Fluorescence Experiments in Fingerprint Detection
When it comes right down to it, there are basically two kinds of people in the world: those who produce good fingerprints and those (like Joanie and me) who do not. I managed to find some volunteers to help with the following experiments.To encourage the production of decent prints, temporarily donning polyethylene gloves of the type food handlers are supposed to wear can help. After a while, gloved hands perspire enough to allow leaving telltale evidence on various articles.
WARNING: Before going any further, let me add this cautionary note to those who want to work with ultrabright LEDs, such as the Luxeons described here. They are LEDs, yes, but they can be hazardous to one's sight if treated carelessly. Though not yet as bright as lasers or alternative light sources, these little rascals are amazingly bright. Always exercise due caution.
DFO Under Blue LED Illumination
An advertising sheet with fresh palm prints and fingerprints was sprayed with DFO twice, allowed to dry in the hood, and placed in an oven @100°C for 10 minutes to speed up the development process. The sheet was placed under a 5-watt, Luxeon royal-blue LED with a peak emission @ 455 nm. An Intor 450/40/60 (center wavelength/bandwidth in nm/transmission at peak wavelength) bandpass filter and Physical Optics Corporation holographic diffuser were placed over the lamp's projection lens to illuminate the paper.
A Sony DSC-F717 digital camera with a Promaster orange filter mounted on its lens was anchored to an optical table mount. The camera lens was set @ F2.8 (in aperture mode) at an equivalent ISO of 100 and carefully focused on the fingerprints. The original image size was set for 2048x1536 pixels. The camera was allowed to automatically determine the exposure, which was around 4-5 seconds for these DFO images.
Below are three views of the fingerprints shown on the lower left: fingerprints developed in DFO; color-inverted image of same; and a grayscale relief image built by overlaying color-inverted and non-inverted images with slight offsets. The relief image is dramatic but may not fare well when using an automated fingerprint identification systems software package (AFIS).
All the images shown here were brightness and contrast enhanced by Adobe Photoshop.
The Setup
Here is a view of Joanie using two LED Fluorescence lamps to examine objects that have been treated with fingerprint developers. If successful, this usually would be followed by photographing any revealed fingerprints, as shown. On the left is a lamp with a 5-watt, Luxeon royal-blue LED, and the lamp on the right is using a 5-watt, Luxeon green LED. The lamp selected will depend upon the developer used and the nature of the article's surface. Fluorescent developers vary in performance under UV, royal-blue, blue, or green light. Here, the camera being used to photograph a glass is equipped with a B+W orange/red filter to block the direct excitation light from the green LED lamp.
The yellow goggles Joanie is wearing are used to block direct light from the royal-blue LED as she scrutinizes a document for prints. Telltale fluorescence comes through very nicely. A pair of orange-hued, laser safety goggles designed to block 532 nm light is worn when working with the green LED lamp. Sometimes, a pair of red goggles is useful to block annoying background fluorescence or bleed-through light from LEDs.
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Indanedione Under Green LED Illumination This one-day old hand print on a sheet of copy paper was sprayed several times with a fresh solution of 1,2-indanedione in ethyl acetate and HFE-7100. After drying for 5 minutes @100°C, the print was evaluated under green light. The print looked weak, so the sheet of paper was sprayed again with indanedione, briefly air-dried, and sprayed with zinc chloride in petroleum ether and MTBE. The sheet was then placed back in the oven for 10 more minutes. For indanedione, the royal-blue LED will not excite very much fluorescence, and the resulting contrast is poor. So, for that reagent, I switched to the green LED lamp. The camera was set as above, but this time the automatic exposure was closer to 10 seconds. (The hand print has been considerably reduced to fit here. The original photo shows the fingerprint and palm print ridges in crisp detail.) |
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The green LED used for the indanedione images has a nominal peak wavelength of 530 nm and a large bandwidth. Because of that bandwidth, excitation filters have to be used with the lamp. I usually employ either of two filters for experiments with indanedione: an Intor 532/10/50 bandpass filter or an Intor 540 SWP T-FAJ lowpass filter (see note). With either filter mounted on the lamp, a pair of laser safety goggles (intended to block 532 nm light) works pretty well, though it works better with the bandpass filter. Ideally, either a Schott OG 570 glass or a Wratten #22 filter should be mounted over the camera lens. The standard orange camera filter, which works well for royal-blue, blue, or blue-green lamps, allows too much 532 nm light through and is useless for fluorescence photos. Schott longpass filters with screw-in mounts for a particular camera lens have to be made-to-order. However, a screw-in mount, B+W 041 orange/red filter (equivalent to the Wratten #22) is readily available for different lens sizes.
Note: I decided to try the lowpass filter because of manufacturing variations of LEDs. As an example, Luxeon states that the wavelength-peak range for the green LED is 520-550 nm with 530 nm typical. The 540 SWP lowpass allows much of the light from any of those peaks to pass through, while blocking longer wavelengths sufficiently to permit both the goggles and the B+W camera filter to work fairly well. Much more light comes through with that filter, which is usually a good thing, but some might leak through the goggles and camera filter. It is ideal for scanning areas or articles for fluorescent regions. For even better contrast for photography, the 532/10/50 filter should be used on the LED lamp with the B+W filter mounted on the camera lens. Since all of this is really dependent on the actual peak wavelength of the LED on hand, experiment away!
