Luminescent devices for taking fingerprints.
Law Enforcement Technology.- 1995 .- March.- P. 50-51.
Luminescent devices for taking fingerprints
When the possibility of detecting latent fingerprints was first discovered in the late 1800s, the main tool used by the investigator using this opportunity was a brush and black fine-grained powder. In the late 1970s, the same brush and powder remained the main tools for taking fingerprints. The exception was paper products that could be treated with chemical reagents.
In the early 1980s, research began on the use of laser radiation to detect latent fingerprints. The results of this research turned out to be practically applicable, and various types of lasers for forensic applications appeared. These lasers and the devices in which they were used were of great help to forensic scientists in detecting and recognizing fingerprints and other traces that could be used as evidence. However, they had a number of significant drawbacks: high cost, reaching $ 100 thousand, large size and weight. Most of them were laboratory installations that required powerful power sources and water cooling systems. The output beam of laser radiation covered an area of no more than 6 cm2.
In the mid-80s, more acceptable portable devices were proposed that used powerful light sources instead of lasers. In the late 80s — early 90s, industrial models of such devices appeared, in which the light flux of the light source passed through light filters that formed an intense directed beam of light. This beam covered a larger area than a laser beam, which accelerated the search. Some devices had an adjustable wavelength of the emitted light. The cost of these portable devices was in the range of 1,000-20,000 dollars.
Since 1993, two US companies have become the main suppliers of optical devices for forensic purposes: Lynn Peavey (Blue Light Special device) and Sirchie (Blue Maxx device). These devices, which have much in common in their design, have a set of light filters attached to the end of a standard battery-powered hand-held flashlight, such as Streamlight or Maglight. The investigator or police officer using such a device directs a beam of light at the search area and observes objects appearing in the illuminated area through a large filter mounted on the flashlight or special glasses with colored filters. Fingerprints are detected as blurry spots that are covered with a special luminescent dye solution or black powder for their processing. Particles of the solution or powder settle on the fingerprints, and when illuminated by a light source, they begin to luminesce. They can be photographed using special filters and lenses.
The most important limitation of these devices is related to battery power sources. Most flashlights with such power sources can operate continuously without recharging for 1 hour. However, in many cases, a longer investigation of crime scenes or incidents is required. To overcome this limitation, it has been suggested to have a sufficient number of backup devices or charged batteries in patrol cars, which is almost always unacceptable, and to use a special power supply for the devices from a standard AC network with a voltage of 120 V.
Long-wave UV light sources, previously used by forensic scientists for other studies, have proven suitable for fingerprint detection. The difference with such devices is that no special filters are required to photograph the fingerprints they reveal. Most commercially available fingerprint detection dye solutions and powders can be used with such UV devices, as well as those that cannot be used with other light sources. One of the best dyes for use with UV devices is the yellow-green dye «ardrox». After drying, the fingerprints covered with this dye solution luminesce when illuminated with UV light.
Documentation of latent fingerprints can be done in several ways. One of them is photography. For this purpose, reflex cameras with a lens that allows obtaining images on a scale of one to one should be used. The best results are given by cameras with programmable shutter speed. Depending on the clarity of the latent fingerprints, the shutter speed can be from 1 sec to 1 min or more. When using slide film, negatives can be compared with black and white images. In many cases, photographed prints are more contrasting and clearer than those simply observed under illumination by an appropriate light source.
UV devices have been used in forensics for about 15 years. During this time, they have developed from expensive laboratory installations to portable devices with low weight. This allows most police stations to independently conduct investigations with the identification of fingerprints that can be used as evidence.