J Forensic Sci, March 2015, Vol. 60, No. 2 doi: 10.1111/1556-4029.12661 Available online at: onlinelibrary.wiley.com
TECHNICAL NOTE CRIMINALISTICS Sarah J. Fieldhouse,1 Ph.D.
An Investigation into the Effects of Force Applied During Deposition on Latent Fingermarks and Inked Fingerprints Using a Variable Force Fingerprint Sampler*
ABSTRACT: Physical factors, including the magnitude of the force applied during fingermark deposition, may affect friction ridge surface
area and clarity, and the quantity of residue transferred. Consistency between fingermarks may be required; for example, in research projects, yet differences between marks are likely to exist when physical factors are not controlled. Inked fingerprints and latent fingermarks were deposited at 1–10 N at 1 N increments using a variable force fingerprint sampler to control the force, angle of friction ridge and surface contact, and the duration of friction ridge and surface contact. Statistically significant differences existed between the length and width measurements of the inked prints (p ≤ 0.05), particularly at lower forces. Scanning electron microscopy and surface plot analysis demonstrated how differences in force applied during deposition affected ridge surface area, displacement of latent residue, and differences in the quantity of residue transferred. Consistency between inked prints was demonstrated at equivalent forces.
KEYWORDS: forensic science, fingerprints, latent fingermarks, inked fingerprints, Cyanoacrylate fuming, scanning electron microscopy
Physical factors, including the magnitude of the force applied, the angle of friction ridge and surface contact, and the duration of friction ridge and surface contact, are all known to affect the appearance of friction ridge skin marks (1). Differences in the magnitude of the force applied during deposition can affect ridge width (2,3), the quantity of friction ridge residue transfer (3), and the clarity of the mark (1), which may affect mark identification (2). Given the role of friction ridge skin in forensic investigations, a considerable amount of research has been dedicated to this area of study, where often many hundreds or thousands of marks are deposited in any single project. Owing to the effects of physical factors on fingermark deposition, the deposition method employed is sometimes described in published research, which has been achieved by asking participants to carry out routine tasks (4) or to apply a specific force during deposition, for example, a “moderate” amount (5). These approaches obviously have the potential to introduce subjective error. On some occasions, different forces have been requested (6), some of which have made attempts to introduce precise forces (6,7). This has not just occurred in the area of friction ridge skin; methods of standardizing footwear mark deposition have been reported (8).
1 Forensic and Crime Science Department, Staffordshire University, The Science Centre, Leek Road, Stoke on Trent, ST4 2DF, U.K. *Presented at the 2012 Fingerprint Society Annual Conference, April 13– 15, 2012, in Swanwick, Derbyshire, U.K. The variable force fingerprint sampler described in the paper was designed by Philip Morton of SciChem, John Whitehead of Lascells Ltd and Sarah Fieldhouse. Received 13 June 2013; and in revised form 15 Mar. 2014; accepted 17 Mar. 2014.
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Previous research has investigated different fingermark deposition methodologies and their effects on the resultant marks (1,9). These have included no control over deposition (i.e., participants were simply asked to deposit a mark onto a slide, without further instruction), the use of a top pan balance to control the force applied, and the use of a fingerprint sampler. Here, evidence was provided to support the existence of significant variations in deposition force, the duration of friction ridge and surface contact, and the angle of friction ridge and surface contact between consecutive depositions for methods involving no control and a top pan balance. Significantly reduced variations of these factors between marks and/or evidence of the effects of force distortion were found when a fingerprint sampler was used (9). It is important to note that some participants seem to have an increased ability to be able to control fingermark deposition than others. This might be because of their prior knowledge of fingermarks and the factors which affect their structure during deposition, or perhaps they regularly deposit fingermarks, for example, as part of research projects. However, if a person would like to control these physical factors associated with fingermark deposition, then fingerprint samplers may offer them a means of being able to achieve this. There may equally be instances where a person does not wish to control fingermark deposition because they desire a natural variation in fingermark deposition force applied within their sample of marks. The effects of different force applications on the appearance of latent marks developed using Ninhydrin have been reported (7), which suggests how such factors might influence the results of a research project. The aim of this research project was to investigate the effects of different forces applied during fingermark deposition on the © 2014 American Academy of Forensic Sciences
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appearance of the resultant marks using a variable force fingerprint sampler to control fingermark deposition.
