Image Processing Reference
trails and the infrared light the prey emits. This is accomplished with specialized
sensors located in the pits on either side of their heads, just below the eyes. The two
pits are divided into two halves by a thin membrane stretched across the pit like
a drum head. The membranes contain two large branches of the trigeminal nerve
(one of the cranial nerves) that are fanned out across the surface of the membrane.
These nerves respond to minute temperature changes of the membranes, which can
be induced by the presence of a warm-blooded animal in front of the snake's head.
The absolute temperature of the membranes is not sensed, only time-dependent
changes, which is why the snake scans its head from side to side. 5 The membranes
serve as platforms for the nerves, and the platforms have a large surface area and
low thermal mass, making them very sensitive to changes in infrared intensity.
Each pit has a field of sensitivity that overlaps the other slightly, and the snake can
tell when the prey is directly in front of its head because the two pits will generate
approximately equal thermal signals in each pit. The infrared perception of the
viper is not truly vision, since the snake only detects the existence and direction of
a heat source, not its shape. This sense is akin to the heat sensitivity of one's skin,
which can detect radiant energy in the IR band; though the pits are much more
sensitive to IR light than our skin, being able to detect IR energy equivalent to that
produced by a human hand at a distance of 30 cm. 6
Most mammals show their highest surface temperature around their eyes
because of the high degree of blood flow and the lack of any insulating layer over
the living eye tissue. Extremities such as the ears and nose tend to have the lowest
surface temperature because they don't receive as much blood flow as the eyes, or
in the case of the nose, they may be wet, cooling the nose by evaporation. Cats and
dogs tend to have hot eyes and cold noses as seen in Fig. 2.27.
Thermal energy is a vital aspect of biological processes, and infrared imaging
allows us to monitor the flow of thermal energy within living tissue. Thermal
imaging can be used in medical applications to detect anomalies in skin and tissue
surface temperatures, anomalies that can often reveal damage or abnormalities in
underlying tissue. Figure 2.28 shows a normal forearm imaged in the visible and
MWIR bands. The surface of the skin is warmed up slightly by blood flow, and
thus one can see the arteries and veins from their heat signature. If there were
vascular damage to an area of the arm, then the skin there would appear colder
than surrounding undamaged areas.
Now consider a vascular abnormality that results in a thermal anomaly.
Figure 2.29 shows a MWIR image of a hand missing an index finger. The finger
was lost in a shotgun accident, and as a result the stump is colder than the other
digits as a result of vascular damage. This image is rendered with pseudocolor
indicated by the color bar to the right of the image: white or red pixels are the
hottest temperatures in the scene, and purple or black is coldest. Thermal images
such as this could be used to indicate surgery to restore blood flow to tissue.
5 Gamow et al., “The infrared receptors of snakes,” Scientific American, 228(5) 94-99 (May 1973).
6 Angus Bellairs, “The Life of Reptiles,” p. 388, Universe Natural History Series, Universe Books