Ultraviolet luminescence from latent fingerprints

Fortmic Science International, 59 (1993) 3 - 14 Elsevier Scientific Publishers Ireland Ltd.

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ULTRAVIOLET LUMINESCENCE FROM LATENT FINGERPRINTS

S.K. BRAMBLEa, K.E. CREERa, W. GUI QIANGb and B. SHEARDa aMetropolitanPolice Forensic Science Laboratory, 109 Lambeth Road, Lo&on SE1 7LP (UK) and bZnstituteof Fore-n&cScience, Building 17 Muxidi Nanliu, Beijing 100038(P.R. China) (Received August 3rd, 1992) (Revision received November 6th, 1992) (Accepted November 23rd, 1992)

Summary Illumination of latent fingerprints on white paper using 266~nm radiation from a Nd:YAG laser and photographic detection of their ultraviolet fluorescence, produces images with good ridge detail. The detection rate was 69% in a survey of fingerprints from 34 people compared with only 23% using an argon-ion laser at 514 nm. Prolonged exposure to UV light decreased the inherent UV fluorescence intensity but no adverse effects were observed on subsequent treatment with l$-diazafluoreng-one or ninhydrin. Key words: Latent fingerprints;

Fluorescence;

Laser; Ultraviolet photography

Introduction Attempts to detect fingermarks have traditionally concentrated on making them visible. The operational benefits in crime investigation are that if a mark can be seen, its quality can be assessed and it can be confidently recorded photographically. Approaches have varied from dusting with powders to treating the fingerprint residue chemically. The use of a laser as a powerful monochromatic light source [l] opened up new and exciting possibilities. Dalrymple et al. [l] showed that the natural luminescence properties of fingerprint residues can be used to form visible fluorescing images under laser illumination. (Following established practice we use the term ‘fluorescence’ rather loosely in this paper in circumstances where the mechanism of luminescence has not been established. Other mechanisms can produce light emission of short duration with a significant red-shift relative to the exciting illumination.)

Correspondace to: SK. Bramble, Metropolitan Road, London SE1 7LP, UK.

Police Forensic Science Laboratory,

0379-0738/93/$06.00

0 1993 Elsevier Scientific Publishers Printed and Published in Ireland

Ireland Ltd.

109 Lambeth

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Building on this early success, research subsequently concentrated on finding chemical pre-treatments to improve the visibility under laser illumination [2] because the natural luminescence of fingermarks is often much weaker than that of the substrate. Although pre-treatment methods have been extremely successful, direct stimulation of natural luminescence retains an important role in routine fingermark detection because it is simpler at the searching stage and, being relatively non-destructive, it leaves other options open. Despite the success of laser methods, few attempts to determine the source of the inherent luminescene have been reported. This is no doubt due, in part, to the complexity of the chemical make-up of sweat residues. However, solutions of extracts of fingerprint sweat residues have been reported to have absorption and corresponding emission peaks in the ultraviolet (UV) region of the spectrum [l - 41. Over 20 years ago, Ohki [3] looked at the possibility of using UV fluorescence photography as a method for latent fingerprint detection. His fluorescence results were inconclusive and he reported no attempt to photograph fluorescing marks. The work reported here explores the possibility of using the inherent UV luminescence of latent fingermarks as an alternative to visible luminescence, using white paper and white card as substrates. The use of fluorescence in the UV is compared with conventional processes currently used in crime casework based on detecting natural fluorescence from fingermarks in the visible region of the spectrum. Experimental

