Image Processing Reference
In-Depth Information
color image. The wax tends to either absorb or scatter UV light (depending on the
illumination angle). The shoe impression changes the texture of the wax surface
and affects how light is scattered from it.
Paint is generally composed of a transparent organic resin or polymer with
pigment particles suspended in it. As we have seen earlier in this chapter, the
pigment particles may not scatter NIR or SWIR light sufficiently to make the paint
opaque. In the UV band, conventional paint is always opaque. The reflectivity of
the paint surface can vary greatly over time, especially for exterior paint exposed to
sunlight, and this property can be exploited to tell old paint from new. Figure 1.30
shows a stucco wall that has been recently repainted near ground level. The fresh
paint is relatively unoxidized, such that UV light is absorbed by the polymer
binder. The older paint that has been exposed to California sunshine is oxidized
and denatured. This inorganic surface layer tends to reflect UV light.
Near-ultraviolet imaging can be used to defeat camouflage because a number
of organic materials reflect visible light, but absorb near-UV light. This can make
visibly white objects appear dark against a white background that reflects near-
UV light. For instance, a polar bear appears white to the mammalian eye because
its fur is highly reflective to visible light. The bears are therefore camouflaged
when viewed against snow and ice. Many other Arctic animals have evolved this
“cryptic coloration”: the Arctic hare, the snowy owl, and the baby harp seal. Two
scientists studying herds of harp seals lying on sea ice found that they could not
easily count the entire herd using aerial photographs, since the white seal pups were
invisible in conventional photographs. These scientists found that photographs
taken with light in the near-UV band revealed the baby seal pups; the pups' fur,
being composed of organic molecules, absorbs light in the near-UV and makes
them appear dark against snow (snow is highly reflective in both the ultraviolet
and visible wavebands). This technique works on polar bears as well, as shown in
Fig. 1.31, a near-UV (350-380 nm) photograph of a female polar bear with three
cubs. The bears appear almost as dark as grizzly bears in the near-UV.
The scientists also found that the standard military-issue Arctic camouflage
issued to Canadian troops was rendered ineffective by their photographic
technique. Soldiers on Arctic duty wear snow-colored parkas and pants and
Figure1.30 Fresh and old paint on a stucco wall: left—VIS; right—near-UV (330-400 nm).
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