METHOD AND APPARATUS FOR DISTINGUISHING PHOTOLUMINESCENT AND REFLECTING SURFACES IN FORENSIC SCIENCE APPLICATIONS

Abstract: In forensic science it may be necessary to distinguish between two photoluminescent sources which emit radiation at similar, but different, wavelengths, or to distinguish between reflected light from surface regions which reflect light at different characteristic wavelengths. Examples of such sources or surfaces are inks on documents and treated latent fingerprints. The prior art technique for distinguishing such sources or regions involves the systematic use of a large number of narrow band pass interference filters. The present invention uses a single interference filter (17, 14) which is tilted to alter the wavelength of the peak radiation transmitted by the filter. If photoluminescent sources are to be distinguished, the filter may be placed between the sources and a monitor (10) of their emitted radiation, or between an illumination lamp (13) and the sources. An angle of tilt of the filter is selected so that radiation from at least one source is rejected while the intensity of radiation from a wanted source is optimised. If a reflecting surface is to be distinguished, the tiltable filter (14) is positioned between an illumination lamp (13) and the surfaces and an angle of tilt is found so that the illuminating light has a wavelength equal to the characteristic reflecting wavelength of an unwanted surface, so that, to an observer, the unwanted surface merges with the background.

Description: TITLE: "METHOD AND APPARATUS FOR DISTINGUISHING PHOTOLUMINESCENT AND REFLECTING SURFACES IN FORENSIC SCIENCE APPLICATIONS" TECHNICAL FIELD This invention concerns photoluminescence and light absorption techniques in forensic science. More particularly, it concerns a technique for distinguishing between emitted light of wavelengths which are close to each other, but which differ by several nanometres, and for distinguishing between surface regions which reflect light at respective wavelengths which are close to each other but which differ by several nanometres. Such a technique may be used to improve the detection and observation of latent fingerprints and in the analysis of in on documents, but it is not limited to such uses. This invention also concerns the selection of the most efficient wavelength from a non-monochromatic source for the illumination of an area of an object which contains a potentially photoluminescent material (or material which reflects light at a particular wavelength) . BACKGROUND The use of fluorescent and phosphorescent tags in biological evaluations is well known. The adoption of photoluminescence (this term includes fluorescence and phosphorescence) techniques by forensic scientists (for example, to detect latent fingerprints and to determine whether different inks have been used for two signatures) is a fairly recent development. The forensic photoluminescence techniques have significantly increased the ability of forensic scientists to obtain evidencial data. For example, the photoluminescent methods have an increased sensitivity which enables the detection and recordal of weak fingerprint latents that would have escaped detection by the conventional methods or that would be been so indistinct after treatment by the conventional methods that their analysis for identification purposes would have been rejected by the courts. One of the most successful methods of detecting latent fingerprints has involved treating the prints with ninhydrin and converting the Ruheman ' s purple compound so formed into a metal complex which fluoresces when illuminated by a narrow band of visible light having a wavelength in the range 450 nm to 550 nm. The zinc, cadmium and mercury complexes are particularly useful for this purpose. The illumination to induce the photoluminescence can be by irradiation of the treated prints with light from a suitable laser (for example, an argon ion laser which produces, inter alia, light having a wavelength of 488 n ) , an arc lamp (such as a xenon arc lamp) with an appropriate filter, or an incandescent lamp (for example, a quartz halogen lamp) with a suitable filter. This technique has been described by • M Stoilovic, H J Kobus, P A Margot and R N Warrener in their paper entitled "Improved enhancement of ninhydrin developed fingerprints by cadmium complexation using low temperature photoluminescence techniques", published in the Journal of Forensic Science, volume 31, pages 432 to 445, 1986. It has also been detailed in the earlier paper by H J Kobus, M Stoilovic and R N Warrener, entitled "A simple luminescent post-ninhydrin treatment for the improved visualisation of fingerprints in cases where ninhydrin alone gives poor results", published in Forensic Science International, volume 22, pages 161 to 170, 1983. Another useful photoluminescence technique involves the secondary staining, with a photoluminescent dye, of fingerprints that have been treated by Superglue (trade mark). When * a surface that bears a latent fingerprint is exposed to the vapour of Superglue, the Superglue forms a deposit (as a