procedure

Two types of fingermark were deposited, labelled F and B. The former were obtained by pressing a finger (F) under moderate pressure against the surface of interest, while the latter required the finger first of all to be wiped across the forehead (brow, B). Two such fingermarks laid down together, type F and type B, were called a ‘set’. Type B are likely to be richer in sebaceous material. UV illumination was provided by the fourth harmonic of a Nd:YAG laser (Sectron Laser Systems SL400). A DCDA type 1 crystal frequency-doubled the laser fundamental to produce the second harmonic at 532 nm, which in turn was doubled by a KD*P type 1 crystal producing the fourth harmonic at 266 nm. The laser was typically run at 10 Hz with a pulse length of 10 ns, producing 3 mJ per pulse at 266 nm. Irradiation of the area of interest was achieved by expanding the laser beam with a quartz lens to produce a beam diameter of about 6 cm. The surface under observation was illuminated at an angle of 45’, an angle chosen for experimental convenience (Fig. 1). There was no obvious change in contrast when the angle of illumination was altered. Photographs were taken with a Nikon F3 35-mm single lens reflex camera using a 105mm UV Nikkor quartz lens with an extension tube. A quartz lens is better than a conventional glass lens, which typically has a transmission of only about 10% near 360 nm, the spectral region of interest in this work. A UV bandpass filter (B & W Blak Ray 403) with the properties shown in Fig. 2 was attached to the front of the lens. It has an experimentally important transmission

5 Substrate with Fingerprint on Surface, Quartz lens 266nm \ /

532nm \ /

I NdYAG

KD’P Crystal

Laser

Fig. 1. Diagram of the equipment used to capture UV fingermark

images.

maximum at 360 nm (70% T, 60 nm FWHM) and a second transmission maximum at 750 nm (22% T, 50 nm FWHM), which is beyond the spectral response of the film used. It also blocks scattered 266-nm laser light. Photographs were taken in the dark because the room light had enough intensity at 380 nm, the upper limit of the bandpass filter, to reduce the contrast. A photographic dark-room lamp would probably be acceptable. Trial and error showed that a typical exposure of 20-40 s at f5.6 was sufficient for good contrast. In addition to the normal laser safety procedures, care was taken to avoid exposing the eyes and skin to harmful ultraviolet radiation. Face visors and cotton

+Y 0

I

200

350

Wavelength Fig. 2. Spectral properties

I

I

500

650

I 800

(nm)

of the B&W Blak Ray 403 filter fitted to the camera lens.

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gloves were worn in addition to laser goggles. Care was also taken to ensure that clothing covered any remaining skin that could become exposed. Ilford FP4, 1.25 ASA, panchromatic film was used, which is particularly sensitive to wavelengths in the region 300-400 nm. It was developed using Agfa Studinol developer (1:X developer:water, ZO”C, 5 min) and Agfa Agefix fixer (1:3 fixer:water, 2O”C, 5 min). UV light generally produces an image of lower contrast than visible light so a development time slightly longer than normal was used. Substrate White paper was selected as a substrate which occurs often in crime investigation. However, the main point of this paper is to illustrate the potential benefits of using UV fluorescence to recover latent fingerprints and white paper was chosen mainly for convenience simply as a realistic substrate which is reasonably well defined. Six well-characterized white paper samples (Table 1) were examined as follows to assess the variation in the ease of finding fingermarks. Little variation was found except for writing paper 3, which repeatedly gave fingerprint images of much lower contrast than the other samples using natural fluorescence in the UV. This article reports results for the 90 pm, 72 g/m2 writing paper sample, which is simply called ‘writing paper’ and the 255 pm, 243 g/m2 white card sample, which is called ‘white card’. Comparison with inherent visible jluorescence Fingerprint ‘sets’ were prepared from 28 men and six women on each of two substrates: writing paper and white card. The donors were taken at random in the laboratory, including security officers and cleaners and the fingerprints were deposited as the donors were encountered - without washing. No systematic differences were observed between the men and the women, nor between the two substrates and this communication reports the results as being from a collection of 68 ‘sets’ of fingermarks, i.e. 136 marks in total. An argon-ion laser (Spectra Physics 20/45, 15 W, 514 nm) and a filtered xenon arc lamp (Rofin Polylight, 300 watt, waveband selectable in the range 320 - 530 nm) provided the excitation light sources for observing fluorescence in the visible

