Introduction to Remote Sensing
First, we need to know that instruments on a spacecraft (such as Terra (EOS-AM), TRMM, GOES) do not "see" in color. Every image is obtained in black and white at a precise wavelength (usually between 0.4 to 12.0 microns). These electronic cameras only collect information in black and white, but they can obtain many images at the same time in different parts of the spectrum.If we look at the diagram below of the spectrum, we see several broad regions that include the ultraviolet (wavelengths between 0.3 - 0.4 microns), visible (0.4 to 0.7 microns), near-infrared (0.7 to 1.2 microns), the solar reflected infrared (1.2 to 3.2 microns), the mid-infrared (3.2 to 15 microns) and the far infrared (longer than 15.0 microns).
Notice that the vertical axis (called % transmission) shows where our atmosphere allows a lot of the Sun's rays to reach the ground. It is easy to see why our eyes work in the visible part of the spectrum, because it is here that almost 100% of the Sun's energy reaches the surface. In contrast, due to water vapor in the atmosphere, almost no energy (%0 transmission) reaches the ground at 1.4 and 1.9 microns.
Our images are obtained as a set of individual black and white images. All the images are obtained at exactly the same time, and each is a computer image where we can assign an 8-bit binary number between 0 (black) and 255 (white). In this diagram, we show six of these images (or "channels"), along with the wavelength of the channel. We show channels 2, 4, 6, 8, 10, and 12 just to cover the whole range, and to show that not all channels have the same wavelength range (the width of the color bar beneath the spectrum). See how Band 12 is very wide (about 4 microns) while band 4 is very narrow (about 0.05 microns).
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Some text/images courtesy of NASA's Virtually Hawaii project. Written by
SHARP student Ian McGlynn.
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