Methods An Overview of the Variable Force Fingerprint Sampler An overview of the variable force fingerprint sampler and its component parts can be viewed in Figs 1a and 1b. The variable force fingerprint sampler was composed of two platforms, the upper fingerprint deposition platform and the lower platform. They were connected at one end by a pivot, which allowed vertical movement of these platforms. In between the two platforms, a spring was positioned. The distance between the two platforms was adjustable, which controlled the compression of the spring during fingermark deposition. The magnitude of the force applied was set by adjusting the compression of the spring. Substrates were placed onto the deposition platform, and the finger was rested directly above. Releasing the spring allowed contact between the finger and the substrate and therefore deposition of the mark. Spring Calibration One end of the spring was secured to a vernier scale where known masses between 100 g and 1 kg were applied to the opposite end. The extension of the spring was recorded, and the data were converted to force data (1 g = 101.97 N [2 dp]) and (a)
(b)
FIG. 1––(a) Overview of the variable force fingerprint sampler (side view). (b) Overview of the variable force fingerprint sampler (rear view).
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THE EFFECTS OF FORCE ON FINGERMARK DEPOSITION
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plotted onto a calibration graph to determine the position of the markers on the force scale. These distances were subsequently verified using a digital Newton meter. The Effects of Different Forces on the Appearance of Latent Fingermarks and Inked Fingerprints Five participants (recruited through opportunity sampling) were asked to deposit three consecutive latent fingermarks from their index fingers onto glass surfaces, followed by three consecutive inked fingerprints onto photocopier paper at approximate forces of between 1 and 10 N at 1 N increments using the variable force fingerprint sampler according to the manufacturer’s instructions (10). For latent fingermarks, participants were asked to refrain from washing their hands for 1 h prior to deposition, and to gently rub their hands together with distribute their skin surface residue. Before the finger was used to deposit latent fingermarks for a different force, it was cleaned using a laboratory tissue to remove any excess residue, and then the finger was wiped against an equivalently sized area of friction skin from the same palmar surface in an attempt to gain consistency between the compositions of the residue of the marks. All of the fingermarks from a single participant were deposited on the same deposition date. The latent fingermarks were then subjected to the following analyses and treatments: 1 Examined using scanning electron microscopy (SEM). 2 Developed using cyanoacrylate fuming in a Foster and Freeman MVC3000 cabinet, following an autocycle within 2 h of deposition (11). An auto cycle was used with approximately 2 g of liquid Cyanobloom (Foster and Freeman Ltd., Evesham, England.). 3 Surface plot analysis using a Foster and Freeman fingerprint digitiser (DCS121), which was used to provide a 3D representation of the fingermark (12). For the inked fingerprints, the fingers were rolled against disposable inked strips (CSI Equipment Ltd., Milton Keynes, England) to ensure that the maximum area of friction ridges available was covered. It was not possible to produce rolled fingerprints using the fingerprint sampler. After the inked depletion series was created (between the depositions of each inked fingerprint), the finger was removed, cleaned with soap and water, and re-inked in an attempt to gain consistency in the quantity of ink present on the fingers. All of the fingermarks from a single participant were deposited on the same deposition date. The duration of friction ridge and surface contact was 2 sec (approximately) for each deposition using the sampler. The inked fingerprints and latent fingermarks were visually examined for differences in surface area between prints deposited under the same force and then between prints deposited at different forces. The maximum length and width of each inked print were measured in mm; mean values were calculated and plotted onto line graphs (the dimensions were plotted against force) to aid analysis. The distribution of the data was investigated using Kolmogorov–Smirnov tests (13). Repeated measures analysis of variance (ANOVA) tests were then used to investigate the existence of statistically significant differences between the data sets for each participant. Homogeneity of variance was investigated using Mauchly’s test of sphericity (13). Alpha levels of 0.05 were used for each test. The effect size was calculated using guidelines proposed by Cohen (13). All statistical testing was carried out using SPSS version 16 (IBM SPSS, New York).
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Results and Discussion Figure 2 illustrates a set of 10 inked fingerprints deposited by the same finger between 1 and 10 N at 1 N increments. Variation in the area of friction ridge and surface contact according to the magnitude of the force applied during deposition was clear, which supports previously published research (1,3). With this sample of marks at depositions of a lower force, there appeared to be no distortion to the friction ridges on account of excessive force applied during deposition, although the intensity of the ridges changed with different force applications, and the ridge widths themselves increased with an increase in force applied. This has been noted in previous studies, where esthetic differences were also attributed to differences in the chemical composition of the marks (3). Contrast between the ridges and furrows generally increased in line with increasing force, which improved the visibility of the ridge detail, particularly from prints deposited at 1 and 2 N compared to those deposited under forces exceeding 2 N. This might suggest that the optimum force for depositing marks exceeds 2 N. It was important to note that these images were of inked fingerprints,
which were highly likely to behave in a different manner to latent fingermarks on account of their different compositions. When the latent marks were considered, it also appeared that marks deposited under smaller forces were more different to one another in terms of surface area than those deposited at higher forces, but also that the ridges themselves were affected by the changes in force applied. The analysis of the length and width measurements of the inked fingerprints can be seen in Figs 4 and 5. Generally, the length and width measurements increased as the magnitude of the force applied to the finger increased. This was expected because the friction ridge skin pad of the distal phalanx is elastic, and therefore, during deposition, the raised portion of the pad will contact the substrate first, and then as the force increases, the pad will compress, allowing peripheral friction ridges to contact the substrate. The results of the repeated measures ANOVA test provided evidence that statistically significant differences existed between the length and width measurements, as shown in Table 1. The eta-squared effect size reported very large effects, which supported the reliability of the data.