polymer) preferentially on the fingerprint. A photoluminescent stain can then be selectively absorbed by the preformed (and otherwise colourless) Superglue-treated print. The stained print is then readily observed when it is illuminated by light of a suitable wavelength. Those who have attempted to use the photoluminescent methods of fingerprint detection will appreciate that although it represents a dramatic increase in the effectiveness of fingerprint detection, it can only be successful when it is possible to distinguish between the photoluminescence associated with the treated latent fingerprints and any other photoluminescence derived from the background, or light of a similar wavelength that is associated with the colour of (or the printing on) the article which bears the fingerprints. Distinguishing between the wanted photoluminescence from the treated latent fingerprint and the unwanted background photoluminescence has been effected, in a limited x^ay, by placing band pass barrier interference filters between the photoluminescing article and the device used to record the fingerprint (for example, a video camera or a photographic camera) . Each barrier filter transmits a narrow band of light of a specific wavelength range and selection of an appropriate filter allows the small difference in wavelength between the photoluminescence from the fingerprint and other extraneous photoluminescence or light to be distinguished. Thus filter design is of paramount importance and is critical to the success of the detection technique. Unfortunately, such barrier filters are expensive to produce and cover only a specific wavelength range. A change in the photoluminescence wavelength of the treated fingerprint, or other radiant source, by only a few nanometres will require a new specific barrier filter. Those who have used the ninhydrin treatment of fingerprints followed by conversion to a metal complex will also be aware that if the light source used to illuminate the object bearing the treated fingerprint is an arc lamp or a quartz halogen lamp or other non-monochromatic source, a different filter is required to obtain the illuminating wavelength which causes the zinc, cadmium and mercury complexes to photoluminesce. The need for multiple filters for the illumination arrangement adds considerably to the cost of the analysing equipment. In another forensic technique, reflected light from a treated latent fingerprint is monitored. The object bearing the treated latent fingerprint is illuminated with monochromatic light and, because the treated fingerprint reflects only light of a characteristic wavelength and absorbs all other wavelengths, the pattern of the fingerprint becomes visible as a dark image against the illuminated background surface of the object. Unfortunately, many objects (such as bank deposit slips and cheques) carry a printed pattern and lettering which have their own characteristic wavelengths. Thus, when the object is illuminated with monochromatic light, both the latent fingerprint and the printed pattern become visible as dark regions against the illuminated background of the surface of the object. In this situation, the latent fingerprint pattern is often obscured by the pattern of the printed regions, the only way to clearly distinguish the fingerprint pattern is to illuminate the object with light which has a wavelength that is precisely the characteristic reflecting wavelength of the printed pattern. If this is done, the printed pattern and the background surface of the object reflect light of the same wavelength, and thus merge in the eye of an observer (and in a photographic film record of the object) , leaving the fingerprint pattern clearly visible. To select the illuminating wavelength of the object to be precisely the reflecting characteristic wavelength of the printed pattern, however, is not easy using a selection ,of monochromatic barrier interference filters - and is impossible if a filter of the required wavelength, is unavailable. DISCLOSURE OF THE INVENTION It is an object of one aspect of the present invention to provide an effective and. simple modification to the technique for distinguishing the radiation from a series of radiant sources which have wavelengths that are so close that their distinguishing would otherwise require (using the prior art techniques) multiple filter systems. It is an object of a second aspect of the present invention to provide a single filter mounting arrangement in the illumination arrangement for an object bearing a plurality of photoluminescent or reflecting regions which enables any one of a predetermined continuous range of monochromatic illuminating wavelengths to be selected. The first objective is achieved by mounting an interference filter between the photoluminescing object and the recording device in such a manner that the angle of incidence on to the filter of the light signal that is to be detected (that is, varying the angle of incidence on to the filter of the radiation being analysed) can be varied. The objective of the second aspect of the invention is achieved by the additional or alternative