Foveon X3 sensor

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The Foveon X3 sensor is an image sensor for digital cameras produced by Foveon, Inc. It is a layered sensor design, in which each location in a grid has layered photosensors sensitive to all three primary colors, in contrast to the mosaic Bayer filter sensor design commonly used in digital camera sensors where each location is a single photosensor (pixel) sensitive to only one primary color.

As of 2005, the Foveon X3 sensor is only used in the Sigma SD9 and SD10 digital SLR cameras and the Polaroid x530 compact digital camera. Another DSLR, the SD14, and another compact, the Sigma DP1, have been announced, both using the same newly announced sensor from Foveon. Both remain unreleased as of September 2006. ("Full Color Capture Direct Image Sensor") ^[1].

The Foveon X3 sensor is difficult to classify by megapixel (MP) count compared to mosaic sensor cameras, in which the location count and photosensor count are the same. The Foveon X3 sensor has three times as many photosensors as grid locations, since each location has photosensors for all three primary colors. Comparing sensors by number of locations underestimates the resolution of the Foveon X3 sensor, since no de-mosaicing interpolation is required to achieve full color information at each location. On the other hand, comparison by photosensor count over-states the resolution in comparison to a Bayer sensor. Independent comparisons of quality tend to rate the Foveon X3 sensor as having resolution equivalent to a Bayer sensor of double the location count, consistent with equal numbers of locations dedicated to luminance resolution (that is, green-sensitive photosensors).[1] [2]

Since demosaicing is not required to produce a full-color image, the color artifacts ("colored jaggies") associated with that process are not seen. Thus, the image-softening anti-aliasing required to mitigate those artifacts is not required. This in turn means that the sharpening filter which most cameras apply to the image is not necessary with the Foveon X3 sensor.

Another Foveon advantage is that more of the photons entering the camera will be detected by a photosensor (almost all of the photons rather than near one-third of them will be sensed). Each pixel of a mosaic sensor is covered by a light filter that passes only one of the primary colors, absorbing the other two. Absorbing the light destroys information about that pixel, making the image fuzzier and grainier. The Foveon's advantage is offset by the matrixing required to reconstruct color information from the sensed raw data, which reportedly results in a Foveon sensor with large photosites being unable to equal the low light performance of more conventional sensors with half the photosite area.

Prospective camera buyers need to understand the pixel counting definition for Foveon X3. The question of whether to count each location (each stack of three photosensors) as a pixel, versus counting each individual single-color photosensor as a pixel as is done in Bayer-mosaic sensors, has been a point of controversy for Foveon X3 sensors and for the specifications of cameras that use them. For example, the Sigma SD10, which produces a native RAW file size of 3.4 Million (2268×1512) RGB pixels, is advertised as a 10.2 MP (2268×1512×3) camera, sometimes with the clarification 3.4 MP Red + 3.4 MP Green + 3.4 MP Blue; an 8 MP Bayer-mosaic camera would similarly be clarified to be 2 MP Red + 4 MP Green + 2 MP Blue.

Since the depth of all three color sensing layers in the silicon crystal of the Foveon X3 sensor is only about three micrometres, the depth dimension has negligible effect on focusing or chromatic aberration. Since the collection depth of the deepest (red) sensor layer is comparable to collection depths in other silicon CMOS and CCD sensors, it does allow some diffusion of electrons and loss of sharpness in the longer wavelengths.

The development of the Foveon X3 technology is the subject of a 2005 book The Silicon Eye by George Gilder.

Regardless of the elegant design, the Foveon X3 sensor has not yet achieved great commercial success because most camera manufacturers use their own image sensors. Up to 2006, there are only five off-brand cameras employing X3 sensors.^[2]