FIG. 2––10 inked fingerprints deposited by the same finger between 1 and 10 N at 1 N increments.
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The results of the Bonferroni post hoc tests suggested that the length measurements of fingerprints deposited under a force ≤4 N were statistically significantly different to one another (p ≤ 0.05), but any fingerprints deposited under a force of ≥5 N were not statistically significantly different to one another (p > 0.05). The width measurements of fingerprints deposited under a force ≤5 N were statistically significantly different to one another (p ≤ 0.05), but any fingerprints deposited under a force of ≥6 N were not statistically significantly different to one another (p > 0.05). The standard error of the mean was only 0.5 mm. Any differences might have been caused by slight differences in the ink quantity on the friction ridges, which was not controlled. As the number of consecutive depositions increased, it was logical to assume that the quantity of ink present on the friction ridges would decrease.
FIG. 3––Three inked fingerprints deposited consecutively at 5 N.
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Differences in the size and shape of the participating fingers, particularly the raised area of the pad, are likely to contribute to differences in the behavior of the friction ridges during compression and therefore the length and width measurements. It was interesting to note that the length measurements generally stopped increasing after 5 N, yet the width measurements continued to increase. The nonisotropic shape of the pad could perhaps aid surface contact across the width of the finger more easily compared to the length generally under increasing forces. Figure 6 shows latent fingermarks from the same finger deposited at 4 and 8 N and developed using cyanoacrylate fuming. Minor differences in the surface area were present, particularly at the top of the marks, but also the marks were affected by differences in force applied, because the residue appears to have been displaced at 8 N to the ridge peripheries (1,2). Efforts were made to standardize the quantity of friction ridge residue present, because it is known that the quantity of residue affects the contact area (3). This was achieved by rubbing of the hands together, and then once the mark was deposited, all remaining residue was removed from that area using a tissue. Soap and water were not used as it was anticipated that any cosmetic product might contaminate the residue. The clean skin was then replenished by rubbing it with a comparable area of the palmar surface. It is acknowledged that the use of a fingerprint sampler will not remove any requirement for a standard chemically consistent fingermark. Further research in this area may look to combine the use of a chemically controlled residue with the control of deposition factors, to build toward the construction of a standard fingermark for research purposes. The use of inked fingerprints as well as latent marks within this research project was also used to standardize the chemical composition of the print, thus removing this variable from the experiment as far as possible. The control of such a physical variable during deposition might suggest that features present within a mark can be attributed to differences in the chemical composition of the marks rather than differences in the force applied. It was possible that the 8 N mark simply contained an increased quantity of residue, but on account of the strategies employed to control residue quantity, this feature was attributed to force. This was apparent with other participants’ marks at higher deposition forces compared to those deposited at lower forces.
FIG. 4––Length measurements of the inked fingerprints at different force applications. TABLE 1––ANOVA test results for length and width measurements Fingermark Measurement
N
F
p (2DP)
Eta Squared
Length Width
15 15
72.485 99.622
0.00 0.00
0.838 (very large) 0.877 (very large)
N: number of samples; F: ANOVA test statistic; p: probability; Eta Squared: estimate of effect size.
FIG. 5––Width measurements of the inked fingerprints at different force applications.
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SEM Analysis The results of the SEM analysis suggested that the appearance of the fingermarks varied at different forces. An example of these results can be viewed in Fig. 7. As the force applied to the finger during fingermark deposition increased, the width of the ridges appeared to increase, as did the quantity of the latent residue, supporting previous research as discussed. The ridges appeared to be more complete in nature and more densely packed with residue. Such differences may affect how the mark responds to its environment over time and also how it reacts to any applied treatment (7). Differences in the appearance of friction ridges can affect the quantity and quality of ridge detail available for analysis, which might affect how the marks are assessed. Surface Plot Analysis The surface plot analysis images for three marks deposited at 1, 5, and 10 N can be seen in Fig. 8.
FIG. 6––Latent fingermarks from the same finger deposited at 4 and 8 N and developed using cyanoacrylate fuming.