step, when the illumination of the photoluminescing or reflecting area is not from a monochromatic light source, of mounting an interference filter in the illumination path so that it may be tilted to alter the angle of incidence upon it of the illuminating light beam. The tilting of this filter alters the wavelength of the light transmitted past the filter assembly and falling on the potentially fluorescent object (or on the reflecting object). Under conventional practice, when a treated fingerprint or a document having ink on it is being examined, the barrier filter located in front of the recording device is mounted in the same way as a filter is attached to the lenses of a photographic camera. Thus the light emitted from a treated fingerprint and its environment strikes the planar barrier interference filter orthogonally to the plane of the filter, immediately before it enters the recording camera (or similar device). In this arrangement, the specificity of the filter remains constant and of measureable efficiency. But if the filter is tilted so that its plane is no longer orthogonal to the light path, its transmission wavelength is varied systematically as a function of the angular tilting of filter. Thus tilting the interference filter performs a "fine tuning" of its transmission wavelength. By adjusting the angle of tilt of the filter, the transmission of light from a photoluminescing, treated fingerprint (or signature, or other region of a surface) can be optimised while reducing the intensity of the unwanted photoluminescence or light. The modification of the apparatus used to observe and record the fingerprint (or other photoluminescing region) is also simple. The filter is mounted on a holder or support in a housing and means are provided to rotate the holder, thus enabling the filter to be tilted within the housing. Usually the means to rotate the holder will be provided with a scale which moves alongside a pointer or mark on the housing to indicate the angular tilt of the holder and hence of the filter. An identical filter mounting arrangement is used to modify the illuminating arrangement for a potentially photoluminescent object. Thus, according to a first aspect of the present invention, there is provided a method of distinguishing between light from a plurality of closely spaced or adjacent photoluminescent sources, each of the sources emitting radiation at a wavelength which is close to, but different from, the wavelength of the radiation emitted by the other source or sources, the method comprising the steps of a) interposing, between the sources and a monitor of the light therefrom, a planar interference filter which transmits orthogonally incident radiation in a predetermined wavelength range, this range either including the wavelengths of- the sources or having limits thereto which are close to and longer than the wavelengths of the sources ; and b) tilting the interference filter to alter the angle of incidence thereon of the light from the sources, to optimise the intensity of light received by the monitor from one or at least one source while simultaneously reducing the light received by the monitor from the other source or sources. According to a second aspect of the present invention, a method of providing a variable wavelength of illumination of an object for stimulating the emission of radiation from photoluminescent sources thereon or for illuminating a plurality of reflecting surfaces thereon comprises the steps of a) illuminating the object with light from a non-monochromatic illumination means; b) interposing a planar interference filter between the illumination means and the object; c) tilting the interference filter to vary the angle of incidence thereon of the light from the illumination means; and d) monitoring the light emitted from the sources during the tilting of the filter, to determine either (i) when the intensity of the light emitted from one or at least one of the sources is maximised while the intensity of the light emitted from the other, or at least one other, of the sources is reduced, or (ii) when an unwanted reflecting surface and the background surface of the object each reflect light of the same wavelength, so that the unwanted reflecting surface becomes indistinguishable from the background surface. If the object has photoluminescent sources on its surface, this variation may be instead of, or in addition to, the interposition of an interference filter between the sources and the monitor. In the special application of this invention to observing and/or recording a latent fingerprint on an object when the latent fingerprint is not inherently photoluminescent, the invention will include the preliminary step of treating the object with chemicals and/or chemical reagents that render the latent fingerprint photoluminescent. The present invention also encompasses a modified form of the apparatus used to distinguish between light from a plurality of simultaneously monitored photoluminescent sources, each of which emits radiation of a wavelength which is close to, but different from, the wavelength of radiation emitted by