Bayer Filter Definition

Bayer filter arrays place color filters directly above the image sensor

The filter contains an alternating pattern composed of one red, one blue and two green patches - each one covering a single photo-site This results in each pixel of the resulting image containing information for only one color. This information is read out of the imager and digitized for further processing.

A Bayer filter mosaic is a color filter array (CFA) for arranging RGB color filters on a square grid of photo sensors. 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 GRGB or other permutation such as RGGB. It is named after its inventor, Dr. Bryce E. Bayer of Eastman Kodak. Bayer is also known for his recursively defined matrix used in ordered dithering.

Explanation

Bryce Bayer's patent called the green photo sensors 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 human eye's greater resolving power with 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 determine color 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 point.

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.

Alternatives

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; or using three separate CCDs, one for each color, which is far more expensive.

Three new Kodak RGBW filter patterns

"Panchromatic" cells

An earlier RGBW filter patternOn 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.

Such cells have previously been used in "CMYW" (cyan, magenta, yellow, and white) "RGBW" (red, green, blue, white) sensors, but Kodak has not compared the new filter pattern to them yet.

Microlenses and dichroic mirrors

Nikon's dichroic color separation patent drawing.On November 21, 2006, Nikon Corporation was granted U.S. patent 7,138,663 which places a microlens over a triplet of photoreceptors. Specific wavelengths of light are separated and passed to specific photoreceptors designated to record red, green, and blue wavelengths. Light separation is achieved with dichroic mirrors. This system emulates three-CCD imaging systems with a single array. Image quality is theoretically improved due to much smaller gaps between photoreceptors assigned to a wavelength (each microlens lens acts as a collector for all pertinent wavelengths of light) and reduced light loss through the absence of wavelength filters.

Foveon's vertical color filter

Foveon's scheme of vertical filtering for color sensingAnother type of color separation, sometimes called vertical color filter, as opposed to lateral color filter, is used in the Foveon X3 sensor. Each location the image sensor array has three stacked active pixel sensors, utilizing the sensor's silicon itself as the color filter, based on the wavelength-dependent absorption of light in silicon.

See also

Front page of Dr. Bryce Bayer's 1976 patent on the Bayer pattern filter mosaic, showing his terminology of luminance-sensitive and chrominance-sensitive elementsUS3,971,065 (PDF version) (1976-07-20) Bryce E. Bayer, Color imaging array. on web

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