Wednesday, May 30, 2007
All About Image Sensors
At the heart of every digital camera is an electronic marvel
The term charge-coupled device comes from the method in which the pixels are linked together. The charge is moved off the CCD much like an old-fashioned “bucket brigade,” whereby water can be transported a long distance passed hand to hand. In the CCD, the individual photosites are the buckets and the charge from one is passed on to the next in line until each is finally collected for readout at the sensor's edge.
Because of their relatively simple design and clean signal, CCDs were the early sensor leader and were constantly compared to film. In fact, they're very efficient in that regard, as a typical CCD responds to 70 percent of the light reaching it. Photographic film can only capture about two percent of that light.
The “bucket brigade” transfer method means there's little interference from other electronic components, so CCDs inherently produce low noise in low-gain situations. Noise is any pixel that misinterprets a scene in an unintended way, and it's commonly caused by heat from electronics or commingling of charges. Lack of additional photodiodes and amplifiers on the sensor tends to give CCDs higher pixel uniformity too, which provides more consistent light sensitivity across the chip. But the charge transfer is never perfect, so some electrons are inevitably lost between capture and readout. Pixel failure and higher operating voltage contribute to CCD noise.
Richard Turner, Vice President of Product Marketing and Applications for chipmaker Foveon, acknowledges that CCDs initially did have an image quality advantage. But at this point, he says, “This is really no longer true. CMOS sensor manufacturers have adopted a CCD-like pixel. The newer CMOS sensors that use this technology are every bit as good as CCDs with respect to noise. CCD had the benefit of 30-plus years of development ahead of CMOS, so it took a little time for CMOS to reach the same level. Most industry experts agree that we have reached the parity point on performance, and the traditional advantages of CMOS are all still true. For this reason, you see most digital-imaging applications migrating to CMOS. The last to do this will be the compact point-and-shoot camera market.”
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