Image Processing Reference
In-Depth Information
absence of infrared excitation modes in the electromagnetic structure of the
material. 9 These excitation modes manifest themselves as a strong response of
the electrons in the material to light of a particular frequency (or equivalently,
wavelength). As I discussed in the introduction, electrons move in response to
the electric field of the lightwave. If the electrons are constrained (bound to a
molecule, for example), then they respond strongly to lightwaves with a particular
frequency. For example, water molecules have a strong absorption band at a
1450-nm wavelength—light at this wavelength tends to be highly absorbed. This
light causes an excitation of the bond between hydrogen and oxygen atoms in a
water molecule. The excitation is akin to a rapid stretching and contracting of this
bond, which is called a stretch mode. Water is only weakly absorbent in the visible
waveband, especially to blue light. We can visualize the infrared absorption of
water with SWIR imaging technology. Figure 1.12 show a glass of water viewed
in both visible light and SWIR light in the 900-1680-nm waveband.
Figure1.12 Visible (left) and SWIR (right) images of water in a plastic cup. (Courtesy of
The water looks like strong coffee because near-IR and SWIR light is strongly
absorbed by liquid water, owing to the presence of molecular absorption bands
in water at various wavelengths, including 760, 970, 1190, and 1450 nm. The
infrared image of the glass of water shows that the light within the camera's
spectral sensitivity (900-1680 nm) is highly absorbed and thus would not be useful
for imaging underwater. The absorption of near-IR light in liquid water continues
into the visible waveband on the red end (as well as into longer-wavelength IR
wavebands). Going deeper underwater, red light is absorbed first, which makes
water look blue at depths beyond several meters.
Criminologists often use near-infrared imaging techniques to examine
documents for possible alterations. Figure 1.13 shows a series of images of a
9 Metals are highly reflective to light over an enormous range of wavelengths. The electrons in
metal are only weakly bound to individual atoms, and thus do not exhibit the same response to
light as electrons bound to individual atoms or molecules. The theory of reflectivity of metallic and
nonmetallic materials is beyond the scope of this topic. It is explained very well in the topic Optics
by E. Hecht (Addison-Wesley, 1998).
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