Practically all microprocessor and memory chips are sensitive to light and so are normally protected by light-tight covers. There are several varieties of CCD and CMOS devices used in digital cameras, and although they do not differ in the basic principle of how they work, they do vary in the way in which the information captured by the sensors is used.
In CCDs, the charges built up in each sensor must be read off serially – one-by-one. Thus, the image is effectively scanned over its whole area to retrieve the data. In addition, the reading must be taken before the sensor array is ready to make another exposure.
CMOS devices are wired differently: each sensor element can be read off individually – they are said to be X-Y addressable – and each one can be reached by giving its reference. As a result, CMOS sensors may be put to many uses – not just image capture but also exposure metering and even autofocusing.
What CCDs lack in versatility they compensate for by being far simpler and cheaper to make, while giving a clean signal that is easier to process than that from CMOS sensors. In digital cameras, however, CMOS has the advantage of working at a single, low voltage, compared with the high and varying voltages of CCD devices.
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