Forensic Light Sources: Four Decades of Detection
Written by Brian Dalrymple   

DETECTION OF FINGERPRINTS by police began in the late 19th Century. At that time, two methods were in mainstream use—dusting powders and silver nitrate—with iodine fuming a niche and distant third. The processing decisions were relatively few. With notable exceptions, powders were used on nonporous surfaces while silver nitrate was the most sensitive reagent for paper (porous) exhibits. A user would select the powder of preference for each surface encountered, and the color or tone that afforded the best contrast with the substrate.

This article appeared in the July-August 2020 issue of Evidence Technology Magazine.
You can view that issue here.

In 1932, Charles Lindberg’s infant son was kidnapped in arguably the most notorious kidnapping case in history, and the same two techniques were employed. By the early 1970s, the only widespread advance in fingerprint detection was the introduction of ninhydrin, which significantly increased the recovery of fingerprints on paper and cardboard. It is reasonable to state that substantial advancement of fingerprint detection science had been somewhat flatlined for almost eight decades, and sequential processing was virtually nonexistent. All methods of this era relied on staining the latent fingerprint to contrast with the surface on which it appeared.

The First Forensic Light Source
In 1977, as a result of curiosity and serendipity, the argon ion laser was found to reveal untreated fingerprints through inherent fluorescence. The first forensic laser in history, acquired in April of that year by the Ontario Provincial Police in Ontario, Canada, was composed of two parts: a plasma tube nine feet in length of considerable weight, and a control center the size of a speaker’s dais (Figure 1). The device emitted 20W of blue and green wavelengths, was water-cooled, and required three-phase power (70 amps per phase).

Figure 1. The first forensic argon-ion laser in 1977 (Image: Brian Dalrymple)

This represented the first time that fingerprint evidence was recovered by a strategy other than staining, and the technique introduced several advantages. It must be emphasized that the power of the laser was not in revealing larger numbers of fingerprints, but in net gain, both in latent fingerprints and in other types of trace evidence.

  • First, using light as a detection strategy is exponentially more sensitive than staining, particularly when the targets are sub-nanogram in size.
  • Second, it revealed fingerprints undetected by chemical techniques, quite obviously targeting components other than salt, amino acids, and sebaceous oil, which do not exhibit fluorescence.
  • Third, it revealed fingerprints on surfaces not amenable to conventional techniques.
  • Fourth, the laser revelations did not stop at fingerprints. Hairs, fibers, body fluids, and chemical traces (invisible in white light) were found to exhibit luminescence under laser light.
  • Finally, the process was nondestructive, meaning that any other technique could be applied after laser scanning.

The forensic laser technique represented the first significant option for sequential processing, the chance to recover more fingerprints (and other evidence) by employing a succession of strategies in the correct order. In an early case—and an early example of sequential processing—an unglazed clay flowerpot, associated to an attempted bombing, was examined by laser. A clear, identifiable impression was located and photographed (Figure 2). Subsequent dusting revealed no trace of it.

Figure 2. Untreated fingerprint on unglazed flowerpot revealed by laser (Image: Brian Dalrymple)

The FBI were curious about this new detection system and in June came to evaluate it. The laser revealed otherwise invisible ridge detail on a range of exhibits they brought, as well as a thumbprint of exemplary quality on the hand of one of the visitors, obviously acquired while shaking hands (Figure 3). The photograph required the subject to hold his hand steady for a 30-second exposure and the resulting image is understandably blurred.

Figure 3. Fingerprint on hand of FBI visitor under laser illumination (Image: Brian Dalrymple)

Within weeks, the FBI had moved to acquire their own laser, and very quickly began to reap forensic benefits. One such case was the high-profile investigation of an alleged war criminal who had entered the United States after World War II. Numerous documents were released by the German government for fingerprint examination, but chemical techniques were not permitted. The FBI laser revealed a single finger impression on a postcard addressed to Heinrich Himmler. This impression was identified to the suspect and he was subsequently deported. The fingerprint had been deposited during the war and had persisted for over four decades.

Now for the challenges: The running costs were high (this first laser had a penchant for breaking down) and the technique was not portable. All items for scrutiny had to be transported to the lab. The hunt was on immediately for an “alternative”—a smaller, less-costly, and portable device to replace the laser. It is important to note that a viable filtered lamp source was not introduced until 1984. For seven years, all fluorescing evidence had been untreated, and revealed by lasers, which were operated exclusively by a handful of large police agencies.

Alternatives: What’s In A Name?
The Lumilite, developed in Canada (Figure 4) and introduced in 1984, was the first filtered lamp, closely followed by the Polilight from Australia (Figure 5) and, later, the Spex Crime Scope, extending the application to a growing number of police agencies.

