Forensic Dentistry, Second Edition

206 Forensic dentistry

Molecules in tissue absorb the energy from light and release that energy
as a fluorescent glow. It only lasts as long as the light’s excitation energy is
applied, usually about 100 nanoseconds (10–9 seconds).^5 Fluorescence of skin
occurs at a very low level of energy and usually cannot be seen by the naked
eye. Filters are generally required. Fluorescent inks and paint can be easily
seen under “black light” (near UV), as well as latent blood and fingerprints,
when treated with chemicals that facilitate viewing the level of excitation.
When light energy of various wavelengths strikes human skin, all four
of the previously mentioned events can occur simultaneously. Depending on
the wavelength of the source of the incident light and the configuration of
the camera, lenses, and filters, it is possible to record, individually, any of the
four reactions of skin to light energy (Figure 11.1).
Ultraviolet light only penetrates a few microns into skin, whereas infrared
light can penetrate skin to a depth of up to 3 mm.^3 What is usually seen when
visible light strikes the skin is reflected light energy. What isn’t seen, however,
is the light energy that is absorbed by the skin. By varying the wavelength of
incident light used for illumination and setting up the appropriate configu-
ration of the camera, lens, filters, and film, it is possible to photograph any
of the four events that occur. This ability creates an opportunity for interest-
ing pictures, especially when looking at bruises and other injuries to skin.
Sharp surface details can be seen with ultraviolet light, while images well
below the surface of the skin can be seen using infrared light. Images created
using reflected visible light and fluorescence allow other potentially different
appearances of the patterned injuries to be captured. The techniques and
photographic protocols for documenting injuries to human skin in visible
and nonvisible light using film and digital imaging are vastly different.

Epidermis

UltravioletVisible

λnm 200 300 400 500 600 700 800 900

Infrared

Dermis

Subcutaneous

Figure 11.1 diagram of levels of light penetration into skin when illuminated
by varying wavelengths.