Latent Print Development - National Criminal Justice Reference ...

More documents

Recommendations

Info

C H A P T E R 7 Latent Print Development FIGURE 7–17 Composite image of a bloody fingerprint on copy paper. The left side was photographed at 415 nm. The right side was photographed under daylight- balanced photographic lighting. 7.10.6 Time- and Phase-Resolved Imaging As mentioned earlier, background fluorescence may be generated intentionally to better visualize a fingerprint that is faintly absorbing light but not fluorescing. This condition will increase contrast by brightening the background, making the darker fingerprint stand out. However, background fluorescence is more often a hindrance, competing with a fluorescing fingerprint for visualization. Time-resolved imaging has been advocated as one possible means to solve this problem. This technique takes advantage of the difference between the time of emission of the substrate and the fluorescing fingerprint (Menzel, 1999, p 126). Early devices utilized a light source with a gated, rotating wheel that “chops” the light to exploit these differences in emissions. The light shines on the fingerprint and substrate when an opening in the wheel is in front of the light. The light source is then effectively turned off when a blade in the wheel passes in front of the light source. Shortly thereafter, an opening in the wheel passes in front of a detector. The size of the openings in the wheel, and the speed with which it turns, will determine the length of time that the print is exposed and the delay between excitation and detection. If the background fluorescence decays faster than the fluorescence of the chemical on the latent fingerprint, the background can be eliminated by adjusting the delay time (Menzel, 2001, p 216; Campbell, 1993, pp 368–377). 7–32 Later designs proved more practical by using an electronic light chopper in conjunction with a gateable charge-coupled device (CCD) camera, each component controlled by a computer with the image displayed on a monitor (Menzel, 1999, p 126). Time-resolved imaging is still considered impractical for widespread application. Phase-resolved imaging stands to be the next technological advance and is currently used in other fields of spectroscopy (Menzel, 2001, p 216). 7.10.7 Conclusion Fluorescence examination is firmly grounded in everyday latent print detection and imaging techniques. The sensitivity of this technique warrants application on all forms of forensic evidence. Specific bandwidths of radiation are shone on untreated prints as well as prints treated with powders and chemical reagents. When viewed with the appropriate barrier filters, sensitivity via photoluminescence detection may be achieved down to nearly the single photon. Absorption at discrete wavelengths, absent fluorescence, is also a beneficial enhancement technique on substances such as the purple impressions from ninhydrin or dried blood, rendering them darker and easier to view and photograph. Because background fluorescence is the biggest hindrance to fluorescence examination, experimental concepts such as time- and phase-resolved imaging have been proposed to address this problem.
Table 7–3 Common reagents and their wavelengths of peak absorption and emission. DFO Reagent/Substance Absorption Maximum (nm) Emission Maximum (nm) 560 (Champod et al., 2004, pp 129–130) 1,2-Indanedione 515 5-MTN Ninhydrin 550 (Wallace-Kunkel et al., 2006, pp 4–13) 415–560* (Champod et al., 2004, pp 117–118) Ninhydrin/ZnCl 490 Ardrox 380 Basic Yellow 40 445 (Champod et al., 2004, pp 142–145, 228–229) 440 (Lee and Gaennslen, 2001, p 124) MBD 465 Basic Red 28 495 Rhodamine 6G 490–530 (Champod et al., 2004, pp 142–145, 228–229) 525 (Lee and Gaennslen, 2001, p 124) Crystal Violet 532** Acid Yellow 7 527 Acid Yellow 7 + Blood 445–480 Untreated Dried Blood * Does not fluoresce but appears dark. ** Weak fluorescence requiring laser illumination. 415* (Champod et al., 2004, p 168) Latent Print Development C H A P T E R 7 580 (Champod et al., 2004, pp 129–130) 540 (Champod et al., 2004, pp 120–124) 500 (Lee and Gaennslen, 2001, p 124) 495 (Champod et al., 2004, pp 142–145, 228–229 ) 490 (Lee and Gaennslen, 2001, p 124) 515 (Lee and Gaennslen, 2001, p 124) 585 (Champod et al., 2004, pp 142–145, 228–229; Lee and Gaennslen, 2001, p 124) 565 (Champod et al., 2004, pp 142–145, 228–229) 555 (Lee and Gaensslen, 2001, p 124) 550 (Sears et al., 2005, pp 741–763) 485–500 (Sears et al., 2005, pp 741–763) 7–33
Page 1: C H A P T E R C O N T E N T S 3 6 1
Page 4 and 5: C H A P T E R 7 Latent Print Develo

Latent Print Development - National Criminal Justice Reference ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?