The Foveon X3 sensor is a CMOS image sensor for digital cameras, designed by Foveon, Inc. (now part of Sigma Corporation) and manufactured by National Semiconductor and 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, i.e., each has a different spectral sensitivity curve. This difference is due to that fact that different wavelengths of light penetrate silicon to different depths. The signals from the three photodiodes are then processed, resulting in data that provides the three additive primary colors, red, green, and blue.
The development of the Foveon X3 technology is the subject of the 2005 book The Silicon Eye by George Gilder.
Read more about Foveon X3 Sensor: Operation, Utilization, Comparison To Bayer Filter Sensors – Operational Differences, Comparison To Bayer Filter Sensors – Noise, Comparison To Bayer Filter Sensors – Actual Samples