one wavelength then emit light at a longer—more red—wavelength). For example, by illuminating a fingerprint that is invisible to the naked eye, with intense blue-green light, the print will fluoresce yellow and become clearly visible without the use of powders or dyes. The same procedure can be used to identify other trace elements including semen, saliva, inks, and fibers. Since different organic materials absorb and fluoresce at different wavelengths, trace evidence like fingerprints, bite marks, can easily be observed on skin, cloth, paper, rubber, and many other materials.

The keys to using a forensic light source are:

• The correct color of illuminating light in order to excite the maximum fluorescence of the trace evidence without causing the substrate to also fluoresce
• Viewing filters designed to block the illuminating light but transmit the fluorescing light (the intensity of fluorescence is much less than the intensity of the illumination source)

The following examples illustrate the versatility of imaging with a forensic light source.

• Locating a fingerprint on a gun: The finger and palm prints are virtually invisible under normal lighting, but under narrow-band green lighting, observed through an orange filter, they become visible. Although fingerprints can be viewed with the light source without treatment, a fluorescent powder or dye is often applied.

• Detecting the presence of physiological (body) fluids: Forensic light sources may be used to detect the presence of blood, saliva, semen, urine, vaginal secretions, and sweat. The example below shows semen observed by using near-ultraviolet light.

• Observing trace evidence: Fiber, hair, paint, glass, and other types of trace evidence are difficult to find. Our forensic light sources can be used to locate minute samples and to help determine if two samples come from the came source. The example below shows small pieces of fiber and glass mixed with grains of methamphetamine.

• Determining if a document has been altered: The inks used by forgers to alter documents usually have somewhat different characteristics (e.g., composition, age) than the original inks. These differences can be observed with a forensic light source. As can be seen in the example below, the 1 has been added to the original $43.24 check to make it $143.24.

• Bite marks, bruises, and other pattern injuries: can be identified on living and deceased victims using 450-nm (blue) illumination.

• Complimentary Light Photography: By using the appropriate wavelengths and filters, the background image and unwanted colors can be removed, allowing the trace evidence to stand out "like a sore thumb," as can be seen in these before and after pictures.

• Finding gunpowder residue: Gunshot residue can easily be observed with a forensic light source, as shown in the image below.

Choosing the Right System

A forensic light source consists of the light itself, filters to obtain the appropriate wavelengths and bandwidths, a means of delivering the light to the evidence, and the appropriate viewing filters (goggles).

If the light source is needed for use at crime scenes, it must be lightweight but rugged. These systems are designed to use discrete filters, located on a selection wheel, to generate the light. The advantage is simplicity, robustness, and reliability. These units operate at six fixed wavelengths that are chosen for their effectiveness in crime-scene detection Our Omniprint 1000A is ideal for field applications.

If the light source is intended for the criminalistics laboratory, having a broad range of wavelengths and bandwidths is desirable. Our Spectrum 9000 has continuously tunable output (an infinite choice of wavelengths) with 1-nm bandwidth from the ultraviolet to the near-infrared. It also includes the same discrete filters found in the field unit for duplicating field results. Users report that they have found prints with the Spectrum 9000 that were undetectable with other light sources.

Choices of delivery systems include direct beam, flexible fiber-optic cable (the normal choice for field work) and a liquid light guide designed for enhanced performance in the deep blue and ultraviolet. All systems come with orange, red, and yellow goggles.