the other, or at least one other, source, the modification comprising the inclusion between the sources and the monitor of a mounting for an interference filter which permits the interference filter to be systematically tilted or rotated to alter the angle of incidence thereon of light from (or to) the sources. The present invention further encompasses a modification of the illuminating arrangement for an object carrying a plurality of photoluminescent regions, or a plurality of reflecting surfaces, when the illuminating means is not monochromatic, the modification comprising the inclusion between the illuminating means and the object of a mounting for an interference filter which permits the interference filter to be tilted to alter the wavelength band of the light that is transmitted therethrough. A description of embodiments of the present invention in its application to the detection of latent fingerprints will now be given, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of the equipment used to enhance the detection and recordal of a fingerprint. Figure 2 is a sectional view of a filter mount that has been used to rotatably support a filter in the equipment of Figure 1. Figure 3 is a perspective sketch of the filter mount of Figure 2. Figure 4 is a graph showing the variation of the central wavelength of an interference filter as the angle of incidence of the light transmitted through the filter is changed. Figure 5 is a series of photographs showing the observed image of a latent fingerprint at different angles of tilt of an interference filter. DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT The equipment illustrated in Figure 1 includes a camera 10 which receives radiation from an object 11 (for example, a bank deposit slip, a cheque or the 5 like) that is supported in a foam plastic tray 12. The object 11, believed to have a latent fingerprint on it, has been treated either with Superglue in the manner indicated above (or by the modified "fastglue fuming" technique, which is also known to those 10 skilled in this art) and subsequently stained with a fluorescent dye, or by the ninhydrin technique followed by conversion to a metal complex (also outlined above). The object 11 is irradiated with light from a "Unilite" (trade mark) xenon arc lamp 15 13. The irradiating light is passed through a first interference filter 14 mounted in a housing 30. The filter 14 transmits light having a wavelength which initiates photoluminescence of the treated fingerprint. A combination of lenses 15 and a mirror 20 16 are used to illuminate the object 11. (A light guide could replace the lenses and mirror.) Some of the light from the illuminated object 11 (including photoluminescence from the fingerprint) passes through a second interference filter 17 before 25 it enters the lens system of the camera 10 and is recorded on film. The camera ' 10 is preferably a through-the-lens type of camera so that the image of the object 11 can be observed by the operator of the equipment before the fingerprint is photographed. If necessary, the camera can be removed from its mount until the required image of object 11 is observed by the operator of the equipment. A television camera, coupled to a video display screen, has been used in place of the photographic camera 10. This alternative arrangement allows the operator of the equipment to observe the image of the fingerprint and to record the data in digital form for storage, transmission or data manipulation (such as image enhancement). Using the prior art techniques, the interference filter 17 in the filter holder 19 would be fixed with the plane of the filter at right angles to the axis 20 of the camera lens system. With the present invention, however, the interference filter 17 is supported in a filter mount of the type illustrated in Figures 2 and 3. The filter mount of Figures 2 and 3 consists of a generally tubular housing 21 adapted to be fitted on to the lens system of the camera 10. Within the housing 21 is an annular filter holder 22 on which a planar interference filter 23 is firmly positioned. The filter holder 22 is rotatabie about an axis in the plane of the filter using one (or both) of a pair of knobs 24 mounted on respective support axles 25 which extend through the wall of the housing 21 from diametrically opposite locations on the filter holder 22. A scale 26 is attached to each knob 24. Each scale 26 passes alongside a pointer or other mark on the housing 21, which provides an indication of the angular tilt of the interference filter 23. The filter 14 is also supported in a filter mount of the type shown in Figures 2 and 3. The filter 14 is rotatabie about an axis in the plane of the filter using knobs 34 to which respective angular scales 36 are attached to provide an indication of the angle of tilt of filter 14 relative to the propagation direction of the collimated beam from the lamp 13. Figure 4 shows the way in which the central wavelength transmitted by the filter 23 of Figure 2 changes as the angle of rotation of the filter in the housing 21 is varied. The graph of Figure 4 illustrates the characteristic