History of Remote Sensing: Multispectral Images

A significant advance in sensor technology stemmed from subdividing spectral ranges of radiation into bands (intervals of continuous wavelengths), allowing sensors that produce several bands of differing wavelengths to form multispectral images. This concept should be familiar to anyone who has used color filters on a photo-camera. Suppose you mount a red filter in front of the lens in a camera with black and white (b & w) negative film. Focused red light entering from an external object that generates red radiation passes through the filter and will activate the film, leaving numerous microflects of metallic silver, after development of the negative, wherever those light rays had struck the film; these form dark spots or patches in the negative. On printing to positive b & w paper, these dark areas in the negative prevent light from passing through; the positive film process produce light tones (a reversal) in the positive b & w print, so that red objects show bright (whitish) patterns that resemble their shapes. Conversely, the red filter absorbs light from green and blue objects, so that their (unpassed) light does not expose the negative. These areas on the film where a green object's image focuses will develop clear (no silver) in the negative and will print dark. Blue shows as bright shades when a blue filter is used. In effect, the color of an object can be identified by using a filter of that color to image it in bright tones.

Colors in color film are produced by stacking multiple layers of emulsions containing light-sensitive compounds (organic dyes) that filter out different wavelengths. In subtractive color film, the dye colors are: cyan, magenta, and yellow. Using the primary colors as reference, yellow subtracts blue, magenta subtracts green, and cyan subtracts red. So, when multicolored light enters a sensor, light from blue areas in the target or source, on striking the color film will bleach out parts of the yellow emulsion. The same pattern holds for green and red light, affecting the magenta and cyan layers, respectively. Then when white light passes through the multiple layers of the resulting transparencies (e.g., 35 mm slides), the now clear yellow areas will appear blue because the remaining cyan and magenta (so colored over these same areas) will filter out (subtract) red and green, leaving blue to display. The same reasoning holds for the other two primaries (red and green).

In color negative film from which color prints are made, the layer sensitive to red produces its complement color in the negative, which when printed onto paper produces red by leaving behind magenta and yellow dyes (from the [subtractive] color system). We won't elaborate further on the printing rationale; suffice to say that the red in print represents red from the source, green represents green, and blue represents blue.

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