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Visualization of Fingerprints in the Scanning Electron Microscope
J. Forens. Sci. Soc. (1975), 15, 281
Visualization of Fingerprints in the Scanning Electron Microscope G. E. GARNER* Electron Microscope Center, Washington State University, Pullman, Washington 99163 C . R. FONTAN and D. Mr. HOBSON** Department oJ'Police Science and Administration, Washington State Uniaersity, Pullman, Washington 99 163
The Scanni~zgElectron Microscope (SEM) was used to visualize latent fingerprints without processing them with developing powders or chemicals. Prints on two types of surfnces were examined: metal (aluminium-conductive) and glass (non-conductive). Prints on metal lend themselves particularly well to SEM analysis. No pretreatment of the print is required in order to obtain excellent photographs. A t times, prints on glass must be coated with a thin conductive layer of gold in order to minimize charging. Prints exposed to the elements (sun, rain, etc.) become coated with an unidentijed material that prevents the success of this method. Introduction When fingerprints are found in the course of a major crime scene investigation, often attempts are made to photograph latent fingerprints without first processing them with powder or chemical agents. These photographs are made to record minute fingerprint characteristics before they are destroyed or obscured by a processing method, and to record evidence in its original form. Often it is extremely difficult to arrange lighting conditions both to eliminate glare and to emphasize the ridge pattern of fingerprints in-the photograph. This report documents efforts to apply the scanning electron microscope ( S E M ) to the initial problem of photographing latent fingerprints before they are nrocessed. r -Owing to its exceptional depth of field, great resolution and high magnifications, the scannlng electron microscope has been applied to many other areas in forensic science. Plant material, hair, tool marks, and paint are just a few of the recent examples of the application of the S E M to these fields. (Devaney and Bradford, 1971; MacDonell and Pruden, 1971; Taylor, 1970; Williams, 1971). - - - - - - -
Scanning Electron Microscopy The S E M uses a narrow (20-100Angstroms diameter) focused beam of electrons (accelerated from 1000 to 50,000 volts) which is scanned over a sample surface in a square pattern similar to the picture-pattern generated in a television tube. When this beam impacts the sample surface, several things occur which can yield information about the surface. Electrons from the structure of the sample (secondary electrons) can be collected a t each point of the scanned pattern (raster) and the intensity of secondary electrons at each point can be amplified by a photomultiplier tube and subsequently displayed on a cathoderay tube whose phosphor surface is scanned in synchrony with the beam . . the sample surface. The result is a topographical picture of the sample scanning area being scanned. The advantage of this kind of visualization system is that high magnifications (up to 50,000 diameters) can be achieved in scanning a smaller area by reducing
the size of the raster. Also a depth of field approximately 300 times greater than with a light microscope can be obtained. Other informational modes are commonly used with the SEM. High energy electrons from the primary beam or probe which are reflected from the sample surface (called back-scattered electrons) can be collected, and are of particular value to fingerprint work because these electrons are less susceptible to charging artefacts that occur on non-conductive or poorly conductive surfaces where prints are often found. Usually samples are coated by evaporating gold or one of its alloys on to the sample surface in a vacuum. The gold coat makes the surface conductive and gives higher numbers of secondary electrons with which to form the picture.
Materials and Methods Two types of surfaces were used in this investigation: non-conductive (glass) and conductive (aluminium). Fingerprints (consistingmostly ofbody oil) were deposited on glass (coverslips, microscope slides, or window glass) which were then cemented to SEM stubs with silver conducting cement. They were examined in the SEM before and after ageing, weathering, and heating, with and without gold coating. Fingerprints placed on aluminium surfaces were photographed without gold coating. The fingerprints were treated or allowed to age as follows: heat, 260°C for 1 hour; time, 10 days in a closed chamber exposed to sunlight (21-40°C) and 10 days out-of-doors exposed to light rain. Photographs were made with a n ETEC R-1 Scanning Electron Microscope.
