Visual Imagery
The electromagnetic energy that your eyes can see,
visible light, ranges from a wavelength of .7 µm for red
light, through the visible spectrum (red, orange,
yellow, green, blue, indigo and violet) to .4 µm for
violet light. Figure 1-10, view B, shows the
distribution of the various wavelengths of solar energy
incident on the earth’s atmosphere. About 44 percent
of the sun’s energy falls on the earth in the form of light.
Although some light is absorbed, much of the light
incident on the earth’s atmosphere and surface is
reflected back into space.
The reflected light from the earth is measured by a
sensor aboard the satellite that is sensitive only to
electromagnetic energy in the visual range. The sensor
measures the energy seen in each pixel and assigns it a
reading from 0 for no energy sensed to 256 for very
high energy sensed. Measurements are transmitted to
the earth, and the consecutive pixels and scan lines are
processed to compose an image.
The more direct sunlight reaching objects, the
brighter they will appear. The amount of reflectivity of
an object is termed albedo, and is dependent on the
object’s surface texture and color. In visual-range
images, areas of low reflected light (low albedo), such
as water and forest regions, appear black. Areas of
high reflected light (high albedo), such as snow, appear
white (fig. 1-11). Cloud tops reflect a lot of light, so
they are usually very light shades. Space surrounding
earth reflects no light, so it appears black.
Visible imagery is very useful in both atmospheric
and oceanographic analysis because reflectivity varies
considerably among atmospheric, land, and oceanic
features. An obvious disadvantage of visible imagery
is that it is only available during daylight hours.
Infrared Imagery
Look again at figure 1-10, view B. You can see
that most of the sun’s energy that falls on the earth is in
the infrared band. Most of the shorter wavelengths of
infrared energy are reflected from the earth much the
Figure 1-11.—GOES visual (VIS) image. Space and low reflective areas appear black, while high reflective areas, such as cloud
tops, appear white.
1-11