TABLE 1 CHARACTERISTICS Paper type Writing paper 1 Writing paper 2 Writing paper 3 Photocopy paper White card 1 White card 2

OF THE SUBSTRATES Thickness (pm) 80

90 82 100

USED Grammage (glm2)

60 72 57 78

254

247

255

243

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region of the spectrum, against which the UV fluorescence under 266-nm stimulation could be compared. Both of these light sources are in regular operational use at this laboratory for latent fingermark detection. The sets of fingermarks were exposed sequentially as shown in Table 2. Only one fingermark gave any visible UV fluorescent image with the Polilight at 340 nm (40~nm bandwidth filter), a success rate of little practical significance and for the sake of clarity this method is not discussed any further. Fluorescing marks were examined individually and they were graded according to the quality of the mark. Where visible fluorescence was being observed, marks were examined at a range within 20 cm using a magnifying glass and wearing laser goggles where necessary. Marks fluorescing in the UV under 266nm excitation were examined as photographic negatives. The marks were assessed in terms of their quality, with a mark considered to be of good quality if most of the ridge detail was clear and unsmudged. Effects of aging Two individuals deposited four ‘sets’ of fingerprints on each of two substrates, white paper and white card, which were then stored in a dark box under laboratory ambient conditions of about 23°C and 35% relative humidity. After periods of 1, 8, 15 and 33 days a previously unexposed ‘set’ was irradiated with 266-nm laser light and the resultant fluorescence was photographed. Effects on Wfluorescence of prolonged exposure to 266-nm radiation Two individuals deposited two ‘sets’ of fingerprints on each of two substrates, white paper and white card. One half of each print was covered and they were exposed to the UV laser light at 10 Hz for 20 min - about 12 000 laser shots. UV fluorescence photographs of all the prints were then taken with both halves exposed, each protected half being a control for assessing the effects on the other half. Effects on subsequent chemical treatment of prolonged exposure to 266-nm radiation A series of nine fingerprints was prepared in which one half of each print was

TABLE 2 CHARACTERISTICSOF THE LIGHT SOURCES AND CORRESPONDINGCAMERA BARRIERFILTERS Light

1. 2. 3. 4.

source

Ar+ Laser Polilight Polilight Nd:YAG laser

Peak m&mum

Bandwidth

Camera lens

(nm)

(nmi

barrier filter

514 530 340 266

<1 20 40 <1

550 570 400 See

nm long pass nm long pass nm long pass Fig. 2

exposed to 266~nm laser illumination for 20 min while the other half of the print was covered. Six of the samples were dipped into ninhydrin solution [5] for 10 s and then removed and allowed to dry. The dry samples were developed at 75°C and 75% relative humidity for 20 - 25 min. Three of the samples were dipped into 1,8-diazafluorene-g-one (DFO) solution [6], lifted directly out again and allowed to dry before repeating the procedure for a second time. Once dry the samples were developed at 75°C and 15% relative humidity for 10 - 15 min. The DFOtreated fingerprints were observed under 514 nm argon-ion laser illumination. Results Intense blue fluorescence was produced when the six samples of Table 1 were excited by 266-nm light and no contrast could be seen when the latent fingerprint and the paper were observed in the visible through W-absorbing goggles. However, five of the six surfaces gave excellent contrast on photographing the UV fluorescence around 360 nm. The writing paper 3 sample gave much lower contrast, which was not due to strong background fluorescence but rather to weak fingerprint fluorescence. This was not investigated further. An example of a fingerprint from the survey of 34 members of staff is shown in Fig. 3. No ridge detail was visible under normal room lighting but Fig. 3 shows that UV fluorescence photography of this sample gave a very clear fingerprint with excellent ridge detail. When fluorescence was occasionally observed also in the visible under 514-nm laser illumination, the detail was generally poor. To check that we were detecting fluorescence and not reflection a simple UV

a

b

Fig. 3. (a) A fingermark on writing paper photographed under room light; (b) the same mark photographed using its inherent luminescence under 266 nm illumination and camera filter as shown in Figs. 1 and 2.

