Image Processing Reference
In-Depth Information
Chapter 2
Thermal Imaging: We All Glow in
the Dark
As we progress past the near-infrared and SWIR wavebands along the wavelength
scale of the electromagnetic spectrum to longer wavelengths, we encounter the
midwave infrared band (MWIR), which starts at 3 micrometers (abbreviated
m) and extends out to about 5 min wavelength; and the longwave IR band
(LWIR), which is generally defined as light in the 7-14- mwaveband. 1 Figure 2.1
shows the relationships between the visible waveband and the infrared wavebands.
For the sake of simplicity, I will refer to the MWIR and LWIR bands as the
thermal infrared waveband, and the corresponding imaging technology as thermal
imaging. Though I am surely biased since I work in the industry, the thermal
infrared band is currently the most fascinating region of the spectrum for invisible-
light imaging because practically everything emits a substantial amount of light in
this waveband. This makes it possible to see in total visible-light darkness because
the scene is self-illuminated! The higher an object's temperature, the brighter it
appears to a thermal imaging camera. For example, in a typical thermal image,
a person standing in a room will appear to glow brightly against a darker, cooler
background, as shown in Fig. 2.2. The child (shown here in pseudocolor) emits
plenty of midwave IR light, while the TV emits almost none because the picture
tube does not heat up in operation significantly and the color phosphors do not emit
significant infrared light—they are designed for human eyes. Notice how the two
objects reverse their appearance in the visible band—the TV is now bright, while
the child is almost completely dark.
The presence or absence of visible light sources like incandescent light or
sunlight does not significantly change the appearance of the thermal image,
particularly in the LWIR band. Sunlight will heat up a scene, causing it to radiate
1 There is no strict definition of the ranges of wavelengths considered to be the MWIR and LWIR
wavebands. Most thermal imaging technology is designed for 3-5 and 7-14 moperation, since
water vapor in the atmosphere is highly absorbing in various wavebands including those around 2
mand 5-7 m. Longer IR wavelengths in the VLWIR (Very Long Wave Infrared) waveband are
used almost exclusively for astronomy applications by orbiting telescopes, because heavy absorption
by molecules in the atmosphere prevents ground-based observation. The extreme end of the infrared
waveband is around 100 m.
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