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Digital Sensors

Introduction:

CMOS and CCD digital imaging sensors (other than the Foveon chip) use the same principle to produce an image. These sensors are found in all digital cameras and as time goes on are getting cheaper and cheaper to produce.

Chip Design:

The idea behind both CMOS and CCD chips are the same. Each chip consists of a grid of pixels, for example a 6 megapixel camera would have a chip that is 3000 x 2000 pixels wide. The chip has two layers. The bottom layer is composed of photosites that capture photons for that pixels information. The layer above the photosites is composed of a mosaic with alternating colors. It looks like this. Each mosaic piece sits directly above one photosite. What this filtering means is that the photosite that lies underneath can only capture the color information for that specific color.

Implications:

This layout of a colored mosaic is call a Bayer pattern. The reason for this arrangement is due to the way our eyes perceive light. The human eye is most sensitive to green light so it is overrepresented in this pattern. There are some drawbacks to using the Bayer filter. Because not every pixel receives all color information, the camera processor has to guess at what the color should be based on the surrounding pixels. Algorithms built into the processor can do a fairly good job of guessing. The more optimized the processor, the more accurate the colors produced by the camera. (It should be worth mentioning here that the processing of the sensor data only applies to JPEG images. If you are shooting RAW, then all the pixel data processing is done on your computer.) Also due to the Bayer interpolation layer, the chip only captures 50% of the green light and 25% of red and blue in the scene. This reduces the sensitivity of the chip and requires much more light to produce an image. As technology progresses and the processors and algorithms used to produce an image improve, so will the resulting photos in terms of color accuracy. Cheers.