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reflection photograph was taken of a sample using a 270-nm interference filter on the front of the camera. The same 45” laser-to-camera angle was used as in the fluorescence experiments. The image had poor contrast and it was a dark fingerprint on a white background, showing that the paper reflects 266~nm light far more than the print. Thus, any leaking of 266~nm light by the near-UV filter would have caused light from the fingerprint to be overwhelmed by light from the background. Since no systematic differences were observed between the writing paper and the white card substrates, the results have been combined in Table 3. In all cases the prints were invisible to the naked eye under normal room lighting. The green 530-nm Polylight and the 514-nm argon-ion laser sources gave similar success at around 30% for type B, mostly sebaceous, fingerprints. When ridge structure was observed with these light sources it was usually of low quality, however. The two light sources tended to enhance the same fingerprint samples. Photography of the inherent UV fluorescence produced a very high yield. A total of 91% of type B fingerprints fluoresced, of which 51% were of good quality. Figure 4 illustrates results from one particular comparison of three of these techniques. Although they are not easily seen in the photograph, the argon-ion laser illumination of the fingerprint does show a few discernable ridges when viewed to within 20 cm with a magnifying glass but the UV result is altogether more impressive. The success rate on this substrate with weaker, identifiable (not smudged) marks was substantially higher in the UV. The trial served its purpose in demonstrating the merits of using UV fluorescence relative to visible fluorescence but the assessments of quality in Table 3 need to be read with care because many of the fingerprints were laid down with too much force, producing fluorescing prints which were very smudged. The performance of an enhancement method can really only be judged from the results it provides with those marks which are in principle capable of showing identifiable ridge detail. Table 3 shows the number of fluorescing prints which were of good quality.

TABLE 3 A COMPARISON OF THE THREE MAIN LIGHT SOURCES CURRENTLY USED TO STIMULATETHEINHERENTFLUORESCENCEOFFINGERMARKS Light sourcea

Type of mark*

Number jluorescing

Number of good quality

Polilight 530 nm Ar+ Laser 514 nm Nd:YAG Laser 266 nm

F B F B F B

12 20 10 21 32 62

0 1 0 2 7 35

aWith detection through the corresponding camera filters given in Table 2. 2Sixty-eight fingermarks of type ‘F’ and 68 fingermarks of type ‘B’.

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a

b

Fig. 4. A single fingermark on writing paper examined using the three configurations 3 with excitation at (a) 530 nm, (b) 514 nm, (c) 266 nm.

shown in Table

Only a slight decrease in UV fluorescence intensity was detected as the latent fingerprint aged while the intensity from the substrate showed no change. This is a subjective assessment made by looking at successive photographs taken at the set time intervals under similar conditions. The definition of the ridges seemed to be less sharp from about 10 days onwards. A slight decrease in the UV fluorescence intensity occurred when latent prints were exposed to 266-nm laser light for as little as two minutes. The effect of twenty minutes’ exposure is more obvious as can be seen in Fig. 5, which shows

Fig. 5. A UV fluorescence photograph of a fingermark on writing paper after the left side had been exposed to 266 nm laser light for 20 min at 10 Hz.

