Image Processing Reference
In-Depth Information
it cannot enter the eye. So even if our retinas were somehow sensitive to MWIR
light, it could not reach them. As discussed in the last chapter, our eyes cannot
see light with a wavelength longer than about 0.75 m(750 nm) because this light
lacks the requisite energy to cause chemical changes in the retina that lead to visual
perception.
The opacity and transparency of materials depend greatly on the waveband of
light used to image the materials. Often, the optical properties of a material in
the visible waveband are quite different in another waveband. This is particularly
true in the MWIR and LWIR wavebands. In the last chapter, we saw that near-IR
and SWIR light can penetrate through thin material that is opaque to visible light,
reflect off what is underneath, and come back through the thin material to a near-IR
imaging device. Thermal imaging can also see through certain thin materials that
block the transmission of visible light. Figure 2.9 shows an example of an opaque
material that transmits MWIR light; in this case, light emitted from the person in
the black plastic garbage bag. There is little absorption in the 3-5- mwaveband
in this black polyethylene, and the material is also quite thin, making it easier for
longer wavelengths of light to pass through it. The walls behind the person appear
dark, as they are colder than his skin, and therefore emit less MWIR light. The
visible image was made with reflected overhead lighting, making the walls appear
bright in the visible band.
Now let us consider the case of a material that is designed to be very transparent
to visible light, yet blocks MWIR and LWIR light. Window glass is designed to
be transparent to visible light, but its transparent properties do not carry over very
Figure2.9 A boy with his arm inside a black polyethylene garbage bag: left—VIS; right—
MWIR (3-5 mm). (CourtesyofFLIR)
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