The surface plots clearly demonstrate how the surface areas of fingermarks change with different forces. They also illustrate how the ridge profiles change with force applied; as the force applied increased the ridge, depth also appeared to increase, which was attributed to the fact that increased forces caused increased quantities of residue to transfer (3). When a latent fingermark is deposited, the droplets of latent residue are sandwiched between the friction ridges and the substrate, which then rupture to transfer the residue to the substrate (14). At light forces, it was hypothesized that the droplets were transferred independently of one another, whereas as the force increases, the droplets were compressed with the deformation of the finger pad, and therefore, they merged together into a continuous stream of residue. This is known to be affected by the chemical composition of the residue (14), although in this study, all of the latent fingermarks were deposited consecutively; therefore, although the composition was unknown, it should have been relatively consistent. Residues from the elevated ridge peripheries were able to contact the substrate at higher forces, providing more residue available for transfer, which built upon the ridge profiles, increasing their depth. It was anticipated that the shape of the finger would affect the degree to which the finger pad deformed, particularly toward the apex, and therefore, there would be differences between fingers. Consecutively deposited inked fingerprints demonstrated a high level of consistency in terms of surface area, ridge width, and structure. Examples of three inked fingerprints deposited consecutively at 5 N are shown in Fig. 3. The fading intensity of the marks was attributed to the reduction in the quantity of ink of the finger following previous depositions. Further research aims to investigate intra- and intervariations in latent fingermark structure at several single forces, which may have further implications for fingerprint research projects. For example, it might be that at a single force, there is considerable variation within donors’ marks on different dates, yet many research projects employ a group of donors who they routinely assume to deposit similar marks. On any one date, there is clearly a limited amount of latent marks that a participant may
FIG. 7––Scanning electron microscopy images of fingermarks deposited at 1, 5, and 10 N.
FIG. 8––Surface plot analysis of fingermarks deposited at 1, 5, and 10 N.
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produce, which highlights another issue with fingermark research in general. The range of forces studied was based on typical variations observed during previous research, where participants were asked to deposit fingermarks onto glass slides using a top pan balance (9). Also, these forces represented those which were achievable with the given spring and the device used. Future research may well seek to extend this range, although as these results have shown, significant effects have been found to exist within this range. Conclusion The aim of the research was met given that the variable force fingerprint sampler facilitated the deposition of latent fingermarks and inked fingerprints at different forces. A study of the marks and prints demonstrated how different forces applied during deposition affect their surface area, with statistically significant differences in the lengths of the prints up to a deposition force of 4 N and statistically significant differences in the widths up to 5 N. Also demonstrated using SEM and surface plot analysis were differences in the quantity of the residue deposited and the ridge profiles, which were seen to generally increase as the force applied to the finger during deposition also increased.
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THE EFFECTS OF FORCE ON FINGERMARK DEPOSITION
2. Ashbaugh DR. Quantitative-qualitative friction ridge analysis. An introduction to basic and advanced ridgeology. Boca Raton, FL: CRC Press, 1999. 3. Scruton B, Robins BW, Blott BH. The deposition of fingerprint films. J Phys D: Appl Phys 1975;8:714–23. 4. Given BW. Latent fingerprints on cartridges and expended cartridge cases. J Forensic Sci 1976;21:587–94. 5. Bohanan AM. Latent’s from pre-pubescent children versus latent’s from adults. Journal of Forensic Identification 1998;48:570–3. 6. Langenburg G. Deposition of bloody friction ridge impressions. Journal of Forensic Identification 2008;58:355–89. 7. Jasuja OP, Toofany MA, Singh G, Singh GS. Dynamics of latent fingerprints: the effect of physical factors on quality of Ninhydrin developed prints – a preliminary study. Sci Justice 2009;49(1):8–11. 8. Farrugia KJ, Riches P, Bandey H, Savage K, NicDaeid N. Controlling the variable of pressure in the production of test footwear impressions. Sci Justice 2012;52(3):168–76. 9. Fieldhouse S. A comparison of fingermark deposition methodologies. Fingerprint Whorld 2011;37:95–101. 10. Scientific and Chemical Supplies. Multi-force fingerprint sampler instructions. West Midlands, U.K.: SciChem, 2011. 11. Mason Vactron. MVC3000 Superglue fuming cabinet instruction manual & user guide. Evesham, U.K.: Mason Vactron, publication date unknown. 12. Media Cybernetics. Image-Pro Plus reference guide for WindowsTM. Bethesda, MD: Media Cybernetics Inc., 1993. 13. Kinnear PR, Gray CD. SPSS 15 made simple. Hove, U.K.: Psychology Press, 2008. 14. Thomas GL. The physics of fingerprints and their detection. J Phys D: Appl Phys 1978;11:722–31.
Acknowledgments I would like to thank Philip Morton of SciChem and John Whitehead of Lascells Ltd. for their collaborative efforts with the design and production of the variable force fingerprint sampler. References 1. Fieldhouse S. Consistency and reproducibility in fingermark deposition. Forensic Sci Int 2011;3:96–100.