A very suspicious advertising postcard arrived in the mail just begging to be fingerprinted. Of course, I was careful to extricate the postcard from the mailbox while holding it by its edges.
The evidence...uh...postcard was heavily sprayed with indanedione, air-dried, and then sprayed with a solution of zinc chloride. It was then placed in the oven @100°C for 10 minutes. Under room lighting, some fingerprints soon became visible—humidity in the air seems to ripen the image—exhibiting a pinkish hue. A selected area was photographed under the green LED lamp and processed using Photoshop. I changed the original color photo to grayscale and applied a flat-field technique to improve the evenness of illumination. Then, after switching back to RGB mode, I selectively colored the fingerprints. Also visible in the picture are tiny fibers present in the postcard.
Next, we leave the ninhydrin analogs to examine a very popular fingerprint development method using Super Glue. If you've ever applied Super Glue in a confined area, you might have noticed a white film coating nearby surfaces. That hazy deposit is produced by polymerizing vapors of cyanoacrylate ester. The coating generally is an annoyance or worse, especially if it has fogged nearby glass or—gulp—optical surfaces. The Super Glue fog can sometimes be removed with active solvents and sometimes not.
Lo and behold, in 1978 technicians at Japan's National Police Agency discovered that Super Glue's troublesome vapors could reveal latent fingerprints on a large number of surfaces! Shortly thereafter, both the U.S. Army Criminal Investigation Department and the ATF introduced the technique into the United States. Anionic polymerization of cyanoacrylate occurs in the presence of hydroxyl ions, hence the benefit of a humid atmosphere for development. Chemicals deposited by fingerprints, including amino acids, proteins, sweat, etc., act as preferential sites on which the fumes settle and polymerize.
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Developing Fingerprints Using Super Glue Fuming Note: The following processing using cyanoacrylate should be performed in a well-ventilated hood to allow for removal of harmful vapors generated during development. A sealable glass container, such as an aquarium, is set up to fume the article for fingerprint detection. A small quantity of Super Glue is placed in an aluminum weighing dish alongside a beaker filled with warm water. The aluminum dish may be placed on a beverage warming plate to produce copious fuming and to speed development of any latent fingerprints. To see details, place the mouse pointer over the photo. (The use of a warming plate rather than a hot plate is usually suggested due to the hazards of operating the latter in a potentially inflammable atmosphere. Also, if the cyanoacrylate gets too hot, it can break down into cyanide products. Investigators would then have another crime to investigate!) An alternative method is to combine sodium hydroxide with the Super Glue when fuming is required. This bypasses the need for a warming plate but is not as easily controlled. The added humidity produced by the water-filled beaker encourages polymerization of the cyanoacrylate fumes. The article being examined is placed in the fuming chamber and allowed to sit for approximately 45 minutes to develop. Often, latent prints become visible during the process and should be monitored to determine the right time to end the fuming. If permitted to continue much beyond this point, areas between ridges begin to fill with polymer, ruining the fingerprints. Today, methods employing a vacuum pump are used to help prevent overdevelopment. |
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Rhodamine 6G for Fluorescent Development
The best dye for fluorescence work has to be rhodamine 6G (R6G). Probably the brightest member in the rhodamine family, this dye has an incredible quantum yield of 95%. It is the de facto standard against which many fluorescence efficiency measurements are compared.
When exceedingly small amounts are dissolved in alcohol or water, R6G produces a highly fluorescent liquid that can highlight fingerprints developed by Super Glue fuming. When applied, the dye intercalates between rows of molecules in the cyanoacrylate polymer staining the fingerprints. R6G responds beautifully to green LED light, generating a bright fluorescence peaking somewhere around 556-590 nm.
R6G Fluorescent Staining of Super Glue Fingerprints A souvenir glass with fingerprints was developed with Super Glue fumes at room temperature. It required an overnight treatment to get a really substantial buildup. Next, the glass was washed in tap water. Then, a 1:1000 solution of R6G in reagent alcohol was sprayed on. After the alcohol evaporated, the glass was rinsed in distilled water and allowed to dry. The green LED lamp was used along with a B+W orange/red filter mounted on the camera. I selected the camera's manual control to set both aperture and shutter speed. Finally, the image was contrast-enhanced using Adobe Photoshop. Not much computer processing was needed. Incidentally, that's water in the glass...not beer. |
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I have developed a whole new respect for fingerprint technicians and detectives who routinely manage to uncover usable prints on the most difficult and varied of surfaces. As previously mentioned, neither Joanie nor I leave good prints, and I'm sure there are numerous others with that characteristic. Unless special precautions were taken in my experiments, most of the time only fragmentary fingerprints were detected, even with the volunteers. Yet from those kinds of partial prints, identities can often be confirmed and criminals subsequently brought to justice. Though I focused here on fingerprint detection, the LED lamps easily detected stains as well as minute fluorescent fibers and particles. I hope that, based on its capabilities in all the areas discussed on this web site, LED Fluorescence will be looked at as an additional and affordable tool for forensic science.
If you incorporate the LED Fluorescence technique into forensics or any other field of study, I certainly would appreciate receiving a note letting me know how you're using the technique and how it's working out for you. (Just click on my name on the home page.)