Results The results are presented in Figures 1-10. Figure 1 shows a fresh fingerprint on a glass coverslip (note the scar running through the print and "charging" artefact a t top of picture). Figure 2 the same as in Figure 1 was coated with gold (2OOAngstroms thick) and rephotographed. Notice how the exposure to high vacuum in the coating process changes the fingerprint appearance by drying. Figure 3 shows a fresh fingerprint on 3mm thick glass slide. Figure 4 shows a fingerprint exposed to sunlight for 10 days in a covered container. Figure 5 shows a print from the same finger as of Figure 4 subjected to 260°C for 1 hour. Figure 6 shows a fresh fingerprint on a glass coverslip. The circular stub and coverslip are distorted a t low magnification. Figure 7 shows the same print as in Figure 6 that has been allowed to weather for 10 days out-of-doors and was exposed to light rain. Note surface debris that obscures much of the print pattern. Figure 8 shows a fresh fingerprint on a n aluminium surface photographed using the standard secondary electron mode of the SEM. Figure 9 shows the same print as in Figure 8 using the back-scattered mode of the SEM. This technique increases contrast but suppresses other detail. Figure 10 shows a fingerprint on a n aluminium surface and exposed to sunlight for 10 days in a dust .free environment.
Figure 1.
Figure 2.
Fresh fingerprint on glass surface.
Same print as Figure 1 except gold coated to eliminate charging. (Note how debris has dried.)
3.
Untreated firigrrprint after 10 days in a dry, dust-licr rnvironment.
Figurr 4.
Firrgrrprint taken a t thr samr timr as Figure 3 rxposed to sunlight for 10 days in a clean, sealed containcr.
!:ig.urr
284
Figure 5.
Fingerprint takrn a t t h r salnr timr as Figurr 3 11ratc.d to 2fi0°C: Ibr 1 hour.
Figurc ti.
Fresh print on glass surface.
285
Figure 7.
Figure 8.
Same print as Figure 6 alter 10 days exposure to weathering, including light rain.
Fresh fingerprint on aluminium surface using standard secondary electron mode of SEM.
Figure 9.
Figure 10.
Same print as Figurc 8 except using back-scatter mode of SEM.
Fingerprint on aluminium surface exposed to sunlight in dust-free environment for 10 days.
Conclusions Under certain conditions fingerprints can be successfully recorded using the S E M where extreme difficulties are encountered using traditional methods. The photographs show that the S E M works well in photographing fingerprints both on conducting and non-conducting surfaces after ageing and heating. However, there are three general areas of difficulty which presently limit the usefulness of the method. Surface Charging O n non-conductive surfaces (e.g. glass), when the surface of the print has not been coated with gold, a surface often builds up such that the print cannot be properly photographed. However, prints on conductive surfaces lend themselves particularly well to S E M examination. Coating Material A coat of material is deposited on the print when the fingerprint is left exposed to the elements. I n the samples left exposed to the sun and to light rain for 10 days, the ridge pattern was clearly visible under light optics, but the S E M view was obscured, probably because electrons could not penetrate the overlying material. Sample Size The size of the sample placed in most S E M ' s is limited to an object usually less than lOcm (4") in any dimension. The lowest magnification permits an area approximately 2.5 x 2.5cm to be photographed.
Acknowledgments We are grateful to Dr. Arthur L. Cohen, Director of the Electron Microscope Center, for suggestions and critical reading of the manuscript. References DEVANEY, J. R., and BRADFORD, L. W., 1971, Application of Scanning Electron Microscopy to Forensic Science at Jet Propulsion Laboratory, 1969-1 970 ; SEMI 1971 (Part 11) Proc. of the Workshop on Forensic Appl.; IITRI, 0.Johari, ed.; Chicago, Ill., 561. T. E., and HAYES,T. L., 1972, The Scanning Electron Microscope, EVERHART, Scientifc American, Vol. 226, No. 1, 55. KIMOTO,S., and Russ, J. C., 1969, The Characteristics and Applications of the Scanning Electron Microscope, American Scientist, Vol. 57, No. 1, 112. MACDONNELL, J. L., and PRUDEN,L. H., 1971, Application of the Scanning Electron Microscope to the Examination of Firearms Markings; SEM/1971 (Part 11) Proc. of the Workshop on Forensic Applications; IITRI, 0. Johari, ed.; -Chicago, Ill., 569. TAYLOR, M. E., 1971. Forensic Ap@lications of Scanning Electron Microscopy ;SEMI 1971 (Part 11) Proc. of the Workshop on Forensic Applications; IITRI, 0. Johari, ed.; Chicago, Ill., 545. THORNTON, P., 1968, Scanning Electron Microscopy, Chapman & Hall, London, Chapter 10. WILLIAMS, R. L., 1971, A n Evaluation of the S E M with X-Ray Micro-AnaGyzer Accessory for Forensic Work; SEM/1971 (Part 11) Proc. of the Workshop on Forensic Applications ; I I T Research Institute, 0. Johari, ed. ; Chicago, Ill., 537.
