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 μm) and extends out to about 5 μm in wavelength; and the longwave IR band (LWIR), which is generally defined as light in the 7-14-μm waveband. 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.