Image Processing Reference
In-Depth Information
other locations that reduce the effects of the atmosphere on observation. Other
instruments are flown on orbiting platforms, eliminating the effects of atmospheric
absorption. We have seen some interesting effects on terrestrial objects caused by
solar loading, and the same effects are seen on other objects warmed by the sun. For
example, during a lunar eclipse, the Earth blocks sunlight from reaching the moon
for as much as 70 minutes. Right before the eclipse, the moon's surface has been
heated to very high temperatures by the sunlight. When the sunlight is blocked, the
moon no longer shines with reflected light, but it glows with its own heat.
Figure 2.33 is an image of the moon made by the SPIRIT III telescope on
the MSX (Midcourse Space Experiment) satellite during the total lunar eclipse
in September of 1996. The image has red pseudocolor added, where the lighter
the shade of red, the higher the surface temperature. SPIRIT stands for SPatial
InfraRed Imaging Telescope, an instrument that has, among other sensors, an
infrared imager consisting of a scanned array of silicon detectors in a cryostat
cooled with solid hydrogen. These cooled detectors can image in six wavebands
between 4.2 and 26 m. 8 This particular image was made through a passband
filter with a center wavelength of 4.29 m. Note the white circular “hot spots”
on the surface. These are impact craters, where denser, subcrustal rock material
was exposed by meteor impacts. The dense rock on the crater floors retains heat
longer than the powdery crust material, and thus the crater floor glows brighter in
IR light for a longer time during an eclipse, when the solar loading is interrupted
by the earth's shadow. The cooling curves obtained from a time series of images
like this one can be used to obtain information about the physical properties of the
surface.
The brightest spot in this image is the crater Tycho, the youngest of the large
impact craters on the moon. Tycho can be easily seen through binoculars when
the moon is full. It has a dramatic pattern of radial lines, known as rays, which
are remnants of the rock ejecta blown out of the moon's surface during the meteor
impact that created the crater. The rock on the bottom of the crater is fairly fresh,
i.e. it has not been exposed long enough to pulverization by micrometeor impacts.
When we look at Tycho, we are seeing the dense rock beneath the crust of the moon
that holds heat longer than the lower-density crust outside the impact area.
Thermal imaging is a powerful tool that enables us to see objects in the absence
of visible light without having to provide illumination, and to see through certain
thin materials that absorb visible light. But thermal imaging requires temperature
differences: it is very hard to discern an object that is at the same temperature as its
surroundings. Seeing through material is fairly limited, since thermal infrared light
cannot penetrate more than a few millimeters of material before it is completely
absorbed. If we want to see objects through greater thicknesses of material, we
need to image with much longer or shorter wavelengths of light. The next chapter
travels to the long-wavelength region of the spectrum, a region where light can
8 Bartschi et al., “The spatial infrared imaging telescope III,” Johns Hopkins APL Technical Digest,
17(2), 215-225 (1996).
 
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