The process by which natural and false color composites made from three multispectral intervals or bands by projection and superposition is discussed. One important conclusion is that when a spectral band which contains reflectances in the wavelength interval between 0.7 - 1.1 mm is projected through a red filter, healthy, active vegetation will appear bright (light tones) in a black and white image or red in a false color image.
We can use color-filtered b & w pictures with color filters to produce color composites by projected superposition. To do this, imagine an experimental setup. Pick any scene containing many features and classes of differing colors. First, replace the prints with positive transparencies (tonally analogous to prints). Work with three b & w transparencies, made with blue, green, and red filters respectively. Shine white light through each one mounted in its own lamp projector (total of three) on to a screen. Project the blue transparency through a blue filter, the green through green, and the red through red. Co-register (line up) the three projections by superimposing several distinctive patterns that are common within the photographed scene. Blue features are clear areas on its (blue) transparency is projected through the blue filter as blue, green project through its filter as green, and red as red. The result will be a simulated natural color image. Other colors present are additive mixes of two or more primaries (e.g., yellow is a mix of red and green; orange is a mix of more red and some green; white is an equal mix of all three primaries, and black is simply the absence of any colored light of any wavelength).
We can change the image if one of the transparencies is a b & w infrared film, which we ordinarily use to emphasize a property of healthy vegetation in which light in the range of 0.7 - 1.1 µm reflects strongly from the internal cells of plants, giving rise to bright tones in the film. We generate a false color composite by projecting a green = light tones transparency through a blue filter, a red through green, and this IR-transparency (with light tones corresponding to vegetation) through a red filter, all onto a screen or on color film.
Let's see what a false color image looks like in the scene below. First we show the area, grassland and a field with natural shrub cover (left), as it appears in natural color in this aerial oblique view:
The companion photo below is a notably different color version (typical false color rendition) in which various kinds of vegetation display in several tones of red, pink, or yellow (the latter two may indicate a degree of stressed or unhealthy vegetation).
I-17: What kinds or types of materials/classes are associated with the white, the blue, the dark red, and the light red features noted in this photo. Account for the color. ANSWER
When this film combination is used in military reconnaissance photography, weapons camouflage simulating vegetation is not red, but another hue (usually dark green), because the high near-IR reflectance from healthy vegetation is absent. Or, in a more familiar mode, color IR photos (and sensor-derived images that include an IR band) show a football field with natural grass in bright red as compared with artificial turf (Astroturf) in a dark, non-red tone.
Primary Author: Nicholas M. Short, Sr. email: [email protected]