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b

Fig. 6. Chemically treated fingermarks: (a) writing paper treated with DFO and photographed using visible fluorescence; (b) white card treated with ninhydrin and photographed under normal room lighting. The left hand side of each mark had previously been exposed to 266 nm laser light for 20 min at 10 Hz.

a UV fluorescence photograph from a latent fingerprint after the left half had been exposed to about 12 000 laser shots of 266~nm light. The marked decrease in fluorescence intensity of the fingerprint could not be compensated for by increasing the exposure time because the fingerprint was then overwhelmed by background fluorescence. A most important result from a crime investigation point of view was that prolonged exposure of the fingerprint residue to 266~nm laser light led to no adverse effect on subsequent treatment with ninhydrin and DFO. Figure 6 shows two fingerprints, one treated with ninhydrin and the other with DFO after the left half of each print had been exposed to 266-nm laser light for 20 min at 10 Hz. No difference was detected between the two halves, a result which was repeated for several other sets of fingerprints. All 136 of the survey prints were finally treated with DFO and 135 of them were developed successfully. Discussion Spectroscopic studies of sweat residues have typically been performed by extracting the sweat with aqueous and other solvents and characterising their properties in solution. From work of this kind, absorption and emission maxima have been reported in the UV [1,3,4]. Ohki extracted sweat residues from the hands and feet with organic solvents [3] and reported an absorption maximum at 277 nm. His fluorescence spectra were solvent dependent. More recently Johnson et al. [4] reported an absorption maximum at 276 nm for extracted

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fingerprint residues. Dalrymple et al. reported an absorption maximum of sweat extracts around 280 nm, but their paper dealt mainly with visible fluorescence and they made little comment on the UV region [l]. Both Ohki and Johnson et al. compared their results with absorption spectra from the aromatic amino acids tyrosine, tryptophan and phenylanaline, which have absorption peaks near 280 nm. Solutions of these aromatic amino acids, like solutions of extracted sweat residues, showed broad-band emission between 300 and 400 nm. Although Ohki opted for urocanic acid, a metabolic product of the amino acid histidine, as the source of the emission from extracts of sweat, Johnson et al. concluded that the aromatic amino acids tyrosine and tryptophan account for about 80% of the UV fluorescence from finger sweat extracts. Whatever the molecular origin of the fluorescence observed in solution, this is only an indication of the fluorescence which might occur in the more solid and possibly heterogeneous environment of a fingermark. Fluorescence spectra can be strongly matrix dependent. From a practical point of view, the biochemical origin of the fluorescence is not critically important. There was enough evidence of significant absorption near 280 nm with emission in the range 300 - 400 nm to provide a basis for exploring whether a useful method of fingerprint enhancement could be developed for substrates which are significant in crime investigation. The result has been very encouraging. Under excitation at 266 nm, a convenient wavelength with the technology available in this laboratory, the fingerprint residues gave strong enough UV fluorescence for images with good contrast to be obtained on five of the substrate samples. This excellent contrast is due to a strong inherent UV fluorescence from the fingerprint residue. It is mainly this high sweat residue fluorescence that gives the method an advantage over enhancement using the inherent visible fluorescence, which is rather weak. One school of thought has it that the use of natural visible fluorescence is rarely, if ever, successful unless the hands carry traces of fluorescing contaminants [6,7]. While this interpretation is controversial [2], the fact is that fingermark fluorescence in the visible region of the spectrum is often very weak. Although detection of UV fluorescence proved to be the best of the fluorescence methods tested for enhancing fingerprints without chemical pretreatment, DFO succeeded in enhancing almost all of the fingerprints used in this survey. This simply underlines the power of the DFO method. However, DFO protocol does have some drawbacks for use at crime scenes, such as the need for treatment at 75°C and as with other chemical methods it produces irreversible effects. This problem is inherent in the chemistry and any future progress will depend on further chemical innovation. An advantage of working with fluorescence in the UV is that the associated difficulty of having marks which are invisible to the naked eye is amenable to a technical solution within the scope of known technology using electronic methods of image capture and display [9]. Given the intensity of the blue fluorescence under 266-nm illumination, we might have expected fluorescence from optical brighteners in white paper to have obliterated any fingermark fluorescence in the region of 360 nm. It was

