Image Processing Reference
In-Depth Information
Chapter1
InfraredandUltraviolet:The
EdgesoftheRainbow
The near-infrared (near-IR) and near-ultraviolet (near-UV) wavebands flank
the visible spectrum of light. Their relationship to the visible waveband is shown in
Fig. 1.1, along with the short wave infrared (SWIR) waveband, described shortly.
The chemistry of our eyes bounds our color vision; near-IR photons do not have
enough energy to stimulate our eyes, and the lenses of our eyes block near-UV
light. Though we cannot see these “colors” of light, they are close cousins to the
ones we can see. The optical properties of glass and photographic film in the near-
IR and near-UV wavebands are similar enough to their properties in the visible
waveband that scientists have been able to image in the near-IR and near-UV
wavebands using special filters and films for over a century. These modified camera
systems reveal a surprising view of familiar objects and materials. The changed
appearance of the familiar or the revelation of things unseen is the essence of “alien
vision” (i.e., the imaging of the world in wavebands of light that human eyes cannot
see). Alien vision suggests extraterrestrial beings that see with invisible light, yet
there are familiar creatures around that see light that we cannot; for instance,
butterflies, birds, and honeybees are sensitive to near-UV light. Ultraviolet vision
is quite common in the animal kingdom, particularly among invertebrates. In fact,
there are many examples of markings and patterns on animals and plants that
appear to act as signals or cues to animals with near-UV vision. These markings
were unknown until the advent of ultraviolet imaging technology in the early part
of the20 th century.
Light in both the near-IR and the near-UV wavebands is easy to generate with a
prism: let a narrow shaft of sunlight enter a darkened room, place a glass prism on a
table so as to intersect the shaft of light, and place a white screen behind the prism.
The prism bends the light rays according to their color, with red bent the least and
violet bent the most. This wavelength-dependent refraction is a property of the
glass known as dispersion. The white beam of light becomes a rainbow projected
onto the screen, with the familiar colors arranged in order of increasing frequency
and decreasing wavelength: red, orange, yellow, green, blue and violet, as shown
in Fig. 1.2. As we move along the pattern, we pass through orange, then red, and
then the red becomes a deeper shade of red, like the color of dying embers in a fire.
Finally, the red light appears to fade out completely; we have reached the near-IR
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