Image Processing Reference
Figure2.3 MWIR (3-5 mm) image of pyramid and Sphinx. (CourtesyofStanLaband)
make objects buried at shallow depths visible to thermal imaging sensors, as we
will see later in this chapter. The lower parts of the pyramid's face that receive
sunlight at a glancing angle are cooler and are represented by magenta color, while
surfaces that are in shadow are cooler still and are represented by blue. The same
heating patterns caused by solar loading are seen in the Sphinx. For example, the
Sphinx's neck and breast are cool, as they are well shaded from the sun by its head.
Solar loading can lead to an effect called thermal scarring. An object that
shades the ground from the sun during the day will create a cooler area underneath
it. Even after the object is moved, the cold spot persists, sometimes for hours,
depending on the sun's position when the object is moved. The black plastic plant
pot shown in Fig. 2.4 was left in position for 30 minutes before it was removed.
The visible image gives no indication that the pot was ever there, but the thermal
scar persists for many minutes as shown in Fig. 2.5. This same technique is used
to look for traces of military vehicles by reconnaissance satellites and surveillance
UAVs—a transport jet can leave a thermal scar long after it has taken off, as can a
ship moving through water.
Thermal imaging technology has advanced to the point where even very subtle
surface temperature differences are quite easy to detect. Recall the example of the
stove burner in the last chapter. It emitted SWIR light even when it appeared black
to the eye. Turn off the burner, wait long enough, and the burner will cool to a
temperature just a little above room temperature. It is then indistinguishable from
the other burner if imaged in the SWIR waveband. Before the burner achieves
thermal equilibrium with its surroundings, it will only be a few degrees hotter
than the one that was never heated. It will not feel hotter than the other burner
to the finger, and yet the temperature differences will be easy to see with a thermal
imaging camera. In fact, the most sensitive cameras can see temperature changes of
0 : 01 Cor smaller. This is possible because the intensities of the MWIR and LWIR
light emitted by “room temperature” objects vary sharply with their temperature.
There can be a great deal of contrast (difference in brightness) between two