Philpot & Philipson: Remote Sensing Fundamentals Color 6.1 W.D. Philpot, Cornell University, Fall 2012 W B = W (R + G) R = W (G + B)

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1 Philpot & Philipson: Remote Sensing Fundamentals olor OLOR The human visual system is capable of distinguishing among many more colors than it is levels of gray. The range of color perception is truly impressive especially given that we have only three distinct color receptors in our eyes. That is a true limitation, and any information presented to us will be limited to combinations of three base, or primary, colors. In remote sensing we commonly deal with high resolution spectral data which includes the visible region but also goes well beyond into other spectral domains. It is often helpful to select three channels and assign them to primary colors in order to produce a color representation of the data (a "False olor" image"). The trick is to understand the way that the primary colors combine in order to effectively interpret the results colors. The purpose of this chapter is to review the basics of color systems as a prelude to interpreting false color imagery. 6.1 Additive olors When dealing with light, blue, green and red are the "primary colors," "additive primaries" or "additive colors." If blue, green and red light are mixed in varying proportions, almost all colors can be produced; if the three are mixed in equal intensities, they will produce white light. In essence, Blue + Green + Red = White. Bue, green and red are the primary colors because objects or filters of these colors reflect or transmit light of their own color, but, they absorb light of the other two colors. Blue + Green + Red = White or B + G + R = W G = W - (B+R) W B = W (R + G) R = W (G + B) Filters: B transmits or reflects B, but absorbs G and R; B = W - (G + R) G transmits or reflects G, but absorbs B and R; G = W - (B + R) R transmits or reflects R, but absorbs B and G; R = W - (B + G) Figure 6.1: haracteristics of additive colors. 6.2 olor-additive Viewing urrently, much of the viewing and color manipulation of imagery occurs while using color displays attached to a computer which are color additive devices. To illustrate how additive colors might be used to display information, consider exposing three aerial images over an area using three identical frame cameras, each using a different filter. Although there is no requirement that these be red green and blue any wavelength combination is acceptable suppose for illustration that one camera uses a filter that transmits only blue light. Only objects that reflect blue light will be imaged. If the area second camera uses a filter that transmits only green light then objects that reflect green light will be imaged. If the third camera is fitted with a red filter, only red light will be imaged. This procedure is shown in Figure 9.1 for an idealized scene of a blue stream, green tree, redroofed structure, and black background. The resulting black-and-white images show only the blue stream (as white), the green tree (as white), and the red roof (as white), all on black backgrounds.

2 Philpot & Philipson: Remote Sensing Fundamentals olor 6.2 The three images could be referred to as "spectral" images; "spectral," because each image in the set was produced by radiation from a distinct range of wavelengths of the electromagnetic spectrum. Figure 6.2: Spectral image set of an idealized blue, green and red scene. Figure 6.3: olor-additive viewing of blue, green and red spectral images. Projections of all photographs are registered in position on the viewing screen. Suppose, now, that the three images were printed as positive transparencies, i.e., dark areas would be opaque (black), and bright areas would be transparent (clear) in this simple image. If white light is projected through the blue image of the scene in Figure 6.2 (i.e., through the blackand-white transparency representing blue light), only the stream and the road will transmit light and be seen. If the projected light were blue instead of white, the stream would appear blue. If a second light is projected through the green transparency in Figure 6.2, and if the scene is projected onto the same screen as the blue stream, a white tree would appear along with the blue stream. The tree and stream will not interfere if the images are positioned correctly (i.e., registered spatially). The color of the tree will be green if the color of the second light is changed to green. Similarly, if the third image is projected onto the same screen with red light, the original scene will appear -- a blue stream, green tree, and red-roofed structure. Reiterating, different colors could be viewed if the colors of the projected lights were changed. For example, a red stream would appear if the light projected through the blue spectral photograph were red instead of blue. olor-additive viewing of the spectral photographs of Figure 6.2 is illustrated in Figure 6.3.

3 Philpot & Philipson: Remote Sensing Fundamentals olor 6.3 ulti-color projection of spectral images of a single scene is essentially how spectral images are displayed on a color monitor. Each of the three selected digital images is assigned one of the primary colors of the computer monitor. Individual pixels consist of a set of red, green and blue radiating elements that are together smaller resolution limit of the eye, but combine to form a color pixel. The patterns vary with the type of display, as shown in Figure 6.4, but the process is functionally the same as the color additive process illustrated above. Figure 6.4: Typical color display patterns (image from Peter Halasz). 6.3 Subtractive olors olor transparencies (color film in which the color is seen when viewing the transparency on a light table) are produced using subtractive colors. agenta, cyan and yellow are the "subtractive primaries" or "subtractive colors". If the three are mixed in equal intensities, they will produce black. The importance of the subtractive colors is that they are complementary to the additive colors. Referring to opposite points of the star in Figure 6.4, it is noted that: agenta + yan + ellow = Black or + + = Black G Black absorbs G, but transmits B and R; = W G absorbs R, but transmits B and G; = W R B R absorbs B, but transmits G and R; = W B Figure 6.5: Additive (RGB) and subtractive (,,) colors. olors at the opposite sides of the star are complementary.

