Image Processing Reference
In-Depth Information
Figure1.8 Night-vision goggles with head mount.
starlight) by amplifying existing light, thus eliminating the need for an infrared
searchlight. These devices are known as image intensifiers, and they image with
visible as well as near-IR light out to a wavelength of about 900 nm. This extended
spectral sensitivity adds to the overall brightness of the scene due to the presence
of near-IR night glow, and allows for the use of covert signaling devices used
primarily in law enforcement applications. For example, there are commercially
available miniature infrared beacons that can be attached to the roof of a vehicle
and then tracked from the air by law enforcement with night-vision devices.
It is interesting to look at a person in total darkness with a near-IR viewer and a
purely near-infrared light source. Human skin reflects equally well no matter what
the skin color, so that dark-skinned people appear the same color as light-skinned
people. Eye pupils dilate completely, giving them a glowing-eye appearance, due to
the human retina's high reflectivity in the near-IR and red regions of the spectrum.
This reflectivity is the reason why flash photographs often show the subject's eyes
glowing with a dull red color. The subject's pupils opened to admit more light into
the eye, enabling better vision in the low-light situation that required the use of the
flash in the first place. Some cameras with built-in flash units attempt to prevent
this “red-eye” effect by emitting a series of flashes right before the picture is
taken, which causes the subject's pupils to close, eliminating or reducing the retinal
reflection. Figure 1.9 shows a near-IR image of the author with dark-adapted eyes
taken with a silicon CCD camera. Illumination was provided by a light-emitting
diode source that emits IR light at wavelengths around 850 nm. This light is totally
invisible to the human eye, so my pupils were fully dilated.
Near-IR imaging in aerial surveillance has had a long and interesting history.
For example, during World War II, a great deal of effort was made to conceal
strategic targets from enemy bombers and observation planes. Often, factories and
other stationary objects were concealed with painted cloth, netting, and artificial
trees to make them difficult for enemy aircraft to detect. It was realized that
this camouflage looks different when imaged with near-IR light, and research at
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