Technical Note

Fluorescent Powders

History of Redwop™

How did Redwop™ get its name? Several years ago, when lasers were first being used to develop latent fingerprints, the usual developing agents were liquid, such as Rhodamine 6G in methanol. Some surfaces could not be treated with a liquid, and there were no suitable fluorescent latent print powders commonly available. To meet this demand, we marketed a red fluorescent powder. We needed a name for this new powder. Earlier we had noticed that the word "powder" spelled backwards had an interesting sound: "Redwop™." Since our new powder was red, we thought Redwop Powder would be a catchy name. Later, an investigator asked us to make a green-colored powder to fluoresce in the shorter bands of light and to provide a different color from red to give a greater contrast on certain backgrounds. He told others about this new powder, and he always referred to it as "Greenwop™." Thus was born the suffix, "-wop."

Advantages

What are the advantages of using Forensic Light Sources and fluorescent powders? The sensitivity of seeing latent prints may be improved by a factor of 100,000. The following is by Dr. E. Roland Menzel from An Introduction to Lasers, Forensic Lights and Fluorescent Fingerprint Detection, pages 2-3. . . . . it is perhaps worthwhile . . . to ask why one would want to detect fingerprints via fluorescence in the first place. To answer, let us consider the most widely used conventional fingerprint detection procedure, dusting, and let us imagine a strong print on a white surface dusted with black powder. Ambient light is reflected (scattered) from the surroundings of the print and from sites between fingerprint ridges, but not from the ridges themselves because there the black powder absorbs the incident light. Absorption/reflection, then, is the basic mechanism by which the print is seen. Let us now imagine a weak print to which only few powder particles have adhered. Ridge sites now reflect light also, but with an intensity slightly lower than the reflection from between ridges. The visualization of the print now amounts to the detection of a small difference between large light signals, and this is a detection mode of inherently poor sensitivity. The detection of a faint fingerprint would then amount to the detection of a small signal, rather than the detection of a small difference between large signals. To place the sensitivity difference in perspective, let us compare the detection of a trace quantity of rhodamine 6G in solution via absorption and via fluorescence under (green) 514.5 nanometer (nm) argon-ion laser excitation, using a 5 watt laser (which is fairly commonly found in crime laboratories). The fluorescence detection sensitivity under these conditions is better by a factor of about 100,000. It is for this reason that fluorescence-based detection has supplanted absorption-based detection in many fields of science.

Description

Redwop™ and Greenwop™ fluorescent powders are very fine powders with a Lycopodium base. They fluoresce differently on the substrate, and some investigators report that they seem to be best for wooden surfaces, such as rifle stocks and wood paneling. All of these powders work well under argon-ion, copper-vapor and Nd:YAG lasers, along with Forensic Light Sources and long-wavelength ultraviolet lights. Whenever liquid dye staining might damage the object being examined or in a field work situations where liquid dye staining is a problem, any of these powders can be used instead.

Magnetic Powders

Blitz™ powders are fluorescent magnetic powders to be used on the same types of surfaces as other magnetic powders, but with a Forensic Light Source. Blitz-Red™ fluorescent magnetic powder is pale red in color but fluoresces a bright red color under several wavelengths of light, including ultraviolet, 450, 485, 525, 530 and 570 nm. Blitz-Green™ fluorescent magnetic powder is a light green when viewed in the jar but fluoresces a bright green when viewed under ultraviolet, 450, 485, 525 and 530 nm.

Application

As with any powders, it is recommended that the piece of evidence is treated with glue fumes initially. It is better to "underfume" than to "overfume" as the powders adhere to the glue residue. If the entire surface is heavy with white residue, the fluorescent powder may adhere to the entire surface and it will glow so strongly that the fine details in the latent prints may be lost. Glue-fuming in a vacuum system causes the glue to adhere to latent print ridge details and prevents the glue residue from adhering to the entire surface. It is recommended that a feather duster be used to apply Redwop™ or Greenwop™ fluorescent powders. A feather duster applies a thin, even coat of these powders without coating the surface too heavily. Since Blitz™ powders are magnetic, they would be used on the same types of surfaces as regular magnetic powders. The same style of magnetic applicator is used.

Latent Prints on Skin

It has been reported that in laboratory conditions, latent prints have been developed on the skin of a live Caucasian person by applying Blitz-Green™ fluorescent magnetic powder directly to the skin and viewing it under 450 nm wavelength of light. Of course, the live person was not treated with glue fumes.

In the past, some agencies have had success with glue-fuming deceased bodies before processing with fluorescent dyes and powders. It is recommended that the glue-fuming process be performed as soon as possible after the body is discovered, even if it must be done in the field. There must be coordination between the Coroner's office and the latent print technicians.

Viewing

The color of powder chosen in either category of powder types depends on the color and fluorescence of the surface to which they will be applied. The goal is to choose the color of powder which will fluoresce at a different wavelength than the background. It is recommended that the examiner experiment with each color to determine its best fluorescence using each wavelength available on the Forensic Light Source in use with each color of viewing goggles. For example, the Redwop™ and Blitz-Red™ are very sensitive to 570-590 nm wavelength when viewing through a red filter. Blitz-Green™ and Greenwop™ are very sensitive to long wavelength ultraviolet, about 365 nm with no viewing filter needed.

Additional Reading

An Introduction to Lasers, Forensic Lights and Fluorescent Fingerprint Detection Techniques by Dr. E. Roland Menzel
Fingerprint Detection by Fluorescence Examination: A Guide to Operational Implementation by the British Home Office

Ordering Information

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