Visualization of Fingerprints in the Scanning Electron Microscope G. E. GARNER* Electron Microscope Center, Washington State University, Pullman, Washington 99163 C . R. FONTAN and D. Mr. HOBSON** Department oJ'Police Science and Administration, Washington State Uniaersity, Pullman, Washington 99 163
The Scanni~zgElectron Microscope (SEM) was used to visualize latent fingerprints without processing them with developing powders or chemicals. Prints on two types of surfnces were examined: metal (aluminium-conductive) and glass (non-conductive). Prints on metal lend themselves particularly well to SEM analysis. No pretreatment of the print is required in order to obtain excellent photographs. A t times, prints on glass must be coated with a thin conductive layer of gold in order to minimize charging. Prints exposed to the elements (sun, rain, etc.) become coated with an unidentijed material that prevents the success of this method. Introduction When fingerprints are found in the course of a major crime scene investigation, often attempts are made to photograph latent fingerprints without first processing them with powder or chemical agents. These photographs are made to record minute fingerprint characteristics before they are destroyed or obscured by a processing method, and to record evidence in its original form. Often it is extremely difficult to arrange lighting conditions both to eliminate glare and to emphasize the ridge pattern of fingerprints in-the photograph. This report documents efforts to apply the scanning electron microscope ( S E M ) to the initial problem of photographing latent fingerprints before they are nrocessed. r -Owing to its exceptional depth of field, great resolution and high magnifications, the scannlng electron microscope has been applied to many other areas in forensic science. Plant material, hair, tool marks, and paint are just a few of the recent examples of the application of the S E M to these fields. (Devaney and Bradford, 1971; MacDonell and Pruden, 1971; Taylor, 1970; Williams, 1971). - - - - - - -
Scanning Electron Microscopy The S E M uses a narrow (20-100Angstroms diameter) focused beam of electrons (accelerated from 1000 to 50,000 volts) which is scanned over a sample surface in a square pattern similar to the picture-pattern generated in a television tube. When this beam impacts the sample surface, several things occur which can yield information about the surface. Electrons from the structure of the sample (secondary electrons) can be collected a t each point of the scanned pattern (raster) and the intensity of secondary electrons at each point can be amplified by a photomultiplier tube and subsequently displayed on a cathoderay tube whose phosphor surface is scanned in synchrony with the beam . . the sample surface. The result is a topographical picture of the sample scanning area being scanned. The advantage of this kind of visualization system is that high magnifications (up to 50,000 diameters) can be achieved in scanning a smaller area by reducing
the size of the raster. Also a depth of field approximately 300 times greater than with a light microscope can be obtained. Other informational modes are commonly used with the SEM. High energy electrons from the primary beam or probe which are reflected from the sample surface (called back-scattered electrons) can be collected, and are of particular value to fingerprint work because these electrons are less susceptible to charging artefacts that occur on non-conductive or poorly conductive surfaces where prints are often found. Usually samples are coated by evaporating gold or one of its alloys on to the sample surface in a vacuum. The gold coat makes the surface conductive and gives higher numbers of secondary electrons with which to form the picture.
Materials and Methods Two types of surfaces were used in this investigation: non-conductive (glass) and conductive (aluminium). Fingerprints (consistingmostly ofbody oil) were deposited on glass (coverslips, microscope slides, or window glass) which were then cemented to SEM stubs with silver conducting cement. They were examined in the SEM before and after ageing, weathering, and heating, with and without gold coating. Fingerprints placed on aluminium surfaces were photographed without gold coating. The fingerprints were treated or allowed to age as follows: heat, 260°C for 1 hour; time, 10 days in a closed chamber exposed to sunlight (21-40°C) and 10 days out-of-doors exposed to light rain. Photographs were made with a n ETEC R-1 Scanning Electron Microscope.
