Image Processing Reference
of strong absorption bands in a material can also lead to strong emission of light at
those same wavelengths when the material is heated. Many molecular compounds
have most or all of their absorption bands in the infrared region of the spectrum.
They are thus transparent to visible wavelengths of light and can only be seen in
For instance, methanol and other alcohols burn with a flame that is quite hard to
see during the day even when impurities are added to make the flames more visible.
But there are a number of overlapping absorption bands present in alcohols in
the infrared region of the spectrum, especially between 1600 and 1900 nm. These
absorption bands correspond to various modes of vibrations between the bonded
carbon and hydrogen atoms. Thus, burning alcohol glows very brightly in the near-
IR and SWIR wavebands, and this characteristic can be used to detect invisible
fires. Many motor sports involve alcohol-based fuels that are highly flammable
and can produce invisible fires. Racecar drivers have been killed after crashes
because no one could see that they were on fire, and small fires in refueling pits
have grown out of control because the flames were invisible. Firefighters equipped
with infrared imaging systems can see these fires and fight them more effectively.
Compare the two views of burning methanol from a leaking fuel hose in Fig. 1.42.
The visible image shows a dark patch where the alcohol has soaked into the metal
deck, but the SWIR image shows bright white patches of flame around the hose.
Near-UV imaging is also useful for the detection of certain flames, rocket
plumes, and hydrogen fires, all of which emit ultraviolet light. Methanol flames
have a UV signature in the 240-280-nm waveband, and this makes it possible to
detect methanol fires in broad daylight. Figure 1.43 shows a composite image of
a pair of small methanol fires taken with a special camera system that consists
of a visible-light camera mounted to a UV-sensitive camera with a solar-blind
filter (solar blind is 240-280 nm). The visible-light channel is necessary because
the solar-blind UV images alone would be nearly all black. The ozone layer
of the Earth's atmosphere, which absorbs shortwave UV, prevents sunlight from
reaching the ground with wavelengths shorter than about 300 nm. The filter passes
240-280-nm UV light emitted by the methanol fire but blocks longer-wavelength
sunlight very effectively.
Because the UV sensor sees virtually no sunlight at all, it can be operated at
an extremely high amplification factor, making it possible to pick up even weak
Figure1.42 Visible (left) and SWIR (right) images of a methanol fire. (Courtesy of FLIR)