Foveon X3 sensor
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The Foveon X3 sensor as used in digital cameras has a layered sensor so that at each photosite three colours are detected by the photons entering the sensor at deeper layers. The resulting image would have just one pixel for each three photosites (unless the image has been interpolated to increase resolution).
One advantage of the Foveon sensor over the more conventional mosaic sensors, such as the Bayer sensor, is that the captured image does not require anti-aliasing to remove the "coloured jaggies" characteristic to an unprocessed mosaic sensor image. This anti-aliasing step is the cause of the soft appearance of images captured with a digital camera, and the cause for most digital cameras to apply a sharpening filter to an image automatically, to add the illusion of sharpness.
Another Foveon advantage is that each pixel can potentially detect more photons, also improving sharpness. 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.
As a rough rule of thumb, an image captured with a Foveon sensor will contain about as much detail as a Bayer sensor with twice as many photosites. For example, a 3 megapixel Foveon sensor and a 6 megapixel Bayer sensor should be roughly equal in measurable detail.
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, contrary to prior statements here.
The question of whether to count 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 controvery 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 RGB pixels, is advertised as a 10.2 MP 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. The word "photosite" is also used in both senses (even in the opening paragraph of this article!).
Foveon sensors are not as widespread as mosaic sensors. They are only supplied by one company (Foveon, Inc., Santa Clara, CA, USA) and are currently used in two cameras manufactured by Sigma and will be used in a Polaroid digital camera.de:Foveon-X3-Sensor ru:Foveon