A definition of Foveon system
The Foveon X3 sensor is employed in all of Sigma’s current compact and DSLR models, and it’s notable for its unconventional design. While standard sensors require a filter over each photosite to determine colour, the X3 has instead layers of silicon which absorb different wavelengths of light to different depths. Shorter wavelengths (such as those associated with blue) are absorbed closer to the surface while longer wavelengths (greens and reds) are absorbed further along
This process means that neither an array of coloured filters nor the subsequent demosaicing process are necessary. Foveon claims that this system has a number of advantages over more conventional sensor designs, such as better colour accuracy and detail retention, as well as fewer artefacts and lower noise.