Satellite imagery is produced by sensors that detect electromagnetic waves emitted or
reflected by a surface and measure their intensity in different parts of the spectrum.
Because all substances absorb and reflect light differently, varying light and tempera-
ture are recorded as black, white, and shades of gray in a image. APT imagery is avail-
able from the visible or infrared segments of the electromagnetic spectrum.
All visible images of Earth record sun light (solar radiation) that is reflected by Earth—
Earth does not emit visible light of its own. APT images show these differences in
absorption and reflectivity (albedo) as different shades of gray. Subjects with highest
albedo (greatest reflectivity) appear white in visible images, clouds show up as white or
gray, outer space is black.
I n f r a red images are derived from two sources of infrared energ y, thermal emissions
( p r i m a ry source) and reflected solar radiation. Thermal infrared emissions are the
e n e rgies which are emitted from the Earth. Reflected solar infrared radiation is the
e n e rgy which is given off by the sun and reflected from the Earth or clouds. Both of
these types of energy are seen on an image as varying shades which correspond to
p a rticular temperatures. The warmer the temperature, the darker the shade in the
image. The cooler the temperature, the lighter the shade. In an infrared image, the
blackness of outer space (cold) is displayed as white. Hot spots, such as urban
sprawl, appear as dark gray or black.
figure 52. albedo: ratio of the outgoing solar radiation reflected by an object to the
incoming solar radiation incident upon it.