Foveon X3 sensor
In the late 1970s, a similar color sensor having three stacked photo detectors at each pixel location, with different spectral responses due to the differential absorption of light by the semiconductor, had been developed and patented by Kodak.
The image on the right shows a layered sensor stack depicting the colors it detects at each absorption level for each output pixel.
[2] The depth of the silicon wafer in each of the three sensors is less than five micrometers that creates a negligible effect on focusing or chromatic aberration.
[5] It used a 20.7 × 13.8 mm, 2268 x 1512 × 3 (3.54 × 3 MP) iteration of the sensor and was built on a Sigma-designed body using the Sigma SA mount.
Constructing a full-color image from a Bayer sensor requires demosaicing, an interpolative process in which the output pixel associated with each photosite is assigned an RGB value based in part on the level of red, green, and blue reported by those photosites adjacent to it.
The separate anti-aliasing filter[14] commonly used[n 1] to mitigate those artifacts in a Bayer sensor is not required; this is because little aliasing occurs when the photodiodes for each color, with the assistance of the microlenses, integrate the optical image over a region almost as big as the spacing of sensors for that color.
However, it has been advertised as a 10.2 MP camera by taking into account that each photosite contains stacked red, green, and blue color-sensing photodiodes, or pixel sensors (2268 × 1512 × 3).
The actual resolution produced by the Bayer sensor is more complicated than the count of its photosites, or its native file size might suggest; the demosaicing and the separate anti-aliasing filter are both commonly used to reduce the occurrence or severity of color moiré patterns that the mosaic characteristic of the Bayer sensor produces.
The effect of this filter blurs the image output of the sensor which produces a lower resolution than the photosite count would seem to imply.
[18] Subsequent X3-equipped cameras have less aliasing because they include micro-lenses, which provide an anti-aliasing filter by averaging the optical signal over an area commensurate with the sample density.
For example, Mike Chaney of ddisoftware says "the SD14 produces better photos than a typical 10 MP DSLR because it is able to carry sharp detail all the way to the 'falloff' point at 1700 LPI, whereas contrast, color detail, and sharpness begin to degrade long before the 1700 LPI limit on a Bayer based 10 MP DSLR.
[30] Sigma's SD14 site has galleries of full-resolution images showing the color produced by the Foveon technology.
Second stage prototyping in this case is the evaluation of a small image sensor prototype with the same pixel size as the product specifications but with a reduced total pixel count to verify the performance characteristics of the image sensor in practice.
Third stage prototyping will evaluate a full-frame image sensor with the same specifications as the mass production devices including the AD converter etc.