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Additional information and reprint requests: Sarah J. Fieldhouse, Ph.D. Forensic and Crime Science Department Staffordshire University The Science Centre Leek Road Stoke on Trent ST4 2DF U.K. E-mail: [email protected]
TECHNICAL NOTE CRIMINALISTICS Sarah J. Fieldhouse,1 Ph.D.
An Investigation into the Effects of Force Applied During Deposition on Latent Fingermarks and Inked Fingerprints Using a Variable Force Fingerprint Sampler*
ABSTRACT: Physical factors, including the magnitude of the force applied during fingermark deposition, may affect friction ridge surface
area and clarity, and the quantity of residue transferred. Consistency between fingermarks may be required; for example, in research projects, yet differences between marks are likely to exist when physical factors are not controlled. Inked fingerprints and latent fingermarks were deposited at 1–10 N at 1 N increments using a variable force fingerprint sampler to control the force, angle of friction ridge and surface contact, and the duration of friction ridge and surface contact. Statistically significant differences existed between the length and width measurements of the inked prints (p ≤ 0.05), particularly at lower forces. Scanning electron microscopy and surface plot analysis demonstrated how differences in force applied during deposition affected ridge surface area, displacement of latent residue, and differences in the quantity of residue transferred. Consistency between inked prints was demonstrated at equivalent forces.
KEYWORDS: forensic science, fingerprints, latent fingermarks, inked fingerprints, Cyanoacrylate fuming, scanning electron microscopy
Physical factors, including the magnitude of the force applied, the angle of friction ridge and surface contact, and the duration of friction ridge and surface contact, are all known to affect the appearance of friction ridge skin marks (1). Differences in the magnitude of the force applied during deposition can affect ridge width (2,3), the quantity of friction ridge residue transfer (3), and the clarity of the mark (1), which may affect mark identification (2). Given the role of friction ridge skin in forensic investigations, a considerable amount of research has been dedicated to this area of study, where often many hundreds or thousands of marks are deposited in any single project. Owing to the effects of physical factors on fingermark deposition, the deposition method employed is sometimes described in published research, which has been achieved by asking participants to carry out routine tasks (4) or to apply a specific force during deposition, for example, a “moderate” amount (5). These approaches obviously have the potential to introduce subjective error. On some occasions, different forces have been requested (6), some of which have made attempts to introduce precise forces (6,7). This has not just occurred in the area of friction ridge skin; methods of standardizing footwear mark deposition have been reported (8).
1 Forensic and Crime Science Department, Staffordshire University, The Science Centre, Leek Road, Stoke on Trent, ST4 2DF, U.K. *Presented at the 2012 Fingerprint Society Annual Conference, April 13– 15, 2012, in Swanwick, Derbyshire, U.K. The variable force fingerprint sampler described in the paper was designed by Philip Morton of SciChem, John Whitehead of Lascells Ltd and Sarah Fieldhouse. Received 13 June 2013; and in revised form 15 Mar. 2014; accepted 17 Mar. 2014.
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Previous research has investigated different fingermark deposition methodologies and their effects on the resultant marks (1,9). These have included no control over deposition (i.e., participants were simply asked to deposit a mark onto a slide, without further instruction), the use of a top pan balance to control the force applied, and the use of a fingerprint sampler. Here, evidence was provided to support the existence of significant variations in deposition force, the duration of friction ridge and surface contact, and the angle of friction ridge and surface contact between consecutive depositions for methods involving no control and a top pan balance. Significantly reduced variations of these factors between marks and/or evidence of the effects of force distortion were found when a fingerprint sampler was used (9). It is important to note that some participants seem to have an increased ability to be able to control fingermark deposition than others. This might be because of their prior knowledge of fingermarks and the factors which affect their structure during deposition, or perhaps they regularly deposit fingermarks, for example, as part of research projects. However, if a person would like to control these physical factors associated with fingermark deposition, then fingerprint samplers may offer them a means of being able to achieve this. There may equally be instances where a person does not wish to control fingermark deposition because they desire a natural variation in fingermark deposition force applied within their sample of marks. The effects of different force applications on the appearance of latent marks developed using Ninhydrin have been reported (7), which suggests how such factors might influence the results of a research project. The aim of this research project was to investigate the effects of different forces applied during fingermark deposition on the © 2014 American Academy of Forensic Sciences
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appearance of the resultant marks using a variable force fingerprint sampler to control fingermark deposition.
