Bayer filter

A Bayer filter mosaic is a color filter array (CFA) for arranging RGB color filters on a square grid of photosensors. The term derives from the name of its inventor, Dr. Bryce E. Bayer of Eastman Kodak, and refers to a particular arrangement of color filters 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 or GRGB.

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Explanation

Bryce Bayer's patent 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 human eye's greater resolving power with green light. These elements are referred to as sensor elements, 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, two-thirds of the color data is missing from each. 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.

"Panchromatic" cells

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.^[1] They present several patterns, but none with a repeating unit as small of the Bayer pattern's 2x2 unit.

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

Microlenses and dichroic mirrors

On November 21, 2006, Nikon Corporation filed patent 7,138,663 which places a microlens over a triplet of photoreceptors. Specific wavelengths of light are separated 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

Another type of color separation, sometimes called vertical color filter,^[2] 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.

References

• US3,971,065 (PDF version) (1976-07-20) Bryce E. Bayer Color imaging array  on web

Notes

1. ^ John Compton and John Hamilton (2007-06-14). Color Filter Array 2.0. A Thousand Nerds: A Kodak blog. Retrieved on 2007-06-15.
2. ^ Thomas Kreis (2006). Handbook of Holographic Interferometry: Optical and Digital Methods. Wiley-VCH. ISBN 3527604928.