J. Forens. Sci. Soc. (1977), 17, 121
Recovery of Latent Fingerprints from Difficult Surfaces by an X-ray Method R. WINSTANLEY Home Ofice Forensic Science Laboratory, Washington Hall, Euxton, Nr. Chorley, Lancashire, England PR7 6 H 3 A procedure for recovery of latent jngerprints is described which consists of dusting with lead powder followed by recording b y use of X-radiation. Back-ground e$ects from the substrate are virtually eliminated. Although proper training and precautions are necessary due to the potential health hazards involved the simplicity of the method commends it for use in a number of circumstances. Introduction Many surfaces are not even considered by the fingerprint officer because conventional methods of latent fingerprint recovery at his disposal are known not to yield satisfactory results. These "difficult" surfaces include multicoloured documents, cloth, polythene, wax, cardboard, hardboard, varnished and untreated wood, rubber, pigskin and the skin of corpses. A technique for recovering latent fingerprints by dusting with lead powder and recording by X-ray methods was described some years ago by Graham (1973). It appeared to have considerable potential for difficult surfaces but practical restraints confined it to the laboratory and it was never widely applied to scenes of crime. Development of a simple film cassette and the availability of transportable industrial X-ray equipment has solved most of the practical difficulties in the original method. The method, generally known as X-ray fluorescence radiography (XFR), or X-ray back-scatter radiography, relies on the emission of low-energy X-rays from heavy metal particles when bombarded with suitable high-energy X-rays. These low-energy X-rays, and consequently the positions of the metal particles, are recorded on a photographic film in contact with the substrate. The film cassette used in this work is similar to that described by Lail and Youker (1975) but employs a simpler and cheaper mode of construction. I t works on dark-slide principles enabling X-ray film to be presented to the treated latent print when required for exposure. At all other times the cassette is sealed and operations may be conducted under normal lighting conditions.
Method -
..- ....
Conventional fingerprint powders (e.g., talc, aluminium, etc.) contain light elements only and are thus totally unsuitable for the application of XFR. The most suitable materials are heavy metals such as tungsten, platinum, gold or lead. Lead dust is relatively inexpensive and is available in a range of particle sizes (Goodfellow Metals Ltd., L10 per 5kg). The results described in this paper were obtained using lead dust; recommended particle sizes being from 10 microns (fine) to 50 microns (coarse). Dust was applied to the latent print via a 54 micron sieve (Endecott), excess being removed by tapping or by a fine air jet. Application by brushing was found to be unsatisfactory. A respirator was worn to prevent inhalation of lead
dust. Having obtained satisfactory adhesion, fixation was achieved by use of an aerosol hair lacquer, spraying from a distance of about twelve inches. Experience was soon gained in selecting a suitable particle size for a given substrate. In general, however, an excessively coarse dust did not adhere sufficiently strongly to prevent its removal and this led to the subsequent use of a finer grade.
Recording Treated prints were recorded by normal photography or by XFR. A lightly adhering film of lead dust, rather than maximum visual contrast, gave the best results with XFR. A further advantage of XFR was that it could be used in situations inaccessible to the conventional camera, the alignment of the X-ray beam being unimportant. A slow, fine-grain film (Ilford Line N4E, 50) and Ilford P Q Universal Developer were used in this work. Exposures were typically from 1 minute to 2 minutes at 200kV/8mA using a source to film distance of 12 to 18 inches. The Cassette and its Operation The XFR system used is shown in Figure 1 and diagramatically in Figure 2. The cassette has a dark-slide and a peripheral sealing strip to exclude light when it is clamped or strapped to the substrate. X-ray film is forced into contact with the substrate by pressurising the internal balloon, and retracted after exposure by suction. A lead mask limits the divergence of the X-ray beam thereby reducing scattered radiation. Unwanted low-energy X-rays are removed from the beam by a copper filter (7mm thick for 200kV operation). Secondary emission from the copper is absorbed by the aluminium back of the cassette. Finally emission from the aluminium together with some of the inevitable scattered radiation is absorbed by a piece of blank film attached to the back of the recording film., The filtered X-rays thus reaching the recording film pass through it without absorption and resultant fogging. Some of the X-rays are absorbed by the lead dust adhering to the latent print. I n this process secondary X-rays, having a lower energy than the primary beam, are emitted by the lead dust and absorbed by the recording film, giving rise to a latent image.
Figure 1.
Cassette used for XFR.
x-IUY
SOURCE
SiXLINC STRIP
BALLOON
BWL:r)( FIU!
Figure 2. Schematic arrangement for XFR.
X-ray Equipment The equipment used was a "Scanray DOA 200" industrial unit having a maximum output of 200kV/8mA. A smaller set designed for 140kV should, however, give very good results, would be lighter in weight, and would require less shielding than the 200kV set.
