Basic Crime Scene Processing Techniques Using Fluorescent Powders

This article is the tenth in a comprehensive series of articles on latent print development techniques. In this issue, the article is contributed by guest author, Mary Nolte of Melles Griot. Watch for future articles in each issue of Minutiæ and save these issues in a binder to compile a manual of fingerprint development techniques.

Since the late 1970s, when lasers were first used as an investigative tool to locate fingerprints on "difficult" surfaces, fluorescence-based fingerprint techniques have become increasingly popular. We have all processed items such as guns, plastic bags, Formica counters, and many other surfaces where black or silver powders alone did not produce results. In such situations, the value of fluorescent techniques is well documented. This paper on fluorescent techniques, will discuss the basics of fluorescent powdering techniques at a crime scene.

At a crime scene, we are often limited to using fluorescent powders. This is due to the contamination and/or destruction of property when we use chemical methods. Fluorescent powders not only yield very good results on a variety of surfaces; they are also much less messy than traditional powders.

There are two types of fluorescent powder: magnetic and nonmagnetic. Both work well, however magnetic powders cannot be used on most ferrous surfaces. Personal preference is also a factor in the decision. Nonmagnetic powders include Redwop and Greenwop. Magnetic powders include Blitz Red and Blitz-Green™.

Application of nonmagnetic powders is best done with a feather brush, as minimal powder is needed for adequate coverage. Touch the tips of the feather brush to the powder adhering to the open lid; do not dip the brush into the jar. Application of fluorescent magnetic powders is the same as with standard magnetic powders, using a magnetic applicator.

For Redwop™ or Blitz-Red™, set your forensic light source at any setting from approximately 400nm (violet-blue) to 550nm (green). For Greenwop™ or Blitz-Green™, set your light source anywhere from 350nm (ultraviolet) to 530nm (green-blue). The optimum setting is <530nm as this gives the best intensity. You may want to select another wavelength if background fluorescence is a problem. Select the appropriate color of goggles for the chosen wavelength (orange is used for <530nm). Darken the location as much as possible.

Illuminate the surface while you are brushing on the powder. Care should be taken so as not to over-process the prints. If this should occur, a clean Zephyr® brush can be used to gently clear away excess powder.

Photograph any results using the same color camera filter as the goggles. Fluorescent photography does require longer exposure times, so a tripod is recommended. Make sure you include a scale in the photograph that will be visible in your photos.

While it is possible to lift fluorescent prints, it is good practice to photograph any results prior to attempting to lift. For lifting, it is recommended that you use frosted, non-glare, tape and a black backing card. Mark on the back of the lift card the wavelength that was used to illuminate the fingerprint.

When selecting light source equipment for the crime scene, here are some points to consider:

Weight: A self-contained system will be lightweight and contain all necessary components.

Durability: Rugged exterior to withstand transport and general wear and tear. Internal filter system protects the filters from breakage and contamination For example, an agency ran over their Omniprint™ 1000 with a crime scene vehicle and the unit still functioned flawlessly.

Cross-contamination issues: Smooth cable is easily cleaned and decontaminated. A fan filter will prevent dirt and fibers from entering the unit, preventing cross-contamination of the next scene.

Ease-of-Use: Consider a system with instructions for use, lamp changing, and goggles selection that are permanently affixed to the unit.

High Intensity: For a portable system, a 400W Halide lamp is very good because it offers very high-intensity in the blue spectrum, where the majority of fingerprint work is conducted.

Our thanks to Mary Nolte for sharing her expertise with us. In the next issue of Minutiæ, Mr. Pat A. Wertheim will write about chemical sequential processing of latent prints.