4 Philpot & Philipson: Remote Sensing Fundamentals olor 6.4 The alternate and intermediate points on the star in Figure 9.3 helps one remember that combining subtractive or additive colors in equal proportions will result in the following: + = R + = G + = B G + R = R + B = B + G = olor prints or printed continuous tone images use a similar set of dyes. Both processes produce a color by absorbing, or subtracting the unwanted colors. The transparency removes unwanted colors during transmission of the light; the dyes of a color print remove unwanted colors by absorption during reflection. 6.4 olor Reversal Films olor aerial photography is commonly acquired with color reversal films. A reversal film is one for which no negative is formed. The film in the camera is exposed and processed directly into a positive image. Familiar examples are Kodachrome and Ektachrome films, manufactured by Eastman Kodak. Figure 6.6: Photomicrograph cross-section of Kodak EXR 5296 color negative camera film showing the emulsion layers. The emulsions of conventional color reversal films have three sets of layers. The upper layers are sensitive to blue light, the middle layers are sensitive to blue and green light, and the layers closest to the film base are sensitive to blue and red light (Figure 6.6). The exposure/development process is illustrated schematically in Figure 6.6. To prevent blue light from exposing all emulsion layers, a yellow filter is incorporated into the emulsion, just below the upper, blue-sensitive layer. As shown in Figure 6.6, blue light will thus expose only the upper layer; green light will expose the middle layer, but have virtually no effect on the blue- or red-sensitive layers; and red light will expose the red sensitive layer, but have virtually no effect on the blue- or green-sensitive layers. Since white light is composed of blue, green and red light, exposing the film to white light will expose all layers. During processing of color reversal films, dyes form in the unexposed portions of the emulsion layers, and the colors of the dyes are complementary to the colors of the layer sensitivity (Figure 6.6). ellow dye forms in the unexposed parts of the upper, blue-sensitive layer; magenta

5 Philpot & Philipson: Remote Sensing Fundamentals olor 6.5 dye forms in the unexposed parts of the green-sensitive layer; and cyan dye forms in the unexposed parts of the red-sensitive layer. When the processed film is viewed over white light, the colors of the original scene will appear. Where the film was exposed to blue light, cyan and magenta dyes formed, but yellow dye did not. The cyan and magenta dyes will prevent the red and green from the white light from passing through the film; since there is no yellow dye, however, the blue light will not be affected. Analogously, where the film was exposed to green light, the cyan and yellow dyes will remove the red and blue from white light, but not green; and where the film was exposed to red light, the magenta and yellow dyes will remove the green and blue from white light, but not the red. ost objects are not purely blue, green or red. They reflect some mix of these colors, and will thus expose all emulsion layers to different degrees. This will result in variable amounts of each of the complementary color dyes in all parts of the processed emulsion. The relative amounts of blue, green and red that are removed from white light on viewing the processed film will be in proportion to the amounts of the complementary color dye. As stated, nearly all colors can be produced by mixing various proportions of blue, green and red light. And this is what occurs when the dyes control the relative proportions of transmitted blue, green and red light. In essence, all colors can be photographed and viewed in the processed film. 6.5 olor Infrared Photography Although color infrared (IR) film has a three-layer emulsion, the layers differ from conventional color film in their spectral sensitivities. The upper layers are sensitive to near-infrared radiation, the middle layer is green sensitive, and the bottom layer is red-sensitive (Figure 6.7). Because all layers are sensitive to blue light, a yellow filter must be placed over the camera lens during photography. The interaction of light with the emulsion layers is shown in Figure 6.7. yan dye will form in the unexposed portions of the infrared-sensitive layer, magenta dye will form in the unexposed portions of the red-sensitive layer, and yellow dye will form in the unexposed portions of the greensensitive layer. With these dye-layer assignments, the processed film will appear blue where exposed to green, green where exposed to red, and red where exposed to infrared. White features in the processed film correspond to objects that reflect balanced amount of green, red and infrared radiation, and all other combinations are possible. 6.6 Suggested Reading Eastman Kodak o. publications: - olor as Seen and Photographed. Pub. No. E Applied Infrared Photography. Pub. No Filters for Black and White and olor Pictures. Pub. No. AB-l. Smith, J.T., Ed anual of olor Aerial Photography. American Society of Photogrammetry, Falls hurch, VA.

6 Philpot & Philipson: Remote Sensing Fundamentals olor 6.6 Exposing light B G R IR W Bk Emulsion Layer Sensitivities yellow filter (removes blue light) B B+G l ayer i s exposed B+R ompensatory dyes f orm in unexposed parts of the emulsion B -> yellow G -> magenta R -> cyan Viewing over white light White B G R Black BGR Black ellow controls blue agenta controls green cyan controls red BGR BGR BGR BGR BGR BGR Figure 6.7: Exposing, processing and viewing a color film.

7 Philpot & Philipson: Remote Sensing Fundamentals olor 6.7 Figure 6.8: Exposing, processing and viewing a color-infrared (IR) film.