Results Paper and Documents Multi-coloured paper surfaces having either a matt or gloss finish presented no problems. XFR almost completely eliminated background, as shown in Figure 3 which compares a treated palm print on a magazine cover recorded by conventional photography (Figure 3 left) and by XFR (Figure 3 right). In the rare event of failure of the lead dusting technique on plain surfaces the lead could generally be removed and the surface subsequently treated with an alternasive reagent such as ninhydrin. Cloth Results were very variable depending on the nature and duration of contact. Cloth made from synthetic fibres generally gave superior results to that composed of natural fibres owing to the reduced tendency of the former to absorb perspiration. As expected, finely woven materials gave the best definition. The best results to date have been obtained from Crimplene fabric, persistence of latent prints in this instance being in excess of 24 hours. Polythene Latent prints have been recovered from polythene after storage for three weeks in a warm, dry atmosphere and after brief immersion in running water. The air jet was generally required for excess lead dust removal, shaking being inadequate. Figure 4 shows prints on a well-used multicoloured polythene
Figure 3.
Lead dusted palm print on a magazine cover recorded by (left) photography and (right) XFR.
Figure 4.
Lead dusted fingerprints on a polythene carrier bag recorded by (top) photography and (bottom) W R .
carrier bag; recorded by conventional photography (Figure 4 top) and by
XFR (Figure 4 bottom). Comparable results have been obtained from wax, cardboard, hardboard, 124
varnished and untreated wood, rubber, pigskin, glass, painted metal and P.V.C
.
Human skin Tests on living subjects using the lead dusting technique (but without X-ray recording!) gave encouraging results in that, under ideal conditions, prints could be developed after periods in excess of half an hour following contact. As anticipated skin in a heated or perspiring condition did not retain latent prints. I t is anticipated that latent prints may persist for some time on skin from dead subjects, though in circumstances where there has been violent physical contact, blurring and distortion of prints is likely to preclude the development of a satisfactory image. XFR could be of particular value in investigations where a body has been moved or handled after death.
Discussion Clearly priorities would have to be established in a particular case since on the one hand dusting for fingerprints could obliterate evidence sought by the biologist and on the other the act of sellotaping for fibres could destroy latent prints. Owing to the toxicity of lead dust the present technique is not advocated for use in enclosed spaces (e.g., private premises or vehicles) since adequate cleaning is unlikely to be achieved. No problems in this respect are likely for use out of doors or in situations where the substrate is disposable. Further investigations using tungsten dust as a less toxic alternative to lead are in progress and will be the subject of a future report. Conclusions The method described has been shown to give good latent print recoveries from a wide range of difficult surfaces. Although ideally a laboratory technique, scene of crime operation is feasible with the special film cassette described and a modern, transportable X-ray set. References GRAHAM, D., 1973, The use of X-ray Techniques in Forensic Investigations, ChurchillLivingstone. J., 1975, Hewlett-Packard Technical Bulletin. LAIL,H. and YOUKER,
Recovery of Latent Fingerprints from Difficult Surfaces by an X-ray Method R. WINSTANLEY Home Ofice Forensic Science Laboratory, Washington Hall, Euxton, Nr. Chorley, Lancashire, England PR7 6 H 3 A procedure for recovery of latent jngerprints is described which consists of dusting with lead powder followed by recording b y use of X-radiation. Back-ground e$ects from the substrate are virtually eliminated. Although proper training and precautions are necessary due to the potential health hazards involved the simplicity of the method commends it for use in a number of circumstances. Introduction Many surfaces are not even considered by the fingerprint officer because conventional methods of latent fingerprint recovery at his disposal are known not to yield satisfactory results. These "difficult" surfaces include multicoloured documents, cloth, polythene, wax, cardboard, hardboard, varnished and untreated wood, rubber, pigskin and the skin of corpses. A technique for recovering latent fingerprints by dusting with lead powder and recording by X-ray methods was described some years ago by Graham (1973). It appeared to have considerable potential for difficult surfaces but practical restraints confined it to the laboratory and it was never widely applied to scenes of crime. Development of a simple film cassette and the availability of transportable industrial X-ray equipment has solved most of the practical difficulties in the original method. The method, generally known as X-ray fluorescence radiography (XFR), or X-ray back-scatter radiography, relies on the emission of low-energy X-rays from heavy metal particles when bombarded with suitable high-energy X-rays. These low-energy X-rays, and consequently the positions of the metal particles, are recorded on a photographic film in contact with the substrate. The film cassette used in this work is similar to that described by Lail and Youker (1975) but employs a simpler and cheaper mode of construction. I t works on dark-slide principles enabling X-ray film to be presented to the treated latent print when required for exposure. At all other times the cassette is sealed and operations may be conducted under normal lighting conditions.
Method -
..- ....
Conventional fingerprint powders (e.g., talc, aluminium, etc.) contain light elements only and are thus totally unsuitable for the application of XFR. The most suitable materials are heavy metals such as tungsten, platinum, gold or lead. Lead dust is relatively inexpensive and is available in a range of particle sizes (Goodfellow Metals Ltd., L10 per 5kg). The results described in this paper were obtained using lead dust; recommended particle sizes being from 10 microns (fine) to 50 microns (coarse). Dust was applied to the latent print via a 54 micron sieve (Endecott), excess being removed by tapping or by a fine air jet. Application by brushing was found to be unsatisfactory. A respirator was worn to prevent inhalation of lead
dust. Having obtained satisfactory adhesion, fixation was achieved by use of an aerosol hair lacquer, spraying from a distance of about twelve inches. Experience was soon gained in selecting a suitable particle size for a given substrate. In general, however, an excessively coarse dust did not adhere sufficiently strongly to prevent its removal and this led to the subsequent use of a finer grade.