Figure 4. Lumilite – the first filtered lamp (Image: R. Rigole, Ontario Police College)

Figure 5. Rofin Polilight - a forensic cornerstone (Image: Rofin Forensic)

These devices, and other light sources based on the same filtered lamp technology, became known as “Alternative Light Sources”, or ALS. Dictionary definitions vary, but the inference in all of them is this: an alternative is the adoption of a different strategy that achieves the same desired result. Herein lies the problem. The argon laser is monochromatic, emitting a single, surgically precise wavelength (514.5nm) at high intensity. The filtered lamps, all of them, deliver selected bands of wavelengths (some approximately the same color as the argon laser, some in the blue region), but with a bandwidth of 40–100 wavelengths. They are about as alternative as fish and bicycles. How could they possibly deliver the same results? Of course, they do not. Frequently, similar results are obtained with both examinations, but it became increasingly apparent under objective comparison that each source revealed things missed by the other.

In the 1990s in Ontario, Canada, the house of a serial killer was examined first with a filtered lamp, which failed to reveal any significant ridge detail. Subsequent examination by argon laser resulted in the detection and photography of more than 40 fingerprints. With apologies to Al Gore, this was an inconvenient truth. Instead of finding a cheaper, portable, and more robust alternative to argon lasers, we were presented with yet another device for revealing more fluorogenic evidence—and more options for sequential processing of objects and crime scenes. For all these reasons, it has long been suggested that “Forensic Light Source” is a more accurate and appropriate title for any device (including laser) that is used to generate fluorescence in evidence.

Fluorogenic Chemistry: Extending Our Reach
Detecting fingerprints by fluorescence was initially a happy accident, but researchers soon began to exploit this property with chemical extensions. In 1983, Menzel introduced dye staining (Rhodamine 6G) as a complement to cyanoacrylate fuming—arguably one of the most significant developments in fingerprint detection history.

Again, fluorescence is simply a far more sensitive detection strategy than staining. This property has been expanded even further by the development of fluorogenic ninhydrin analogs like DFO (reported to be 2.5-times as sensitive as ninhydrin) and 1,2 indanedione (reported to be 10-times as sensitive as DFO). Great attention has been devoted to exploiting this property in new chemistry wherever possible.

Diagnosis and Triage
We in the forensic identification discipline are frequently the first, if not the only, experts to examine crime scenes and exhibits; are trained in the collection of an open-ended range of physical evidence; and must photographically optimize physical evidence of all types. One could say that we are experts in recognizing the presence of, or the potential for, visible forensic signal. Our goal is to optimize the signal-to-noise ratio—to obtain the best possible images (not to mention the most possible images) of impression evidence for evaluation. We must attempt to recover every possible impression, not just the low-hanging fruit. In the writer’s crime scene experience, the exemplar fingerprint is frequently not the most significant one.

There are several factors affecting this goal when using forensic light sources:

  1. The absorption spectrum of the target (where it absorbs light)
  2. The emission wavelengths of the target
  3. The emission wavelengths of the substrate

There is literally no limit to the range of physical evidence that may be detected or optimized by forensic light source examination. The key point to remember is that one specific combination of excitation wavelength and barrier filter may be the best—or the only—way to obtain optimum signal-to-noise ratio (that is, the clearest, most complete image).

Narrow Band Filters: Gains At The Threshold
Inherent fingerprint fluorescence (and, routinely, the response of chemistry like DFO and indanedione) can be weak and easily obscured by the fluorescent response of the substrate. The laser goggles (a personal safety device for eye protection when using lasers) blocked the laser reflection in a darkened room and, coincidently, revealed fluorescing fingerprints. To photograph these impressions, the filter from the goggles was removed and fastened to the camera lens. In the early days, the filter was affixed using duct tape. The goggles filter, however, was not selective. Its purpose was to block laser emissions, which it did very effectively. It transmitted all fluorescence created by the laser however, including any emitted by the substrate. In cases where the fingerprint fluorescence was weak, it could easily be overpowered by background noise.

Unlike the high-pass filter in the laser goggles, narrow-band filters transmit a very select band of wavelengths. They proved most effective in eliminating background fluorescence and increasing the number of comparable fingerprints revealed by the laser and, subsequently, by other forensic light sources. Introduced in 1982 (at a time when only a handful of police agencies possessed laser capability), they remained virtually unnoticed by the identification community for several decades and now, with the increasingly wide acquisition of all light sources, lasers, and filtered lamps, they are enjoying a comeback to prominence. Routinely, they rescue fingerprints that teeter on the brink of forensic value by removing obstructive background fluorescence and revealing ridge detail. In Figure 6 (left side), when photographed by laser with the standard orange barrier filter, just the suggestion of ridge detail can be seen. It was developed by indanedione on a portion of an envelope previously covered by a postage stamp. In Figure 6 (right side), where the same impression is photographed with the narrow-band and the orange filters together, a clearly comparable fingerprint can be seen. One such filter is the FF 1.0, available from Arrowhead Forensics.