of a 4-band interference filter (central transmission wavelength of 590 nm, and half band width of 37 nm) produced by Omega Optical, Inc. of Brattleboro, Vermont, U.S.A. Using the apparatus (which the inventors have termed the "TILTFILT" apparatus) illustrated in Figures 1, 2 and 3, the present inventors examined the ridge details of a number of latent fingerprints placed on coloured and patterned bank deposit slips. The deposit slips were treated first with ninhydrin and then converted to the zinc or cadmium complex using known techniques. They were then illuminated with a "Unilite" xenon arc lamp through an appropriate filter which was tilted to a first position to transmit light to stimulate the photoluminescence of the zinc complex and to a second position to transmit light to stimulate the photoluminescence of the cadmium complex. The initial illumination of the deposit slips revealed the coloured background pattern of the deposit slip and the location of the fingerprints, but only a little of the ridge detail of the fingerprints could be observed (due partly to the poor contrast between the photoluminescence from the weakly coloured prints and the light from the coloured pattern printed on the bank deposit slip and partly to " the inherent weakness of the photoluminescence) . The deposit slip was cooled to the temperature of liquid nitrogen (77 K) to improve the photoluminescence from the fingerprints, but this did not eliminate the coloured pattern. By tilting the filter used to observe the photoluminescence, however, it was possible to find a filter position which significantly reduced the light from the pattern printed on the deposit slip and enabled the fingerprints to be seen clearly (and to be photographed) . The series of photographs of Figure 5 show the image of part of a bank deposit slip on which there is a latent fingerprint. The first photograph of the series is the image of the ninhydrin treated, cadmium complexed, print under white light illumination (the region of the fingerprint has been marked by a line). Little ridge detail of the print is seen in this photograph. The other eight photographs are of the images of the fingerprint when the deposit slip was illuminated with light of a wavelength that caused the treated fingerprint and part of the printing on the deposit slip to photoluminesce at almost the same wavelength. The angle of rotation (tilt) of the interference filter is shown under each photograph. As will be apparent, the best ridge detail of the fingerprint is observed with the filter tilted at 30 and 35 , when the photoluminescence from the printing on the deposit slip is mainly, and almost ' totally, suppressed. In some cases, as a modification of this method, an object bearing a latent fingerprint was cooled to 77 K and the filter was tilted to find a position of fair definition of the fingerprints. The object was then allowed to warm up continuously to room temperature while the location of the prints was observed with a television camera and recorded on a video tape. In this way, a record of the fingerprints at different times (and thus at different temperatures) was made. Because the photoluminescing compounds have differing photoluminescent efficiencies at different temperatures, there was a temperature at which the ridge detail of the fingerprint was optimised relative to the background. Observation of the display of the video tape enabled the record at the optimum performance temperature to be easily selected. In another experiment, a ninhydrin treated latent fingerprint on a bank deposit slip was examined using monochromatic light. The monochromatic light was the light from a xenon arc lamp transmitted through a planar barrier interference filter positioned between the lamp and the deposit slip. The light reflected from the deposit slip was observed. The bank deposit slip appeared to the observer as a uniformly illunimated background surface, on which dark patterns of the printing on the slip and the fingerprint were visible. The printing pattern, however, obscured the details of the fingerprint. By tilting the interference filter, a position of the filter was found where the light transmitted through the filter had a wavelength which was precisely the reflected wavelength of the printing on the deposit slip. At this position of the filter, the printed pattern of the bank deposit slip merged with the background reflected light to show a uniformly illuminated surface with the fingerprint pattern appearing as a dark (light-absorbing) pattern. Photographs were taken of this dark fingerprint pattern. A change of only 5 nanometres in the peak transmitted wavelength of the filter was sufficient to re-introduce the dark pattern of the printing on the deposit slip and to obscure, again, the fingerprint details. Those familiar with this art will appreciate that when implementing the present invention, any suitable illuminating wavelength, illuminating light source, recording device and interference filter may be used, and that modifications to the technique and apparatus may be made without departing from the inventive concept.

