A Bayer filter mosaic is a color filter array (CFA) for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camcorders, and scanners to create a color image. The filter pattern is 50% green, 25% red and 25% blue, hence is also called RGBG, GRGB, or RGGB.
Alternatives to the Bayer filter include both various modifications of colors and arrangement and completely different technologies, such as color co-site sampling, the Foveon X3 sensor or dichroic mirrors.
Bryce Bayer's patent (U.S. Patent No. 3,971,065) in 1976 called the green photosensors luminance-sensitive elements and the red and blue ones chrominance-sensitive elements. He used twice as many green elements as red or blue to mimic the physiology of the human eye. The luminance perception of the human retina uses M and L cone cells combined, during daylight vision, which are most sensitive to green light. These elements are referred to as sensor elements, sensels, pixel sensors, or simply pixels; sample values sensed by them, after interpolation, become image pixels.
The raw output of Bayer-filter cameras is referred to as a Bayer pattern image. Since each pixel is filtered to record only one of three colors, the data from each pixel cannot fully specify each of the red, green, and blue values on its own. To obtain a full-color image, various demosaicing algorithms can be used to interpolate a set of complete red, green, and blue values for each pixel. These algorithms make use of the surrounding pixels of the corresponding colors to estimate the values for a particular pixel.
Different algorithms requiring various amounts of computing power result in varying-quality final images. This can be done in-camera, producing a JPEG or TIFF image, or outside the camera using the raw data directly from the sensor.
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The Bayer filter is almost universal on consumer digital cameras. Alternatives include the CYGM filter (cyan, yellow, green, magenta) and RGBE filter (red, green, blue, emerald), which require similar demosaicing. The Foveon X3 sensor (which layers red, green, and blue sensors vertically rather than using a mosaic) and arrangements of three separate CCDs (one for each color) don't need demosaicing.
On June 14, 2007, Eastman Kodak announced an alternative to the Bayer filter: a color-filter pattern that increases the sensitivity to light of the image sensor in a digital camera by using some "panchromatic" cells that are sensitive to all wavelengths of visible light and collect a larger amount of light striking the sensor. They present several patterns, but none with a repeating unit as small as the Bayer pattern's 2×2 unit.
Another 2007 U.S. patent filing, by Edward T. Chang, claims a sensor where "the color filter has a pattern comprising 2×2 blocks of pixels composed of one red, one blue, one green and one transparent pixel," in a configuration intended to include infrared sensitivity for higher overall sensitivity. The Kodak patent filing was earlier.
Fujifilm "EXR" color filter array
Fujifilm's EXR color filter array are manufactured in both CCD (SuperCCD) and CMOS (BSI CMOS). As with the SuperCCD, the filter itself is rotated 45 degrees. Unlike conventional Bayer filter designs, there are always two adjacent photosites detecting the same color. The main reason for this type of array is to contribute to pixel "binning", where two adjacent photosites can be merged, making the sensor itself more "sensitive" to light. Another reason is for the sensor to record two different exposures, which is then merged to produce an image with greater dynamic range. The underlying circuitry has two read-out channels that take their information from alternate rows of the sensor. The result is that it can act like two interleaved sensors, with different exposure times for each half of the photosites. Half of the photosites can be intentionally underexposed so that they fully capture the brighter areas of the scene. This retained highlight information can then be blended in with the output from the other half of the sensor that is recording a 'full' exposure, again making use of the close spacing of similarly colored photosites.
Fujifilm "X-Trans" filter
The Fujifilm X-Trans CMOS sensor used in many Fujifilm X-series cameras is claimed to provide better resistance to color moiré than the Bayer filter, and as such they can be made without an anti-aliasing filter. This in turn allows cameras using the sensor to achieve a higher resolution with the same megapixel count. Also, the new design is claimed to reduce the incidence of false colors, by having red, blue and green pixels in each line. The semi-random arrangement of these pixels is also said to provide grain more like film.
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