Gray scale
Most raw unprocessed satellite imagery is stored in a gray scale
format. A gray scale is a color scale that ranges from black to
white, with varying intermediate shades of gray. A commonly used
gray scale for remote sensing image processing is a 256 shade
gray scale, where a value of 0 represents a pure black color,
the value of 255 represents pure white, and each value in between
represents a progressively darker shade of gray.
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[256 level gray scale]
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Objects in a gray tone display have a brightness value (or digital
number), which represents the measured energy level of the item.
Contrast refers to the difference in relative
brightness between an item and its surroundings as seen in the
image. A particular feature is easily detected in an image when
contrast between an item and its background are high. However,
when the contrast is low, an item might go undetected in an image.
The following two images illustrate differences in contrast between
images taken in different portions of the electromagnetic spectrum.
The image on the left is a clip from a Landsat 7 Thematic Mapper
Band 4 image, which depicts the spectral response in the near
infrared portion of the spectrum. This spectral band is especially
sensitive to young vegetative growth, which contains pigments
that reflect near infrared radiation from its leaf surfaces. The
image on the right is a clip from the exact same scene as the
first image and is a Landsat 7 Thematic Mapper Band 3 image, which
depicts the spectral response in the visible red portion of the
spectrum. This channel is not very sensitive to vegetation. In
both images, contrast between the deep reservoir water (which
appears black in the lower center part of each image) and land
is relatively high. In the first image, however, the contrast
between the agricultural fields (which are bright white and light
gray) and the surrounding land use classes (pasture, suburban
developments, forested areas) is much higher than in the visible
red image. This is caused by the heightened sensitivity to reflected
near infrared in the channel 4 sensor.
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Thematic Mapper Band 4 (RED)
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Thematic Mapper Band 3 (NEAR IR)
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Images of raw, unprocessed data streams are often not particularly
useful to a human interpreter, since the contrast is often very
low and the human eye can only distinguish between a few dozen
shades of gray. Image processing techniques can be used to enhance
the contrast between the most important shades of gray that make
up an unprocessed image.
