CMOS Camera Modules for OEM and Embedded Integration

Pixel Size and Photon Collection

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A pixel collects light in proportion to its area. Pixel size therefore sets the ceiling on how much signal is available before any amplification, which is why a lower resolution sensor of the same physical size often outperforms a higher resolution one in dim conditions: fewer, larger pixels each gather more photons. The AR2020 and AR0544 both use 1.4 micron pixels, the AR0821 uses 2.1 micron pixels, and that difference is the reason the AR0821 reaches wider dynamic range at lower resolution. Resolution and sensitivity trade against each other on a fixed die, and a CMOS camera module should be chosen with that trade explicit rather than assumed away. 

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Backside Illumination

In a conventional front-illuminated sensor the metal wiring sits above the photodiode and blocks part of the incoming light. A backside illuminated sensor moves that wiring behind the photodiode so more of each pixel is exposed. A BSI CMOS sensor therefore collects substantially more light per pixel at the same pixel size, which is the single largest architectural gain available in low light imaging. The AR2020 and AR0821 use BSI structures, as do the Sony STARVIS and STARVIS 2 parts in the Merlin and Innova ranges. 

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Global Shutter and Rolling Shutter 

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A rolling shutter CMOS sensor reads the array line by line, so different rows are exposed at slightly different moments. Anything moving during that interval appears skewed, and a fast enough subject can be distorted beyond use. A global shutter CMOS sensor exposes every row simultaneously and reads them together, removing the effect entirely at the cost of a more complex pixel and usually lower resolution for the same die area. The global shutter parts here are the onsemi AR0234 and AR0235 at 2.3MP, the Sony IMX900 at 3.2MP and the Sony IMX568 at 5MP. Everything else in the range is rolling shutter, which remains correct wherever the scene is static at the moment of capture. 

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Color, Monochrome and Bayer Raw

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A color CMOS sensor carries a Bayer color filter array, one filter per pixel in a repeating red, green and blue pattern. That means each pixel measures only one color and the other two are interpolated, so a color sensor delivers less true spatial detail and less sensitivity than the same sensor without the array. A monochrome camera module removes the filter entirely, raising effective resolution and sensitivity wherever hue carries no information. A Bayer raw camera goes a third way: it keeps the filter but sends the unprocessed sensor output, leaving demosaic, white balance and tone mapping to the host.