Image Processing Reference
Figure1.25 CD jewel case with scratches: left-visible; right-near-UV.
optics—the surface finish tolerances are much looser than with visible light lenses
because they can be. For example, a sanded piece of metal that is a very diffuse
reflector of visible light can look like an almost perfect mirror to a thermal infrared
Ultraviolet imaging is useful for some types of forensic analysis, which is often
concerned with seeing invisible traces of a crime that may have been overlooked
by naked-eye inspection. For example, shortwave ultraviolet imaging has proven to
be quite useful in locating and recording latent fingerprints (fingerprints that have
not been processed with powder or fuming and that are not readily apparent to the
naked eye). Fingerprint analysis has long been a vital tool to law enforcement, since
fingerprints are often left behind at crime scenes and are unique to an individual.
There are two types of prints, oily and sweaty. Both are often very difficult to
see with the unaided eye, even with a magnifying glass, because the skin oils and
dried sweat tend to be transparent to visible light. These skin oils do not transmit
shortwave UV light readily, and the salt crystals in sweaty prints tend to scatter
shortwave UV light more than visible light. These two UV optical properties can
be exploited to detect both types of fingerprints on nonporous surfaces without
physical contact. This is sometimes crucial to preserve evidence, as dusting for
fingerprints can destroy or blur them out.
Figure 1.26 shows a magazine cover imaged in visible light as well as a
magnified view of an area of the magazine imaged in both the visible band and with
shortwave UV light. The red box superimposed on the magazine cover shows the
area with the sweaty fingerprints, which appear brighter than the paper background.
The salt crystals in the prints diffusely reflect the UV light into the camera, while
the paper absorbs the UV, giving the resulting image a great deal of contrast.
These images were taken with an electronic viewing scope and a UV light
source that illuminated the magazine with 254-nm light. The viewing scope is
an image intensifier consisting of a light-sensitive photocathode (a glass window
coated with a special electrically charged conductive coating) that converts incident
light into electrons, which then are amplified and converted back into visible light
by a phosphor screen. Note that the fingerprint in the near-ultraviolet image is
not fluorescing (emitting light at a visible wavelength in response to an ultraviolet
excitation); rather, the salt crystals are reflecting UV light back into the ultraviolet