Wednesday, October 08, 2008

Bayer filter, image sensors

Still image sensor tend to have deeper bit depth, i.e., 12-14 bits per pixel (bpp) rather than 8 bpp common with video sensors. (2^8=256 levels (color image has 24 bits), depth resolution, usually we have 2Y, 1U, 1V for two pixels = 32 bits. studio video camera may be higher).

Still image sensor can read out frames rather slowly, 1-3 fps. Video sensor must output a faster rate, i.e. 24-100 fps.

In some cameras there is only one sensor for the 3 primary colors. In this case some kind of color checkerboard pattern is applied to the sensor.The most used color mosaic filter is Bayer color filter array(CFA). The 2x2 cell is repeated to tile the sensor surface, effectively subsampling each of the 3 color primaries R,G, and G. The filter pattern is 50% green, 25% red and 25% blue. It favors green as does the human visual system.

350px-Bayer_pattern_on_sensor.svg 350px-Bayer_pattern_on_sensor_profile.svg

In raw file (raw output data), each pixel is filtered to record only one of three colors, 2/3 of color data is missing from each. To get a full color image, demosaicing algorithm (also called color reconstruction, CFA interpolation) is used to interpolate(bicubic, etc., this part is kind of difficult) a set of R,G,B values for each point.

Sometimes it is known as 4:2:2RGB, the following shows the Bayer pattern image sensor arrangement:

[update: 6/30/2009]#

In professional video cameras there is a prism together with 3 color filters, each with its own sensor, one channel for each of R, G, and B.

Two types of image sensors: CCD and CMOS. CCD tend to higher quality and price, while CMOS generally have lower quality but much lower price.

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