Image Processing Reference
In-Depth Information
Figure6.3Midwaveinfrared( = 3-5m)photo.(CourtesyofFLIR)
In the shortwave IR band, water absorbs very strongly. My water-filled skin
looks completely black in Fig. 6.4, an image taken in the 2-2.5 micrometer
band. The near-infrared band is too short in wavelength to convey any thermal
information about a face—the skin is far too cold. Instead, we see reflected infrared
light from hair and skin with no regard to visible color—light and dark skin appears
equally reflective, and hair color disappears. The blue shirt looks white—near-IR
light reflects off the cotton fibers without absorption by blue dye. The whites of
the eyes are not nearly as “white” as they appear in visible light. The net result is
a visible image gone wrong—I look like an old man with demon eyes! This image
was taken indoors under tungsten lights with an indium gallium arsenide infrared
Figure 6.5 is a near-infrared image taken with a silicon CCD camera that has
a black glass infrared pass filter in front of the lens. The platform is a ten-year-
old Nikon Coolpix camera with the internal blue glasshotmirrorremoved and
replaced with a clear glass piece of the same size so that the camera becomes
sensitive to both visible and infrared light. 1 I used a black glass filter in Fig. 6.6
that transmits light above 830 nm or 0.83 micrometers, making the image infrared
only. The resulting image is pretty close to a monochrome visible-light image, with
the exception that the blue shirt looks light grey.
The visible image in Fig. 6.7 returns to familiar territory, and the pigmentation in
the hair and eyes asserts its effect on our perception of life-like color and contrast.
The dye molecules in the shirt absorb the reds and greens in the room lights, letting
only the blue component reflect back to the silicon CCD digital camera.
Sunblock absorbs heavily in the near-ultraviolet waveband. My face is coated
with it in Fig. 6.8, all except for a few spots in the right ear and on the chest.
This image was taken with a Wratten 18A filter over a standard 35-mm camera
lens, with Kodak Tri-X black-and-white film, which has plenty of sensitivity in the
0.3-0.4mwaveband. If you are wondering why my face looks different in some
1 The hot mirror is designed to block very near-infrared light from reaching the sensor. This is
desirable in a color camera, where one wants the color response to closely match that of the human
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