Monday, March 3, 2008
Turbo'd Image Sensors
At the heart of it all, the tiniest technology makes every picture possible
All digital cameras use one of two types of sensors: a Charge-Coupled Device (CCD) or a Complimentary Metal Oxide Semiconductor (CMOS). The older technology, CCD sensors, has been in use since the 1960s for scientific purposes, while CMOS came about in the '70s as a component in computer hardware. Ultimately, it was decades behind CCD in terms of imaging applications. CCDs had an early edge in quality and reputation among photographers, although CMOS sensors have made great strides in recent years, led largely by innovations in CMOS manufacturing for high-end digital SLRs.
CCDs had a head start in imaging applications, but advances in CMOS noise processing have made the technology viable for professional photographic applications. Today, both CCD and CMOS sensors are found in top-of-the-line digital-capture devices—from the Canon EOS-1Ds Mark III and Nikon D3 (both use CMOS) to Phase One's P45+ (which utilizes CCD). It appears that CMOS is gaining favor among D-SLR makers, however, thanks to everything from cost (CMOS sensors often are less expensive due to the economies of scale created by manufacturing versatility) to quality (demonstrated by the powerful noise reduction incorporated in the latest CMOS chips) to simple trends (more camera makers, such as Pentax, Samsung and Sony, are incorporating the ever-maturing CMOS technology into their flagship models). Both technologies will remain vital in high-quality imaging applications for the foreseeable future, and to understand why is to understand the inherent differences between the two.
The charge in a CCD, created when light photons strike each photosite, is carried off the sensor in what's commonly called a “bucket brigade” method. Imagine each pixel representing a bucket—the charge from each is passed to the next, on down the line until every last pixel's charge has been delivered step by step off the sensor for processing. The method is slow, methodical and accurate. The design is simple, making for a clear signal because on-chip electronics are minimized—meaning interference is reduced and the image-making charge is collected by highly uniform pixels and delivered relatively unadulterated from the sensor.
The drawbacks of the CCD are largely found in collateral damage. Because they operate at a higher voltage, some noise is created. The more dramatic impact, though, is that the sensor is a greater drain on power, significantly shortening camera battery life. Because the sensors have been so good for so long, they haven't required the innovations in noise reduction and general design that have occurred with the red-headed stepchild of imaging sensors, the CMOS.
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