Image Processing Reference
In-Depth Information
Can conventional infrared film detect hot objects in total darkness? Yes, but
only if the object is hot enough to emit near-IR light, which requires a temperature
of hundreds of degrees centigrade. As an object is heated to increasingly higher
temperatures, it glows with light of increasing photon energy and intensity. The
intensity of the light emitted by an incandescent object can be a very strong
function of wavelength. The glowing coal on the end of a cigarette is at a
temperature of about800 C(1472 F) when it is just sitting idle, as in Fig. 1.46.
There is approximately 10,000 times more light emitted by that coal in the
900-1700 nm band than there is in the visible band!
When an electric stove burner is first turned on, the heating element appears
black at first. Then it begins to glow with a dull red color, gradually becoming
reddish-orange. If we could keep heating the element, it would glow with yellow
light and then finally appear white-hot to our eyes (and then the kitchen would burn
up). Reversing this process, the element returns to a dull red glow that gradually
disappears as the burner cools. The burner would still be emitting near-IR light.
Figure 1.47 contains two images of a pair of stove burners taken simultaneously,
one with a visible-light camera, the other with a SWIR camera sensitive in the
950-1700-nm waveband.
The visible-light image on the left suggests that both gas burners are cold, but
clearly the back burner is hot enough to glow with SWIR light, as shown in the
SWIR image on the right. Also, the glowing burner is reflected in the stainless-
steel splash panel behind it. That splash panel has a brushed finish, and is not very
reflective to visible light, since the brush marks and ridges on the surface are larger
than a wavelength of visible light. However, the surface is a much better reflector
of the longer wavelengths of SWIR light emitted by the burner. As the burner
cools further, the SWIR glow disappears. The light emitted by the burner becomes
increasingly longer in wavelength as it cools. A different imaging system is needed
to see the emitted mid-wave and long-wave infrared light. These wavebands of
Figure1.46 Shortwave IR image of a lit cigarette illuminating a bathroom. (Courtesy of
FLIR)
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