Figure 6. Fingerprint on manila envelope, developed by indanedione – Left) with orange barrier filter; Right) with orange barrier filter plus FF 1.0 narrow-band filter (Image: Brian Dalrymple)

In the 40-year quest for the ideal forensic light source, many new forensic light sources have emerged. During the 1990s, the Ontario Provincial Police introduced the Mobile Crime Unit, a 12-ton, 24-foot truck, housing an argon laser, complete with 50-kW generator, cooling system, and a 100-ft. fiberoptic light guide. The mainframe laser could now be transported to crime scenes, albeit at considerable expense and effort. This resulted in a 40% increase in evidence recovery at major crime scenes over two years. The frequency-doubled, solid-state YAG laser was portable, much smaller than its argon predecessor, emitted light at 532 nm, and could be plugged into a standard electrical outlet. It became a popular choice, although repeated technical breakdowns tarnished its overall performance.

The Coherent Tracer, a semiconductor laser emitting at 532 nm, was one of the most significant game-changers in this evolutionary path in 2006 (Figure 7). Powerful, portable, reliable, and affordable, the Tracer continues today to be a crime scene and lab workhorse. It has been joined by cordless handheld light-emitting diode sources (LEDS), including the ROFIN Flare Plus 2 (Figure 8), and the Foster Freeman Crime-lite, which feature several broadband emission choices akin to those of the larger filtered lamps.

Figure 7. Coherent Tracer semiconductor laser (Image: Coherent)

Figure 8. Rofin Flare Plus 2, powerful cordless LED source (Image: Brian Dalrymple)

These can be characterized as even more easily and pleasantly portable for crime scene work. The most recent addition to the forensic light source list is the powerful and portable diode laser (BrightBeam and Dual 77) which includes both blue and green wavelengths in a single cordless user-friendly device—a far cry from the laboratory laser in 1977 (Figure 9).

Figure 9. Dual 77, cordless, 8W diode laser with blue and green wavelengths (Image: Arrowhead Forensics)

Sequential Processing
In 1975, two mainstream choices existed for the detection of fingerprints, with a few niche techniques occasionally used. Sequential processing—that is, the strategy of using multiple techniques on the same exhibit—did not exist. In today’s world, dozens of detection methods are in common use, many of them fluorogenic. A forensic specialist can use as many as five or six different techniques on the same exhibit (provided they are conducted in the correct order) and achieve success with any or each of them. These include different laser emissions, wavelength bands throughout the actinic spectrum (ultraviolet, visible and near infrared), and chemical treatments. The direct result of this extension of examination is obvious: many more fingerprints are being revealed on more substrates than ever before.

The Ideal Forensic Light Source
What is the best forensic light source? We are not much closer to answering that question than we were in 1977, but we now know much more about detection of fluorescing evidence—enough to know that there is no single right answer, no magic button. We search for a huge range of evidence targets, with differing and unknown absorption/emission properties—all on substrates with the same range of variables. We know the fluorogenic properties of chemical techniques like indanedione and Rhodamine 6G, but they may require different excitation wavelengths on different surfaces for optimum results. Forensic lasers have featured emissions at 445, 488, 514.5, 520, 532, and 577 nm. The writer is unaware of any research results that establish any of these as the best or only option, although the mid-green range (500–532 nm) has yielded consistently positive results both in terms of untreated evidence and chemical extensions. It has been proven repeatedly that any light source and filter combination has the potential to be the optimal or exclusive means of visualizing any piece of evidence.

A banded source may be indistinguishable in color from a monochromatic laser, but experience has taught us that they do not always offer the same results. One single wavelength from a laser is surgically precise, and may excite only the desired target, where the filtered lamp, comprising many wavelengths, frequently creates background fluorescence. One analogy would be that the laser can tiptoe into the room without waking up the kids. On the other hand, the broadband, “shotgun” approach of the filtered lamp or LED may be advantageous in other circumstances where the laser fails to reveal something, possibly chemical residue or body fluid.

In the writer’s opinion, particularly when our goal is to recover every trace of physical evidence that the perpetrator has been gracious enough to leave behind, a sequential examination that includes both monochromatic and broadband light, from ultraviolet to green, will greatly reduce the chances of missing something critical to the case. The cost of lasers and other forensic light sources now places them well within the reach of all police agencies, and they have shrunk to easily manageable size, at a crime scene or in the lab. Shop carefully and shop well!

About The Author
This e-mail address is being protected from spam bots, you need JavaScript enabled to view it was part of the original research team that introduced lasers in 1977. He retired in 1999 from the Ontario Provincial Police as Manager, Forensic Identification Services. He initiated the first computer evidence enhancement system in Canada in 1991. He is currently a forensic consultant (Brian Dalrymple & Associates), an instructor for Ron Smith and Associates, and an adjunct professor at Laurentian University. He is the recipient of the Dondero Award (International Association for Identification), the Foster Award (Canadian Identification Society), and the Lewis Minshall Award (The Fingerprint Society).

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