Claims: CLAIMS 1. A method of distinguishing between light from a plurality of closely spaced or adjacent photoluminescent sources, each of the sources emitting radiation at a wavelength which is close to, but different from, the wavelength of the radiation emitted by the other source or sources, the method comprising the steps of a) interposing, between the sources and a monitor (10) of the light therefrom, a planar interference filter (17) which transmits orthogonally incident radiation in a predetermined wavelength range, said range either including the wavelengths of the sources or having limits thereto which are close to and longer than the wavelengths of the sources; and b) tilting the interference filter to alter the angle of incidence thereon of the light from the sources, to optimise the intensity of light received by the monitor from one or at least one source while simultaneously reducing the light received by the monitor from the other source or sources. 2. A method as defined in claim ' 1, in which the sources are illuminated by a non-monochromatic illumination means (13), including the additional steps of i) interposing a second planar interference filter (14) between the illumination means and the sources, ii) tilting the second interference filter to vary the angle of incidence thereon of the light from the illumination means while monitoring the radiation emitted by the sources to determine when the intensity of radiation emitted from one or at least one of the sources is maximised while the intensity of radiation emitted from the other, or at least one other, of the sources is reduced. 3. A method of providing a variable wavelength of illumination of an object (11) for stimulating the emission * of radiation from photoluminescent sources tHereon, or for illuminating a plurality of reflecting surfaces thereon, said method comprising the steps of a) illuminating the object with light from a non-monochromatic illumination means (13); b) interposing a planar interference filter (13) between the illumination means and the object; c) tilting the interference filter to vary the angle of incidence thereon of the light from the illumination means; and d) monitoring the light emitted from the sources during the tilting of the filter, to determine either (i) when the intensity of the light emitted from one or at least one of the sources is maximised while the intensity of the light emitted from the other, or at least one other, of the sources is reduced, or (ii) when an unwanted reflecting surface and the background surface of the object each reflect light of the same wavelength, so that the unwanted reflecting surface becomes indistinguishable from the background surface. 4. A method as defined in claim 1, claim 2 or claim 3, in which one of the sources or reflecting surfaces comprises a latent fingerprint which has been treated to render it fluorescent. 5. A method as defined in claim 1, claim 2 or claim 3, in which one of the sources or reflecting surfaces comprises ink on a document. 6. Apparatus for distinguishing between the light emitted by a plurality of simultaneously monitored photoluminescent sources, each source emitting radiation having a wavelength which is close to, but different from, the wavelength of the radiation emitted by the other, or at least one other, source, said apparatus comprising a) illumination means (13) for illuminating the sources to stimulate the emission of radiation therefrom; b) monitoring means (10) for monitoring the radiation emitted by the sources; and c) a planar interference filter (17) interposed between the sources and the monitoring means; characterised in that d) the interference filter is mounted for systematic tilting thereof to alter the angle of incidence thereon of radiation from the sources. Apparatus as.defined in claim 6, also including a second planar interference filter (14) mounted between the illumination means and the sources, said second planar interference filter being mounted for systematic tilting thereof to alter the angle of incidence thereon " of radiation from the illumination means. Apparatus for providing a variable wavelength of illumination for an object carrying a plurality of photoluminescent regions, or a plurality of reflecting surfaces, said apparatus comprising a) a non-monochromatic illumination means (13), adapted to illuminate the object; and b) a planar interference filter (14) interposed between the illumination means and the object; characterised in that c) the interference filter is mounted for systematic • tilting thereof to alter the angle of incidence thereon of light from the illumination means. 9. Apparatus as defined in claim 6, claim 7 or claim 8, wherein one of the sources or reflecting surfaces comprises a latent fingerprint which has been treated to render it fluorescent. 10. Apparatus as defined in claim 6, claim 7 or claim 8, wherein one of the sources or reflecting surfaces comprises ink on a document. 11. Apparatus as defined in any one of cla ' ims 6 to 9, including means (19, 26; 36) to indicate the angle of tilt of the or each interference filter. 12. A method of distinguishing between light from a plurality of closely spaced or adjacent photoluminescentsources, each source emitting radiation at a wavelength which is close to, but different from the radiation emitted by the other, or at least one other, of the sources, substantially as hereinbefore described with reference to Figures 1, 2 and 3 of the accompanying drawings. 13. Apparatus for use in distinguishing between light from a plurality of closely spaced or adjacent photoluminescent sources, each source emitting radiation at a wavelength which is close to, but different from the radiation emitted by the other, or at least one other, of the sources, substantially as hereinbefore described with reference to Figures 1, 2 and 3 of the accompanying drawings .

Inventors: STOILOVIC MILUTIN, WARRENER RONALD NORMAN, STOILOVIC, MILUTIN, WARRENER, RONALD, NORMAN
Applicants: UNIV AUSTRALIAN, THE AUSTRALIAN NATIONAL UNIVERSITY
Application year: 1987
Main classification: G01J9
IPC codes: A61B5, G01J3, G01N21, G02B5, G02B26, G06K7, G06T7