Foveon X3 sensor

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 sensor was first deployed in 2002 in the Sigma SD9 DSLR camera, and most recently in Sigma SD14 which was released in 2007 and Sigma DP2 from 2012. The development of the Foveon X3 technology is the subject of the 2005 book The Silicon Eye by George Gilder.

The diagram to the right shows how this works in graphic form. Depicted on the left is the absorption of colors of the spectrum according to their wavelength as they pass through the silicon wafer. On the right, a Foveon X3 layered sensor stack in the silicon wafer for each output pixel is shown depicting the colors it detects at each absorption level. The color purity and intensity of blue, green and red depicted for the sensors are for ease of illustration. In fact, the attributes of each output pixel that are reported by a camera using this sensor result from the camera's image processing algorithms that employ a matrix process to construct the single RGB color from the data sensed by the photodiode stack. The results, in terms of color accuracy (metamerism index), were state of the art at the time of their invention.

Because the depth in the silicon wafer of each of the three layer Foveon X3 sensors is less than five micrometres, it has negligible effect on focusing or chromatic aberration. However, because the collection depth of the deepest sensor layer (red) is comparable to collection depths in other silicon CMOS and CCD sensors, some diffusion of electrons and loss of sharpness in the longer wavelengths occurs.

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