TYPES OF IMAGERY
All things (with a temperature above absolute
zero) emit radiation in the form of electromagnetic
waves. The wavelengths emitted by each object
depend primarily on the object’s temperature. Higher
temperatures cause electrons to vibrate faster and
therefore produce shorter wavelengths. The sun emits
radiation at several different wavelengths, and the
range of these wavelengths is known as the
electromagnetic spectrum (fig. 1-10, view A).
Electromagnetic wavelengths in the visual and
infrared region are usually measured in micrometers.
A micrometer is equal to one-millionth of a meter and
is represented by the symbol µm. Micrometers are also
referred to as microns.
The sun emits a maximum amount of radiation at
wavelengths near 0.5 µm. The earth, which is
obviously much cooler, emits most of its radiation at
longer wavelengths of between 4 and 25 µm. For this
reason, the earths radiation is referred to as long-wave
radiation and the sun’s energy is referred to as
shortwave radiation. The atmosphere is a strong
absorber of radiation at certain wavelengths and is
relatively transparent to others.
Generally, the
atmosphere is transparent to wavelengths associated
with incoming solar radiation; but because of the
presence of water vapor, carbon dioxide, and other
elements, the atmosphere is largely opaque to the
outgoing terrestrial wavelengths.
When data signals from a satellite sensor scan are
compiled at the satellite receiver on earth, the pixels
and scan lines form an image. The image composed of
measurements of energy in the visual range forms a
visual image. The data from sensors that measure
energy in the infrared band compose an infrared
image.
ELECTROMAGNETIC SPECTRUM
Figure 1-10.—(A) The electromagnetic spectrum, and (B) the distribution of solar energy incident to the earth.
1-10