Results The results are presented in Figures 1-10. Figure 1 shows a fresh fingerprint on a glass coverslip (note the scar running through the print and "charging" artefact a t top of picture). Figure 2 the same as in Figure 1 was coated with gold (2OOAngstroms thick) and rephotographed. Notice how the exposure to high vacuum in the coating process changes the fingerprint appearance by drying. Figure 3 shows a fresh fingerprint on 3mm thick glass slide. Figure 4 shows a fingerprint exposed to sunlight for 10 days in a covered container. Figure 5 shows a print from the same finger as of Figure 4 subjected to 260°C for 1 hour. Figure 6 shows a fresh fingerprint on a glass coverslip. The circular stub and coverslip are distorted a t low magnification. Figure 7 shows the same print as in Figure 6 that has been allowed to weather for 10 days out-of-doors and was exposed to light rain. Note surface debris that obscures much of the print pattern. Figure 8 shows a fresh fingerprint on a n aluminium surface photographed using the standard secondary electron mode of the SEM. Figure 9 shows the same print as in Figure 8 using the back-scattered mode of the SEM. This technique increases contrast but suppresses other detail. Figure 10 shows a fingerprint on a n aluminium surface and exposed to sunlight for 10 days in a dust .free environment.
Figure 1.
Figure 2.
Fresh fingerprint on glass surface.
Same print as Figure 1 except gold coated to eliminate charging. (Note how debris has dried.)
3.
Untreated firigrrprint after 10 days in a dry, dust-licr rnvironment.
Figurr 4.
Firrgrrprint taken a t thr samr timr as Figure 3 rxposed to sunlight for 10 days in a clean, sealed containcr.
!:ig.urr
284
Figure 5.
Fingerprint takrn a t t h r salnr timr as Figurr 3 11ratc.d to 2fi0°C: Ibr 1 hour.
Figurc ti.
Fresh print on glass surface.
285
Figure 7.
Figure 8.
Same print as Figure 6 alter 10 days exposure to weathering, including light rain.
Fresh fingerprint on aluminium surface using standard secondary electron mode of SEM.
Figure 9.
Figure 10.
Same print as Figurc 8 except using back-scatter mode of SEM.
Fingerprint on aluminium surface exposed to sunlight in dust-free environment for 10 days.
Conclusions Under certain conditions fingerprints can be successfully recorded using the S E M where extreme difficulties are encountered using traditional methods. The photographs show that the S E M works well in photographing fingerprints both on conducting and non-conducting surfaces after ageing and heating. However, there are three general areas of difficulty which presently limit the usefulness of the method. Surface Charging O n non-conductive surfaces (e.g. glass), when the surface of the print has not been coated with gold, a surface often builds up such that the print cannot be properly photographed. However, prints on conductive surfaces lend themselves particularly well to S E M examination. Coating Material A coat of material is deposited on the print when the fingerprint is left exposed to the elements. I n the samples left exposed to the sun and to light rain for 10 days, the ridge pattern was clearly visible under light optics, but the S E M view was obscured, probably because electrons could not penetrate the overlying material. Sample Size The size of the sample placed in most S E M ' s is limited to an object usually less than lOcm (4") in any dimension. The lowest magnification permits an area approximately 2.5 x 2.5cm to be photographed.
Acknowledgments We are grateful to Dr. Arthur L. Cohen, Director of the Electron Microscope Center, for suggestions and critical reading of the manuscript. References DEVANEY, J. R., and BRADFORD, L. W., 1971, Application of Scanning Electron Microscopy to Forensic Science at Jet Propulsion Laboratory, 1969-1 970 ; SEMI 1971 (Part 11) Proc. of the Workshop on Forensic Appl.; IITRI, 0.Johari, ed.; Chicago, Ill., 561. T. E., and HAYES,T. L., 1972, The Scanning Electron Microscope, EVERHART, Scientifc American, Vol. 226, No. 1, 55. KIMOTO,S., and Russ, J. C., 1969, The Characteristics and Applications of the Scanning Electron Microscope, American Scientist, Vol. 57, No. 1, 112. MACDONNELL, J. L., and PRUDEN,L. H., 1971, Application of the Scanning Electron Microscope to the Examination of Firearms Markings; SEM/1971 (Part 11) Proc. of the Workshop on Forensic Applications; IITRI, 0. Johari, ed.; -Chicago, Ill., 569. TAYLOR, M. E., 1971. Forensic Ap@lications of Scanning Electron Microscopy ;SEMI 1971 (Part 11) Proc. of the Workshop on Forensic Applications; IITRI, 0. Johari, ed.; Chicago, Ill., 545. THORNTON, P., 1968, Scanning Electron Microscopy, Chapman & Hall, London, Chapter 10. WILLIAMS, R. L., 1971, A n Evaluation of the S E M with X-Ray Micro-AnaGyzer Accessory for Forensic Work; SEM/1971 (Part 11) Proc. of the Workshop on Forensic Applications ; I I T Research Institute, 0. Johari, ed. ; Chicago, Ill., 537.