Methods An Overview of the Variable Force Fingerprint Sampler An overview of the variable force fingerprint sampler and its component parts can be viewed in Figs 1a and 1b. The variable force fingerprint sampler was composed of two platforms, the upper fingerprint deposition platform and the lower platform. They were connected at one end by a pivot, which allowed vertical movement of these platforms. In between the two platforms, a spring was positioned. The distance between the two platforms was adjustable, which controlled the compression of the spring during fingermark deposition. The magnitude of the force applied was set by adjusting the compression of the spring. Substrates were placed onto the deposition platform, and the finger was rested directly above. Releasing the spring allowed contact between the finger and the substrate and therefore deposition of the mark. Spring Calibration One end of the spring was secured to a vernier scale where known masses between 100 g and 1 kg were applied to the opposite end. The extension of the spring was recorded, and the data were converted to force data (1 g = 101.97 N [2 dp]) and (a)
(b)
FIG. 1––(a) Overview of the variable force fingerprint sampler (side view). (b) Overview of the variable force fingerprint sampler (rear view).
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plotted onto a calibration graph to determine the position of the markers on the force scale. These distances were subsequently verified using a digital Newton meter. The Effects of Different Forces on the Appearance of Latent Fingermarks and Inked Fingerprints Five participants (recruited through opportunity sampling) were asked to deposit three consecutive latent fingermarks from their index fingers onto glass surfaces, followed by three consecutive inked fingerprints onto photocopier paper at approximate forces of between 1 and 10 N at 1 N increments using the variable force fingerprint sampler according to the manufacturer’s instructions (10). For latent fingermarks, participants were asked to refrain from washing their hands for 1 h prior to deposition, and to gently rub their hands together with distribute their skin surface residue. Before the finger was used to deposit latent fingermarks for a different force, it was cleaned using a laboratory tissue to remove any excess residue, and then the finger was wiped against an equivalently sized area of friction skin from the same palmar surface in an attempt to gain consistency between the compositions of the residue of the marks. All of the fingermarks from a single participant were deposited on the same deposition date. The latent fingermarks were then subjected to the following analyses and treatments: 1 Examined using scanning electron microscopy (SEM). 2 Developed using cyanoacrylate fuming in a Foster and Freeman MVC3000 cabinet, following an autocycle within 2 h of deposition (11). An auto cycle was used with approximately 2 g of liquid Cyanobloom (Foster and Freeman Ltd., Evesham, England.). 3 Surface plot analysis using a Foster and Freeman fingerprint digitiser (DCS121), which was used to provide a 3D representation of the fingermark (12). For the inked fingerprints, the fingers were rolled against disposable inked strips (CSI Equipment Ltd., Milton Keynes, England) to ensure that the maximum area of friction ridges available was covered. It was not possible to produce rolled fingerprints using the fingerprint sampler. After the inked depletion series was created (between the depositions of each inked fingerprint), the finger was removed, cleaned with soap and water, and re-inked in an attempt to gain consistency in the quantity of ink present on the fingers. All of the fingermarks from a single participant were deposited on the same deposition date. The duration of friction ridge and surface contact was 2 sec (approximately) for each deposition using the sampler. The inked fingerprints and latent fingermarks were visually examined for differences in surface area between prints deposited under the same force and then between prints deposited at different forces. The maximum length and width of each inked print were measured in mm; mean values were calculated and plotted onto line graphs (the dimensions were plotted against force) to aid analysis. The distribution of the data was investigated using Kolmogorov–Smirnov tests (13). Repeated measures analysis of variance (ANOVA) tests were then used to investigate the existence of statistically significant differences between the data sets for each participant. Homogeneity of variance was investigated using Mauchly’s test of sphericity (13). Alpha levels of 0.05 were used for each test. The effect size was calculated using guidelines proposed by Cohen (13). All statistical testing was carried out using SPSS version 16 (IBM SPSS, New York).
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Results and Discussion Figure 2 illustrates a set of 10 inked fingerprints deposited by the same finger between 1 and 10 N at 1 N increments. Variation in the area of friction ridge and surface contact according to the magnitude of the force applied during deposition was clear, which supports previously published research (1,3). With this sample of marks at depositions of a lower force, there appeared to be no distortion to the friction ridges on account of excessive force applied during deposition, although the intensity of the ridges changed with different force applications, and the ridge widths themselves increased with an increase in force applied. This has been noted in previous studies, where esthetic differences were also attributed to differences in the chemical composition of the marks (3). Contrast between the ridges and furrows generally increased in line with increasing force, which improved the visibility of the ridge detail, particularly from prints deposited at 1 and 2 N compared to those deposited under forces exceeding 2 N. This might suggest that the optimum force for depositing marks exceeds 2 N. It was important to note that these images were of inked fingerprints,
which were highly likely to behave in a different manner to latent fingermarks on account of their different compositions. When the latent marks were considered, it also appeared that marks deposited under smaller forces were more different to one another in terms of surface area than those deposited at higher forces, but also that the ridges themselves were affected by the changes in force applied. The analysis of the length and width measurements of the inked fingerprints can be seen in Figs 4 and 5. Generally, the length and width measurements increased as the magnitude of the force applied to the finger increased. This was expected because the friction ridge skin pad of the distal phalanx is elastic, and therefore, during deposition, the raised portion of the pad will contact the substrate first, and then as the force increases, the pad will compress, allowing peripheral friction ridges to contact the substrate. The results of the repeated measures ANOVA test provided evidence that statistically significant differences existed between the length and width measurements, as shown in Table 1. The eta-squared effect size reported very large effects, which supported the reliability of the data.
