A CCD or Charge-Coupled Device is an electronic device that can be used as the image sensor in place of film in a digital camera, or in devices like microscopes, telescopes, and scanners. CCDs are also used for certain autofocus and light metering systems in film or digital cameras. A CCD is a rectangular semiconductor chip with a surface built of an array of light-sensitive areas (pixels). Each pixel is a kind of combination of photo diode for light-sensitivity and capacitor for analogue value storage. The charge levels for each pixel are read out sequentially and digitized by separate circuitry.
A different semiconductor technology used in sensors is known as CMOS. As this is more adaptable to providing video as well as still images, by about 2010 CMOS had come to dominate the consumer marketplace. CCDs remain the preferred technology for certain uses, such as scientific imaging. The 570-megapixel Dark Energy Camera uses specialized Dalsa CCDs for its astronomical observations.
The CCD was invented in 1969 at Bell Labs by Willard Boyle and George Smith. Since that CCD was sensitive to light it was used as image sensor for video and TV cameras. It could also be used as a memory device or analogue signal delay since it's principally a kind of analogue shift-register circuit. Thus the image information given in parallel to the whole pixel matrix during exposure is read sequentially by the camera's image processor after exposure.
In 1972 semiconductor maker Fairchild introduced the first experimental CCD for photography which made exposures of the format 100×100 pixels. Production was launched in 1974. In 1986 Kodak introduced the first 1 Megapixel CCD.
|3.2 megapixel CCD device of a Canon PowerShot A75 digital compact camera|
image by Andrea Fistetto (Image rights)
The sensitivity to light of the sensor controls the ISO range that the camera can emulate. More sensitive sensors can also permit greater pixel bit widths, and so give better colour rendition.
The physical size of the sensor is important in several respects, for example:
- Larger sensors perform better at lower light levels - larger pixels can gather more photons at any given light level - so allowing lower-light photography and generally reducing noise.
- Just like larger film formats, larger sensors require longer focal-length - and hence larger - lenses, thus dictating the size (and weight) of the camera. Larger lenses may also increase the cost of the camera. By contrast, smaller sensors and hence shorter lenses can have their own problems; very short length lenses can restrict the use of very small apertures - e.g. a 5mm lens stopped to f16 would have an aperture of only 0.3125mm; such small apertures can be hard to control accurately and can be subject to diffraction problems.
- Larger sensors are likely to use more power - increasing battery requirements.
Many of the big camera makers or consumer electronics companies have their own CCD production facility, for example Sony and Fujifilm. Kodak was originally a leading company in sensor technology, providing high resolution CCDs for the likes of Leica. During Kodak's collapse the CCD division was spun off under the name Truesense Imaging; and in 2014 this was acquired by ON Semiconductor. Other companies specialise in making image sensors, among them Omnivision and Dalsa. Dalsa already offers a low-noise still camera CCD with 33 megapixels, still based on the above-mentioned Bayer pattern.