What Does CCD Stand for in Cameras?by Scott Shpak
The sensor inside a digital camera is a fascinating device, holding millions of tiny photosites, the individual light-sensitive diodes that collect image data. Each of these defines a picture element and, together with all pixels generated from the image sensor, comprise a digital photograph. Image sensors come in two common constructions; the charge-coupled device and the complementary metal oxide semiconductor. CCD and CMOS sensors work in different ways; each have their own strengths and weaknesses.
Image sensors convert the activity of light photons into a small electrical charge, a process known as the photoelectric effect. In a CCD chip, the charge from each pixel is transferred off the sensor through one or two output nodes, where these charges are converted to voltage and buffered before being transferred off the sensor chip as an analog signal, where later stage processing digitizes the photograph's information. Each photosite on a CCD chip does nothing but collect light, while CMOS sensors process information and convert to digital at each photosite, limiting the space devoted to light collection.
A key advantage to the CCD image sensor is its ability to use a global shutter, exposing all pixels to the scene of a photograph at the same time. In many regards, this permits the CCD sensor to produce results similar to film. For example, if a photographer is panning to photograph a racing cyclist, the background shows blur on both film and CCD sensors, while a CMOS sensor, using a rolling shutter that exposes pixels sequentially, stretches the image with panning motion.
Compared to CMOS sensors, CCD chips excel at light collection but don't do much with the data apart from offloading it from the image sensor chip. Since the data from a CCD sensor is limited to two readout channels, data exchange is much slower than CMOS, which not only converts pixels to digital form, but can output over many readout channels. Think of a readout channel as a faucet. With one or two taps, the CCD can fill a sink reasonably fast. However, with 8 or 16 taps, the CMOS sink fills in an instant.
Other CCD Tidbits
CCD sensors can exhibit a unique trait with bright points of light overwhelming an entire column of pixels, an effect called smearing. Similar to lens flare in appearance, it is a flaw that may put to use creatively, or it may be minimized with camera technique. CMOS sensors are immune to smearing. CMOS technology has been improved very quickly in recent years, due to its use in smartphone cameras. However, properties of CCD sensors cover applications that CMOS sensors cannot, such as infrared and ultraviolet photography.
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