Recording Treated prints were recorded by normal photography or by XFR. A lightly adhering film of lead dust, rather than maximum visual contrast, gave the best results with XFR. A further advantage of XFR was that it could be used in situations inaccessible to the conventional camera, the alignment of the X-ray beam being unimportant. A slow, fine-grain film (Ilford Line N4E, 50) and Ilford P Q Universal Developer were used in this work. Exposures were typically from 1 minute to 2 minutes at 200kV/8mA using a source to film distance of 12 to 18 inches. The Cassette and its Operation The XFR system used is shown in Figure 1 and diagramatically in Figure 2. The cassette has a dark-slide and a peripheral sealing strip to exclude light when it is clamped or strapped to the substrate. X-ray film is forced into contact with the substrate by pressurising the internal balloon, and retracted after exposure by suction. A lead mask limits the divergence of the X-ray beam thereby reducing scattered radiation. Unwanted low-energy X-rays are removed from the beam by a copper filter (7mm thick for 200kV operation). Secondary emission from the copper is absorbed by the aluminium back of the cassette. Finally emission from the aluminium together with some of the inevitable scattered radiation is absorbed by a piece of blank film attached to the back of the recording film., The filtered X-rays thus reaching the recording film pass through it without absorption and resultant fogging. Some of the X-rays are absorbed by the lead dust adhering to the latent print. I n this process secondary X-rays, having a lower energy than the primary beam, are emitted by the lead dust and absorbed by the recording film, giving rise to a latent image.
Figure 1.
Cassette used for XFR.
x-IUY
SOURCE
SiXLINC STRIP
BALLOON
BWL:r)( FIU!
Figure 2. Schematic arrangement for XFR.
X-ray Equipment The equipment used was a "Scanray DOA 200" industrial unit having a maximum output of 200kV/8mA. A smaller set designed for 140kV should, however, give very good results, would be lighter in weight, and would require less shielding than the 200kV set.
Results Paper and Documents Multi-coloured paper surfaces having either a matt or gloss finish presented no problems. XFR almost completely eliminated background, as shown in Figure 3 which compares a treated palm print on a magazine cover recorded by conventional photography (Figure 3 left) and by XFR (Figure 3 right). In the rare event of failure of the lead dusting technique on plain surfaces the lead could generally be removed and the surface subsequently treated with an alternasive reagent such as ninhydrin. Cloth Results were very variable depending on the nature and duration of contact. Cloth made from synthetic fibres generally gave superior results to that composed of natural fibres owing to the reduced tendency of the former to absorb perspiration. As expected, finely woven materials gave the best definition. The best results to date have been obtained from Crimplene fabric, persistence of latent prints in this instance being in excess of 24 hours. Polythene Latent prints have been recovered from polythene after storage for three weeks in a warm, dry atmosphere and after brief immersion in running water. The air jet was generally required for excess lead dust removal, shaking being inadequate. Figure 4 shows prints on a well-used multicoloured polythene
Figure 3.
Lead dusted palm print on a magazine cover recorded by (left) photography and (right) XFR.
Figure 4.
Lead dusted fingerprints on a polythene carrier bag recorded by (top) photography and (bottom) W R .
carrier bag; recorded by conventional photography (Figure 4 top) and by
XFR (Figure 4 bottom). Comparable results have been obtained from wax, cardboard, hardboard, 124
varnished and untreated wood, rubber, pigskin, glass, painted metal and P.V.C
.
Human skin Tests on living subjects using the lead dusting technique (but without X-ray recording!) gave encouraging results in that, under ideal conditions, prints could be developed after periods in excess of half an hour following contact. As anticipated skin in a heated or perspiring condition did not retain latent prints. I t is anticipated that latent prints may persist for some time on skin from dead subjects, though in circumstances where there has been violent physical contact, blurring and distortion of prints is likely to preclude the development of a satisfactory image. XFR could be of particular value in investigations where a body has been moved or handled after death.
Discussion Clearly priorities would have to be established in a particular case since on the one hand dusting for fingerprints could obliterate evidence sought by the biologist and on the other the act of sellotaping for fibres could destroy latent prints. Owing to the toxicity of lead dust the present technique is not advocated for use in enclosed spaces (e.g., private premises or vehicles) since adequate cleaning is unlikely to be achieved. No problems in this respect are likely for use out of doors or in situations where the substrate is disposable. Further investigations using tungsten dust as a less toxic alternative to lead are in progress and will be the subject of a future report. Conclusions The method described has been shown to give good latent print recoveries from a wide range of difficult surfaces. Although ideally a laboratory technique, scene of crime operation is feasible with the special film cassette described and a modern, transportable X-ray set. References GRAHAM, D., 1973, The use of X-ray Techniques in Forensic Investigations, ChurchillLivingstone. J., 1975, Hewlett-Packard Technical Bulletin. LAIL,H. and YOUKER,
