A chromatic modulator has been proposed to enable the separate detection of the red, green, and blue (RGB) color components of the same scene by a single charge- coupled device (CCD), active-pixel sensor (APS), or similar electronic image detector. Traditionally, the RGB color-separation problem in an electronic camera has been solved by use of either (1) fixed color filters over three separate image detectors; (2) a filter wheel that repeatedly imposes a red, then a green, then a blue filter over a single image detector; or (3) different fixed color filters over adjacent pixels. The use of separate image detectors necessitates precise registration of the detectors and the use of complicated optics; filter wheels are expensive and add considerably to the bulk of the camera; and fixed pixelated color filters reduce spatial resolution and introduce color-aliasing effects. The proposed chromatic modulator would not exhibit any of these shortcomings.

Red, Green, and Blue Filter Strips would be registered with pixel rows in a repeating pattern. The filter would be repeatedly placed in the upper, middle, and lower positions to repeatedly expose each pixel to each color.
The proposed chromatic modulator would be an electromechanical device fabricated by micromachining. It would include a filter having a spatially periodic pattern of RGB strips at a pitch equal to that of the pixels of the image detector (see figure). The filter would be placed in front of the image detector, supported at its periphery by a spring suspension and electrostatic comb drive. The spring suspension would bias the filter toward a middle position in which each filter strip would be registered with a row of pixels of the image detector. Hard stops would limit the excursion of the spring suspension to precisely one pixel row above and one pixel row below the middle position.

In operation, the electrostatic comb drive would be actuated to repeatedly snap the filter to the upper extreme, middle, and lower extreme positions. This action would repeatedly place a succession of the differently colored filter strips in front of each pixel of the image detector. At each filter position, each detector pixel would thus acquire information on the local brightness in the momentarily selected color. The frequency of actuation of the comb drive would be three times the frame rate of the camera, so that over one frame period, each pixel would acquire full color information. Hence, the camera would acquire full color information at full pixel resolution.

Of course, it would be necessary to time-multiplex the outputs of the pixels for processing in a manner consistent with the spatial and temporal periodicity of the color information acquired by each detector pixel. To simplify the processing, it would be desirable to encode information on the color of the filter strip over each row (or at least over some representative rows) of pixels at a given instant of time in synchronism with the pixel output at that instant. This could be accomplished by means of an alternating pattern of opaque patches over the last two pixel-column positions of each filter strip: for example, nonzero illumination at both of these column positions could signify the presence of the red filter strip, zero illumination at one of these column positions could signify the presence of the green filter strip, and zero illumination at both of these column positions could signify the presence of the blue filter strip.

This work was done by Frank Hartley and Anthony Hull of Caltech for NASA's Jet Propulsion Laboratory.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, NASA Management Office–JPL. Refer to NPO-20896.

This Brief includes a Technical Support Package (TSP).
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Chromatic Modulator for a High- Resolution CCD or APS

(reference NPO-20896) is currently available for download from the TSP library.

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This article first appeared in the June, 2008 issue of NASA Tech Briefs Magazine.

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