FIG. 2––10 inked fingerprints deposited by the same finger between 1 and 10 N at 1 N increments.
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The results of the Bonferroni post hoc tests suggested that the length measurements of fingerprints deposited under a force ≤4 N were statistically significantly different to one another (p ≤ 0.05), but any fingerprints deposited under a force of ≥5 N were not statistically significantly different to one another (p > 0.05). The width measurements of fingerprints deposited under a force ≤5 N were statistically significantly different to one another (p ≤ 0.05), but any fingerprints deposited under a force of ≥6 N were not statistically significantly different to one another (p > 0.05). The standard error of the mean was only 0.5 mm. Any differences might have been caused by slight differences in the ink quantity on the friction ridges, which was not controlled. As the number of consecutive depositions increased, it was logical to assume that the quantity of ink present on the friction ridges would decrease.
FIG. 3––Three inked fingerprints deposited consecutively at 5 N.
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Differences in the size and shape of the participating fingers, particularly the raised area of the pad, are likely to contribute to differences in the behavior of the friction ridges during compression and therefore the length and width measurements. It was interesting to note that the length measurements generally stopped increasing after 5 N, yet the width measurements continued to increase. The nonisotropic shape of the pad could perhaps aid surface contact across the width of the finger more easily compared to the length generally under increasing forces. Figure 6 shows latent fingermarks from the same finger deposited at 4 and 8 N and developed using cyanoacrylate fuming. Minor differences in the surface area were present, particularly at the top of the marks, but also the marks were affected by differences in force applied, because the residue appears to have been displaced at 8 N to the ridge peripheries (1,2). Efforts were made to standardize the quantity of friction ridge residue present, because it is known that the quantity of residue affects the contact area (3). This was achieved by rubbing of the hands together, and then once the mark was deposited, all remaining residue was removed from that area using a tissue. Soap and water were not used as it was anticipated that any cosmetic product might contaminate the residue. The clean skin was then replenished by rubbing it with a comparable area of the palmar surface. It is acknowledged that the use of a fingerprint sampler will not remove any requirement for a standard chemically consistent fingermark. Further research in this area may look to combine the use of a chemically controlled residue with the control of deposition factors, to build toward the construction of a standard fingermark for research purposes. The use of inked fingerprints as well as latent marks within this research project was also used to standardize the chemical composition of the print, thus removing this variable from the experiment as far as possible. The control of such a physical variable during deposition might suggest that features present within a mark can be attributed to differences in the chemical composition of the marks rather than differences in the force applied. It was possible that the 8 N mark simply contained an increased quantity of residue, but on account of the strategies employed to control residue quantity, this feature was attributed to force. This was apparent with other participants’ marks at higher deposition forces compared to those deposited at lower forces.
FIG. 4––Length measurements of the inked fingerprints at different force applications. TABLE 1––ANOVA test results for length and width measurements Fingermark Measurement
N
F
p (2DP)
Eta Squared
Length Width
15 15
72.485 99.622
0.00 0.00
0.838 (very large) 0.877 (very large)
N: number of samples; F: ANOVA test statistic; p: probability; Eta Squared: estimate of effect size.
FIG. 5––Width measurements of the inked fingerprints at different force applications.
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SEM Analysis The results of the SEM analysis suggested that the appearance of the fingermarks varied at different forces. An example of these results can be viewed in Fig. 7. As the force applied to the finger during fingermark deposition increased, the width of the ridges appeared to increase, as did the quantity of the latent residue, supporting previous research as discussed. The ridges appeared to be more complete in nature and more densely packed with residue. Such differences may affect how the mark responds to its environment over time and also how it reacts to any applied treatment (7). Differences in the appearance of friction ridges can affect the quantity and quality of ridge detail available for analysis, which might affect how the marks are assessed. Surface Plot Analysis The surface plot analysis images for three marks deposited at 1, 5, and 10 N can be seen in Fig. 8.
FIG. 6––Latent fingermarks from the same finger deposited at 4 and 8 N and developed using cyanoacrylate fuming.