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something of a surprise to find otherwise. It is clear, therefore, that either the fluorescence of these agents falls off in the near UV (near 360 nm) or that the fluorescence of the fingermarks is sufficiently intense to produce good contrast. The decrease of fluorescence intensity with increasing exposure is probably due to photochemical degradation. Photodegradation is likely to be limited to the region of the chromophore, however. Amino acids lacking an aromatic side chain are less likely to be affected and they remain capable of reacting with ninhydrin and DFO. Aging of the prints up to about 1 month produced no serious effect upon the UV fluorescence intensity but some loss of clarity in the ridge detail was seen after about 10 days, which gave the appearance of being due to diffusion of the sweat residues across the paper. However, the prints used in this work were stored under laboratory ambient conditions and they were not exposed to any extremes in environmental conditions. These observations may have implications for sample storage if it turns out that control of the ambient conditions is able to slow whatever processes are responsible for reducing the image clarity. An important aspect of the use of ultraviolet light is that research is still needed before UV imaging is used in routine casework where biological stains such as blood or semen might be important. DNA can be damaged by exposure to ultraviolet radiation. Indeed, exposure of reagents to UV is part of this laboratory’s standard protocol for protecting polymerase chain reaction experiments against the inadvertent introduction of contaminating DNA. Proteins may be denatured by UV irradiation and fabric dyes and inks may also be affected. Conclusions UV fluorescence provides a sensitive and relatively nondestructive approach to fingerprint detection. It will not replace current chemical treatments but it does offer a valuable supplement to them. Detection of 69% of the latent fmgerprints on paper and card when excited with 266 nm laser light is impressive compared with a 23% detection rate using an argon-ion laser at 514 nm. In combination with electronic imaging, a valuable tool is available to detect many fingerprints which currently escape detection using visible fluorescence. Also, being a method which leaves the sample apparently undisturbed, UV fluorescence can be used covertly. More research is needed before the method can be used with confidence in circumstances where subsequent analyses could be affected, particularly DNA profiling since DNA is known to be susceptible to UV photodamage. References 1

2

B.E. Dalrymple, J.M. Duff and E.R. Menzel, Inherent fingerprint luminescence-detection by laser. J. Foren.sk Sti., 22 (1) (1977) 106- 115. Ski. Rev., 1 (1) E.R. Menzel, Pretreatment of latent prints for laser development. Forensic (1989) 43 - 66.

14 3 4

5 6

7 3 9

H. Ohki, Physicochemical studies on latent fingerprints: I. Ultraviolet absorption and fluorescence of human epidermal secretions. Rep. Natl. Res. Inst. Police Sci., 23 (1) (1970) 33-40. G.A. Johnson, C.S. Creaser and J.R. Sodeau, Fluorescence Spectroscopic and HPLC Studies of Intrinsic Fingerprint Residues, In A.M.C. Davies, and C.S. Creaser (eds.), Analytical Ap plications of Spectroscopy .z, Proceedings of an ZntemzationalConference on Spectroscopy Spectrum, Royal Society Chemistry, Cambridge, 1991, pp. 207-212. J.R. Morris and G.C. Goode, NFN an improved ninhydrin reagent for detection of latent fingerprints. Police Res. Bull., 24 (1974) 45 - 54. CA. Pounds, R. Grigg and T. Mongkolaussavaratana, The use of l&diazafluoren-g-one (DFO) for the fluorescent detection of latent fingerprints on paper. A preliminary evaluation. J. Forensic Sci., 35 (1) (1990) 169- 175. V.R. Salares, On the detection of fingerprints by laser excited luminescence. Forensic Sci. Znt., 14 (1979) 229-237. K.E. Creer, Some applications of an argon ion laser in forensic science. Forensic Sci. Znt., 20 (1982) 179 - 190. M.H. West, R.E. Barsley, J. Frair and F. Hall, Reflective ultraviolet imaging system (RUVIS) and the detection of trace evidence and wounds on human skin. J. Forensic Zokntz$cation,40 (5) (1990) 249 - 255.