I've been following the development of the unique Foveon X3 sensor since its first appearance in the Sigma SD9 back in 2002. Sigma now owns the Foveon company, and has continued to use this innovative technology in its cameras, including the DP2s compact camera which I reviewed back in June, as well as Sigma's most recent DSLR the SD15.The Foveon X3 sensor works in a different way to the standard CCD and CMOS sensors found in every other digital camera.
Conventional camera sensors only record brightness, relying on a red, green and blue mosaic filter to provide colour information, and interpolating the full colour image from the results. The X3 sensor records full RGB colour at every photocell site on the sensor, thanks to three photocells embedded at different depths in a silicon wafer. You can find a fuller explanation with a diagram here. This difference causes some confusion when it comes to describing the true resolution of the sensor. Since it contains approximately 14 million individual photocells Sigma insists on calling it a 14 megapixel sensor. However the final image size is only 2640 x 1760 pixels, or 4.64 megapixels.
This lack of final resolution is also hard to ignore when one considers the price of the SD15. It is currently selling for £789 body only, which is very expensive compared to other digital SLRs. For the same price you could get an 18 megapixel Canon EOS 550D with an 18-135mm lens, a 14.6MP Pentax K-7 with a weather-resistant 18-55mm kit lens, or a Nikon D90 with an 18-105mm VR lens.