The surface plots clearly demonstrate how the surface areas of fingermarks change with different forces. They also illustrate how the ridge profiles change with force applied; as the force applied increased the ridge, depth also appeared to increase, which was attributed to the fact that increased forces caused increased quantities of residue to transfer (3). When a latent fingermark is deposited, the droplets of latent residue are sandwiched between the friction ridges and the substrate, which then rupture to transfer the residue to the substrate (14). At light forces, it was hypothesized that the droplets were transferred independently of one another, whereas as the force increases, the droplets were compressed with the deformation of the finger pad, and therefore, they merged together into a continuous stream of residue. This is known to be affected by the chemical composition of the residue (14), although in this study, all of the latent fingermarks were deposited consecutively; therefore, although the composition was unknown, it should have been relatively consistent. Residues from the elevated ridge peripheries were able to contact the substrate at higher forces, providing more residue available for transfer, which built upon the ridge profiles, increasing their depth. It was anticipated that the shape of the finger would affect the degree to which the finger pad deformed, particularly toward the apex, and therefore, there would be differences between fingers. Consecutively deposited inked fingerprints demonstrated a high level of consistency in terms of surface area, ridge width, and structure. Examples of three inked fingerprints deposited consecutively at 5 N are shown in Fig. 3. The fading intensity of the marks was attributed to the reduction in the quantity of ink of the finger following previous depositions. Further research aims to investigate intra- and intervariations in latent fingermark structure at several single forces, which may have further implications for fingerprint research projects. For example, it might be that at a single force, there is considerable variation within donors’ marks on different dates, yet many research projects employ a group of donors who they routinely assume to deposit similar marks. On any one date, there is clearly a limited amount of latent marks that a participant may
FIG. 7––Scanning electron microscopy images of fingermarks deposited at 1, 5, and 10 N.
FIG. 8––Surface plot analysis of fingermarks deposited at 1, 5, and 10 N.
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produce, which highlights another issue with fingermark research in general. The range of forces studied was based on typical variations observed during previous research, where participants were asked to deposit fingermarks onto glass slides using a top pan balance (9). Also, these forces represented those which were achievable with the given spring and the device used. Future research may well seek to extend this range, although as these results have shown, significant effects have been found to exist within this range. Conclusion The aim of the research was met given that the variable force fingerprint sampler facilitated the deposition of latent fingermarks and inked fingerprints at different forces. A study of the marks and prints demonstrated how different forces applied during deposition affect their surface area, with statistically significant differences in the lengths of the prints up to a deposition force of 4 N and statistically significant differences in the widths up to 5 N. Also demonstrated using SEM and surface plot analysis were differences in the quantity of the residue deposited and the ridge profiles, which were seen to generally increase as the force applied to the finger during deposition also increased.
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THE EFFECTS OF FORCE ON FINGERMARK DEPOSITION
2. Ashbaugh DR. Quantitative-qualitative friction ridge analysis. An introduction to basic and advanced ridgeology. Boca Raton, FL: CRC Press, 1999. 3. Scruton B, Robins BW, Blott BH. The deposition of fingerprint films. J Phys D: Appl Phys 1975;8:714–23. 4. Given BW. Latent fingerprints on cartridges and expended cartridge cases. J Forensic Sci 1976;21:587–94. 5. Bohanan AM. Latent’s from pre-pubescent children versus latent’s from adults. Journal of Forensic Identification 1998;48:570–3. 6. Langenburg G. Deposition of bloody friction ridge impressions. Journal of Forensic Identification 2008;58:355–89. 7. Jasuja OP, Toofany MA, Singh G, Singh GS. Dynamics of latent fingerprints: the effect of physical factors on quality of Ninhydrin developed prints – a preliminary study. Sci Justice 2009;49(1):8–11. 8. Farrugia KJ, Riches P, Bandey H, Savage K, NicDaeid N. Controlling the variable of pressure in the production of test footwear impressions. Sci Justice 2012;52(3):168–76. 9. Fieldhouse S. A comparison of fingermark deposition methodologies. Fingerprint Whorld 2011;37:95–101. 10. Scientific and Chemical Supplies. Multi-force fingerprint sampler instructions. West Midlands, U.K.: SciChem, 2011. 11. Mason Vactron. MVC3000 Superglue fuming cabinet instruction manual & user guide. Evesham, U.K.: Mason Vactron, publication date unknown. 12. Media Cybernetics. Image-Pro Plus reference guide for WindowsTM. Bethesda, MD: Media Cybernetics Inc., 1993. 13. Kinnear PR, Gray CD. SPSS 15 made simple. Hove, U.K.: Psychology Press, 2008. 14. Thomas GL. The physics of fingerprints and their detection. J Phys D: Appl Phys 1978;11:722–31.
Acknowledgments I would like to thank Philip Morton of SciChem and John Whitehead of Lascells Ltd. for their collaborative efforts with the design and production of the variable force fingerprint sampler. References 1. Fieldhouse S. Consistency and reproducibility in fingermark deposition. Forensic Sci Int 2011;3:96–100.
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Additional information and reprint requests: Sarah J. Fieldhouse, Ph.D. Forensic and Crime Science Department Staffordshire University The Science Centre Leek Road Stoke on Trent ST4 2DF U.K. E-mail: [email protected]
