Emily C. Lennert
fingerprint, latent, visualization, noninvasive, speculative, thermal paper, receipt paper
Bond, J. W. A noninvasive and speculative method of visualizing latent fingerprint deposits on thermal paper. Journal of Forensic Sciences. 2015, 60(4), 1034-1039.
The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.
Fingerprint visualization methods that are noninvasive are desirable so that there is no physical contact with the fingerprint, loss of material, or chemical reaction. Additionally, noninvasive methods are preferred to minimize the risk of damaging DNA evidence, or to allow for subsequent analysis by other methods. Speculative methods are methods which allow for the analysis of a large area in a short time period. One method of noninvasive and speculative fingerprint visualization that is being explored is the use of ultraviolet (UV) light. UV light has the greatest effect on DNA in the UV-B and UV-C range, with the least pronounced effect in the UV-A range. The authors of this study therefore sought to visualize latent fingerprints on thermal, e.g. receipt, paper using an array of lights including visible white light, visible blue light, and UV-A light.
Fifty male and fifty female fingerprint donors contributed to the samples studied. Samples were divided into age ranges, with ten male and ten female donors per age range: 21-30, 31-40, 41-50, 51-60, and 61 and up. Fingerprints were collected by pressing a finger onto the paper surface with light pressure for 1-2 seconds. Fingerprints were examined 24 hours after deposition, as well as weekly up to 12 weeks. The paper was stored in an office environment between examinations, exposed to natural and artificial light but not to direct sunlight. Light sources used for examination included natural daylight, white Crime-Lites (400-700 nm), blue Crime-Lites (peak 450 nm), and LED torches (peak 365 nm), i.e. UV-A light. UV-A light is light in the 315-400 nm range. Light examinations took place in a dark room to minimize interference from other light sources. Samples were photographed while under the specified light.
Samples were first analyzed under multiple light sources. Samples were analyzed under natural light, followed by white and blue Crime-Lites and the LED torch at a specified intentisty of 60 W/m2. Fingerprint visibility was graded on a scale of 0-4 using the Bandey scale, seen in Table 1 within the study. Based on this grading system, best visibility of fingerprints at 24 hours was obtained under the LED light. While most fingerprints rated a 0 or 1, 28% of the fingerprints were graded as a 3 or 4, which has previously been reported as sufficient for identification. At 1 week, the visibility of prints appeared to have improved, with 34% ranking a 3 or 4, however this change was not deemed significant after a Pearson’s chi-square test. Like the 24 hour mark, best visibility at 1 week and all weeks later was observed for the LED light.
Next, the effect of the intensity of the LED light was examined on the lower graded fingerprints to determine whether lower graded fingerprints, i.e. less than 3, would improve in visibility with increased intensity of light. A LED light with an intensity of 250 W/m2 was compared to the initial grading obtained with an intensity of 60 W/m2. With increased light intensity, the percentage of higher graded fingerprints increased from 28% to 34% at the 24 hour mark. This change was not observed at 1 week and later. However, it was concluded that increased light intensity tended to increase the visibility of fingerprints.
The authors then investigated the effect of visible light being emitted alongside UV-A light. The LED torch emitted both visible and UV-A light in previous stages. In this stage, the torch was fitted with a shortpass filter to reduce the emission of visible light from the torch but allow the emission of UV-A light. This was to determine whether the visible light was an important factor in the visualization of the latent fingerprints. It was observed that even with the filter, i.e. with reduced visible light emission, fingerprints were still visible, suggesting that the UV-A wavelengths were responsible for the visualization of the fingerprints.
The authors sought to explain why UV light allowed the visualization of latent fingerprint on thermal paper. The authors state that the fingerprint is visible due to a lack of emission from the paper in the areas where the fingerprint is present, whereas in areas where the fingerprint is not present the paper will re-emit visible wavelengths of light, giving the fingerprint a black appearance against a blue background. The authors propose two explanations for this: UV absorption by either the sweat components of the fingerprint or by the dye that is within the thermal paper. If it is due to the sweat deposit, then fingerprints would be visible on other papers, such as regular office paper. To test this, split fingerprints were taken, where half of the fingerprint was on office paper and half on thermal paper, and the fingerprints were examined using LED lights both with and without the shortpass filter. The office paper showed no fingerprint visibility. To ensure that surface roughness was not a factor, Magno Satin paper was also studied in this manner, and again no fingerprint visibility was observed, suggesting that fingerprint visibility on thermal paper is due to the dye present in the thermal paper. Previous work has shown that the ink in thermal paper may be visualized due to the protonated amino acids present in sweat, which facilitate a color change in the dye at elevated temperatures. The authors proposed that a similar mechanism was at work, with a weak color change in the dye facilitated by the protons present in the sweat deposit and the UV-A irradiation.
- Latent fingerprints were visualized most successfully with UV-A light sources, particularly at higher intensities.
- Filtering out the visible light from the UV-A emission did not adversely effect the visualization of fingerprints.
- The mechanism for this color change was proposed to be a result of the a color change in the paper’s dye caused by the protons present in the sweat deposit.
Noninvasive, speculative fingerprint visualization methods allow for the visualization of fingerprints without physical contact.
UV-A light may provide a noninvasive and speculative method that allows for the visualization of latent fingerprints.
Photo by Aman Jha from Pexels.