Wednesday, May 30, 2007
All About Image Sensors
At the heart of every digital camera is an electronic marvel
Basic Background Information
There are two primary types of sensors used in digital cameras today: CCD (Charge-Coupled Device) and CMOS (Complimentary Metal Oxide Semiconductor). Often thought of as 21st-century innovations, they were actually developed in the 1960s and immediately shaped the digital revolution—CCDs in astronomical and scientific uses and CMOS in everything from cell phones to computer RAM.
Approximately 40 companies make CCDs, and just as many manufacture CMOS. Some of the more familiar names include Foveon, Fujifilm, Kodak, Panasonic and Philips, which makes sensors for companies like Hasselblad, Leaf, Leica and Phase One. Sony's sensors can be found in Sony cameras, as well as those from Canon, Nikon and others. Canon also manufactures sensors for its own SLRs.
Image sensors essentially work like solar cells. Light photons are captured by individual electrodes (also called photosites and pixels) on the sensor and converted into an electronic charge, which is converted into voltage, amplified and transferred off-chip for processing. Both CCD and CMOS sensors do this, but they differ in the methods used—namely, how and when the charge is converted to voltage and transferred. The charges on a CCD are transported across and off the sensor before they're converted to voltage and amplified, whereas for CMOS, conversion and amplification happen on the chip itself.
This basic difference shaped the entire course of digital camera development. Because CCDs don't include the on-chip electronics of CMOS, they quickly gained the reputation of providing better image quality. But because CMOS can do more on-chip than CCD, the image processing is easier and faster. Designers realized that these processes translated into real-world issues for photographers, so they invested considerable effort into making CCDs consume less power and operate quicker, while improving the image quality of CMOS designs. Thanks to years of innovation, technology has advanced to the point where each delivers exceptional image quality and functionality.
Chuck Westfall, Canon's Director of Media and Customer Relationship, explains that sensor differences go beyond electronics. “Broadly speaking,” he says, “there are technical differences and there are market share differences. CCDs are still dominant in the compact digital camera and video camcorder categories, which account for over 95 percent of all digital cameras sold worldwide. CMOS has made its biggest impact in digital SLRs.”
Nikon uses the CCD as the sensor of choice in most of its digital cameras, with the exception of the D2 models, which incorporate a Sony-made CMOS sensor. Westfall believes, though, that there's an industry-wide trend away from CCD. “Sony has announced their intention to migrate their entire product line to CMOS,” he says, “and now that Canon has increased its CMOS manufacturing capacity, it seems clear enough that Canon will eventually expand its use of CMOS image sensors across its own product lines.”
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