False Color Rendition - Lecture Material - Completely Remote Sensing tutorial, GPS, and GIS - facegis.com
False Color Rendition

We are now in a position to apply image processing to generating our first color composite combination. Review page I-13, if need be, to recall the discussion of how color composites are made.

In practice, to make a color composite one needs to project on to color film an illuminated black and white image of each of three different spectral bands through a set of color filter or if computer-generated using color beam produced by three electron guns on to a monitor*. Let's start by producing photographically the now-conventional standard false color composite, made by assigning the TM Band 2 (green) image to the blue filter , Band 3 (red) to the green filter, and Band 4 (the near or photographic reflective IR) to the red filter.

The standard false color composite that is the most common of the colorized images made from Landsat MSS or TM (and SPOT and other satellite systems); TM Band 2 = blue; TM Band 3 = green; TM Band 4 = red; for MSS images the usual combination is MSS 4 = blue; 5 = green; 7 = red.

Two color patterns dominate the land classes: reds, depicting vegetation, and medium grayish-browns, found mainly along the bright sun-facing slopes. The ocean and the bay are evinced in deep blues that, near the shore (a), become a bit lighter where thicker sediments add reflectance. The breakers are presented in mottled blue and white patterns.

We can place the various expressions of vegetation in several categories based on their specific red tints and in most cases also from the spatial patterns they occupy. The continuous and rather deep red at g represents a segment of the forested areas in the Los Padres National Forest near the crest of the Santa Lucia Mountains. Elsewhere, as at i, we can attribute thin strings of red or irregular red patches (j) to trees and/or scrub vegetation (as at l), lining stream channels or scattered as copses and patches along the slopes. In the valleys, bright red areas (at k and other points), some rectangular and others more uneven, are primarily examples of field crops, hay meadows, or other types of cultivation. Areas believed to be barren to varying degrees (as at m and o), have darker gray-brown tones, but may have faint pink overtones implying limited vegetation cover. Where vegetation is sparse and scattered on the hills, particularly where well-illuminated by the sun, the prevailing tan to grayish brown colors imply joint contributions from underlying soils, combined with reflectances from the brown grasses (with much diminished Band 4 input). However, we temper this last statement by the fact that the image produced for this same scene by EOSAT (not reproduced here but examined by the writer) shows more pinks over most of the grassland areas than does this image. At the time of this scene acquisition (mid-November), if winter rains had started early, and had been only moderate to this date, the hills would still be relatively brownish. However, the EOSAT rendition suggests some greening had started.

Once again, identify the urban areas from the street patterns. The streets, as well has Highway 1 and other major roadways, have bluish tones, expected because they are especially light-toned in Bands 1 and 2 (assigned here to blue). The mixed tones in Los Osos, with some yellow-brown scattered about, give it a subtly different color signature when compared with Morro Bay. We'll explain more about this difference when we examine the next color composite. Both town areas contain segments that have red blotches, which correspond to residential sections, park land, or other places where trees or grass grow. The extraction pits (u) are very bright, tinged with blue.

* A brief discussion of how Color TVs (and with variations Color Monitors such as your computer screen) is appended here. A color vidicon produces an encoded signal which combines primary colors into a video version that can be transmitted. The signal upon reception at the receiver passes through a tuner and then electronic circuitry that separates audio from visual. The visual signal then goes through a decoder to extract three primary color signals that are each assigned to an electron gun that produces a color beam. Now, look at this figure:

Schematic of a TV picture tube.

As they enter the picture tube, these three electron beams are passed from the guns through an accelerator, than a focusing device, and then through a scanner which moves the beams through parallel lines (a high resolution picture tube displays in succession 700 horizontal lines or more on its screen face) that sweeps very rapidly across the picture tube's front surface. This diagram shows that surface assemblage:

Shadow collimator and phosphors in the picture tube's front assemblage.

The shadow masks serves as a collimator. In one arrangement color phosphors of different compositions, each sensitive to one of the primary colors, are activated by whichever electron beam yields a response to that color. If one of a color phosphor triad (RGB) is so excited, it will glow in that color as seen by an external viewer (such as you). Combinations of 2 or 3 beam color/phosphor responses, in varying intensitys, produce other colors than the primaries. In another arrangement, the phosphors are thin linear bands, red, green, and blue in parallel, that yield the same result. The phosphors are individually so small (a full screen can have more than 300000 dots), the eye does not see them as discrete, only the color they impart to individual pixels.

Source: http://rst.gsfc.nasa.gov