Remote Sensing

Color Images Another essential ingredient in most remote sensing images is color. While variations in black and white imagery can be very informative, the number of different gray tones that the eye can separate is limited to about 20 to 30 steps (out of a maximum of about 200) on a contrast scale. On the other hand, the eye can distinguish 20,000 or more color tints, enabling small but often important variations within the target materials or classes to be discerned. Since different bands (or wavelengths) have a different contrast, computers can be used to produce a color image from a black and white remote sensing data set. Remember, satellites record the reflected and emitted brightness in the different parts of the spectrum, as is demonstrated in the figure above. Similar to the screen on a color television set, computer screens can display three different images using blue light, green light and red light. The combination of these three wavelengths of light will generate the color image that our eyes can see. This is accomplished by displaying black and white satellite images corresponding to various bands in either blue, green, or red light to achieve the relative contrast between the bands. Finally, when these three colors are combined, a color image—called a "false color image"—is produced (it's called "false color" because colors are assigned that we can see and easily interpret with our eyes). In order to understand what the colors mean in the satellite image, we must know which band (or wavelength) is used for each of the blue, green and red parts of the computer display. Without detailed knowledge of how each band has been changed for contrast and brightness, we cannot be sure why the colors are what they are.