The Foveon X3 sensor is an image sensor for digital cameras, designed by Foveon, Inc. (now part of Sigma Corporation) and manufactured by Dongbu Electronics.
It uses an array of photosites, each of which consists of three vertically stacked photodiodes, organized in a two-dimensional grid. Each of the three stacked photodiodes responds to different wavelengths of light; that is, each has a different spectral sensitivity curve. This difference is because different wavelengths of light penetrate silicon to different depths.
The signals from the three photodiodes are then processed, resulting in data that provides the amounts of three additive primary colors: red, green, and blue.
The X3 sensor technology was first deployed in 2002 in the Sigma SD9 DSLR camera, and subsequently in the SD10, SD14, SD15, SD1 (including SD1 Merrill), and the compact Sigma DP2 series from 2012, the Sigma dp2 Quattro series from 2014, and the Sigma SD Quattro series from 2016. The development of the Foveon X3 technology is the subject of the 2005 book The Silicon Eye by George Gilder.
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