220 Forensic dentistryThe technique requires an alternate light source that is capable of pro-
ducing the monochromatic beam. Most forensic light sources are capable
of emitting several frequencies of visible light by using different filters, but
they are limited in purity since generally each color band is 30 mm in width.
Fortunately, there are a large number of less expensive, less complicated, and
more portable light sources available that make photography at a remote
location easier. Several manufacturers now produce what are called personal
light sources that can be handheld, are reasonably priced, and are very
portable (Figure 11.18). These allow the user to individually use several differ-
ent frequencies of light, specific for each forensic application. The particular
wavelength one uses depends upon what evidence the forensic investigator is
seeking. There are optimal wavelengths for different applications; therefore,
the color (frequency) of the light and blocking filters will vary. Research and
investigation of pattern injuries on human skin has shown that peak fluo-
rescence of the epidermis occurs at 430 to 460 nm14,15 and is deep blue in
coloration. Most light energy striking the surface of the skin is reflected.^16
Of the rest, about 30% penetrates below the surface. Some of it gets scattered,
some is absorbed, and some is remitted as fluorescent light. When illumi-
nated with an alternate light source, the electrons of these excited molecules
return to their normal state by releasing energy in the form of light. The
light that is emitted during this transition is of a lower frequency and weaker
than the incident light. The phenomenon causes the tissue to appear to glow,
or fluoresce. The scientific explanation for this phenomenon was described
many years ago by Professor G.G. Stokes and is referred to as Stokes law or
the Stokes shift.^35 Since the fluorescent light is always less bright than the
incident light, one must observe the fluorescence of an object with the use of
Figure 11.18 personal handheld light source. (photo courtesy of rC Forensic.)