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CMOS Image Sensor Characteristics
There are number of phrases and terms for describing the functional capability, physical features or competitive characteristics of an Imager:
1. Active Pixel Sensor (also APS)
An active CMOS Imager pixel has its own amplifier for boosting the pixel's signal. Active Pixels are the dominant type of CMOS Imagers in the commercial market today. The other type of CMOS Imager, a passive pixel sensor (PPS), consists of only the photo detector without a local amplifier. While very sensitive to low light conditions, these types of sensors are not suitable for commercial applications due to their high amount of noise and poor picture quality when compared to active pixels.
2. Fill Factor
The amount of a CMOS Pixel that is actually capturing light. In an active pixel, both the photo detector and the amplifier take up "real estate" in the pixel. The amplifier is not sensitive to light, so this part of the pixel area is lost when taking a picture. The fill factor is simply the percentage of the area of the pixel that is sensitive to light. In the picture above, this is about 40%. As semiconductor process technologies get smaller and smaller, the amount of area taken up by the amplifier is taking up less space, so low fill factors are becoming less of an issue with active pixels. Note that in passive pixels - where there is no amplifier at all - fill factors typically reach over 80%. The reason they do not reach 100% is due to routing and pixel selection circuitry that are also needed in a CMOS imager.
3. Microlenses
In some pixel designs, the fill factor becomes too small to be effective. For example, if a fill factor in an imager was 25%, this would mean that 75% of the light falling on a pixel would be lost, reducing the pixel's capability. To get around this situation, some CMOS imagers have small lenses manufactured directly above the pixel to focus the light towards the active portion that would otherwise fall on the non-light sensitive portion of the pixel. Microlenses typically can increase the effective fill factor by two to three times.
4. Color Filter Array (also CFA or just "color filter")
CMOS Pixels are sensitive to light photons but are not, by themselves, sensitive to color. Unaided, the pixels will capture any kind of light, creating a black and white image. In order to distinguish between colors, filters are put on top of a pixel to allow only certain colors to pass, turning the "rods" of the array into "cones". Since all colors can be broken down into an RGB or CMYk pattern, individual primary or complementary color schemes are deposited on top of the pixel array. After being read from the sensor, software takes the different values of the pattern and recombines the colors to match the original picture. There are a variety of different filters, the most popular being the Bayer Filter Pattern (also known as RGBG). Note the large amount of green in the pattern, due to the fact that the eye is most sensitive to color in the green part of the spectrum.
Bayer Color Filter
Pattern
