Tuesday, November 24, 2009
Misinformation: Sensor Tech
There are fundamental differences between CCD and CMOS, but both are capable of pro-quality imagery
|An image sensor wafer during the fabrication process. CCD or CMOS? Both have plusses and minuses.|
As is the habit in technology, there once was a war brewing between two competing technologies, CCD and CMOS sensors. Unlike the Betamax vs. VHS and HD DVD vs. Blu-ray wars, however, there has been no clear outcome in the battle for supremacy in camera sensors. Both CCD (charge-coupled device) and CMOS (complementary metal oxide semiconductor) sensors work in the same intrinsic manner. They convert photons of light as they strike the sensor into electrical charges, which are then read from the chip as digital information. The way in which they perform said function is how they differ, and once upon a time, there were notable advantages and disadvantages to both.
CMOS sensors utilize a charge-to-voltage conversion at each individual pixel for individualized data at the pixel level, which can involve amplifiers, noise correction and digitization circuits. CCD sensors transport information across the chip to an analog-to-digital converter via a limited number of output nodes, sometimes even just one. Values are determined here from the analog signal and converted into digital information. In CMOS sensors, these variety of components required reduce the real estate of the sensor, which results in a minimized area for light-gathering when compared to a CCD sensor. CCD sensors use high-quality, specialized manufacturing that can transport charges across the chip with minimized distortion and uniform readouts taken from the sensor as a whole. Thanks to a sensor array that isn’t obstructed by the numerous transistors that populate a CMOS sensor, a CCD sensor also minimizes light falloff between photodiodes for higher light sensitivity overall.
Myth: CCD vs. CMOS: Only one can be bestCMOS sensors take advantage of these various components, however, to utilize their innate ability for combining image sensor functions and image processor functions on the same chip. Sony’s DSLR-A900 and A850, for instance, perform on-chip analog noise reduction, which is then followed by BIONZ image processing for noise reduction that’s done digitally, which results in faster image processing without a loss to image sharpness. CMOS sensors also are usually credited as having lower power consumption, which results in extended battery life on a camera, although CMOS also requires additional components, which can offset the power savings on the chip.
In the early days of manufacturing, CMOS sensors also were considered less expensive to produce than CCDs. With manufacturing processes similar to commonly produced microprocessors, they can be made on standard silicon production lines. This is one of the reasons that D-SLR prices have fallen, compared to what digital SLR technologies cost. Now that both technologies are commonly found in cameras, and quality imaging from CMOS chips requires extra components when compared to CCD sensors, costs have balanced out and are now more or less similar at the chip level.
Traditionally, choosing one or the other was a choice between image quality and device cost. Now that digital photography has matured, neither technology, at this point, is the clear forerunner when it comes to image quality. It’s purely a matter of how a manufacturer will employ the advantages and mitigate the disadvantages. The entire technology of the camera, from the front lens element all the way through the pipeline to the capture medium, must be perfected to provide a perfect image. CCD and CMOS are both capable of providing excellent results, which is why both sensor types still are being actively produced, and many D-SLR manufacturers, such as Canon and Nikon, will choose a sensor type that will work best in a camera based on the unique needs of that D-SLR model. So while there may be no clear victor in the war between CCD and CMOS, the winner is us, because we’ve reaped the benefits of two technologies caught in a bitter battle toward perfection.