X3 sensor
CMOS and CCD sensors generate their voltages only from the light intensity with which light falls on the individual CMOS and CCD elements. In order to achieve the highest possible light yield of the image sensor with the lowest possible image noise, the manufacturers use various processes with which the active pixel areas are optimally utilized.
These methods include making the wiring as narrow as possible at the edges or making the wiring plane independent of the exposure plane. Other methods optimize the position and distribution of the color filters in front of the image sensor. Since the image sensor cannot interpret color, the incident light must be separated into the primary colors before it hits the photodiodes. This can be done with optical filters, such as the Bayer filter, or with layered, multilayer image sensors, such as Foveon's X3 sensor, which does not use color filters.
The X3 sensor developed by Foveon Inc. takes advantage of the effect that light waves with different wavelengths have different penetration depths in silicon. Long-wavelength light penetrates deeper than short-wavelength light. If the silicon wafer is sufficiently thin, then long wavelength light can penetrate it.
The X3 sensor is therefore sandwiched from three thin CMOS sensor arrays. In the uppermost semiconductor array, short-wave blue light is absorbed and converted into voltages. The longer wavelength portion of the light beam passes through this first sensor layer and enters the second. In this, light of medium wavelength is absorbed and converted. These are the wavelengths in the green range. The long-wave red component also penetrates this layer and is converted in the third sensor layer.
The voltage values of the superimposed pixels are used to generate the voltage that corresponds to the hue and light intensity of the incident light. Since there is no optical filter in front of the sensors, the light is not attenuated. In addition, the resulting voltage does not have to be determined via interpolation, as is the case with the Bayer filter. However, the image resolution and the image noise at low ambient light are disadvantageous.