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Charge-coupled device

From Academic Kids

A charge-coupled device (CCD) is a sensor for recording images, consisting of an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to one or other of its neighbours. CCDs are used in digital photography and astronomy (particularly in photometry, optical and UV spectroscopy and high speed techniques such as lucky imaging).

Contents

Operation

When a photon strikes an atom, it can elevate an electron to a higher energy level, in some cases freeing the electron from the atom. When light strikes the CCD surface, it frees electrons to move around and they accumulate in the capacitors. Those electrons are shifted along the CCD by regular electronic pulses and "counted" by a circuit which dumps the electrons from each pixel in turn into a capacitor and measures and amplifies the voltage across it, then empties the capacitor. This gives an effective grayscale image of how much light has fallen on each individual pixel.

CCDs containing a single row of capacitors can be used as delay lines. An analogue voltage is applied to the first capacitor in the array, and at regular intervals a command is given to each capacitor to transfer its charge to its neighbour. Thus the entire array is shifted by one location. After a delay equal to the number of capacitors multiplied by the shift interval, the charge corresponding to the input signal arrives at the last capacitor in the array, where it is amplified to become the output signal. This process continues indefinitely, creating a signal at the output that is a delayed version of the input, with some distortion due to sampling. A CCD used in this way is also known as a bucket-brigade delay line. This application of CCDs has now been mostly superseded by digital delay lines.

CCDs with several rows of pixels shift the charge down in the fashion of a vertical shift register and only the last row is read out in a horizontal shift register. The speed of the measuring circuit must be enough to count out the entire bottom row, then shift the rows down and repeat for every other row, until it has read the entire frame. In video cameras this entire process takes place about 40 times a second.

Several factors can affect whether a photon will cause an atom to release an electron: circuits on the CCD surface can block light from entering; longer wavelengths can penetrate certain depths of the CCD without interaction with the atoms; some shorter wavelengths may reflect off the surface, and so on.

Knowing how many of the photons which fall on the photoreactive surface will release an electron is an accurate measurement of the CCD's sensitivity. This figure is called "quantum efficiency" and is given as a percentage.

Applications

CCDs containing grids of pixels are used in digital cameras, optical scanners and video cameras as light-sensing devices. They commonly respond to 70% of the incident light (meaning a quantum efficiency of about 70%,) making them more efficient than photographic film, which captures only about 2% of the incident light. As a result CCDs were rapidly adopted by astronomers.

An image is projected by a lens on the capacitor array, causing each capacitor to accumulate an electric charge proportional to the light intensity at that location. A one-dimensional array, used in line-scan cameras, captures a single slice of the image, while a two-dimensional array, used in video and still cameras, captures the whole image or a rectangular portion of it. Once the array has been exposed to the image, a control circuit causes each capacitor to transfer its contents to its neighbour. The last capacitor in the array dumps its charge into an amplifier that converts the charge into a voltage. By repeating this process, the control circuit converts the entire contents of the array to a varying voltage, which it samples, digitises and stores in memory. Stored images can be transferred to a printer, storage device or video display. CCDs are also widely used as sensors for astronomical telescopes, and night vision devices.

An interesting astronomical application is to use a CCD to make a fixed telescope behave like a tracking telescope and follow the motion of the sky. The charges in the CCD are transferred and read in a direction parallel to the motion of the sky, and at the same speed. In this way, the telescope can image a larger region of the sky than its normal field of view.

CCDs are typically sensitive to infrared light, which allows infrared photography, night-vision devices, and zero lux (or near zero lux) video-recording/photography. Because of their sensitivity to infrared, CCDs used in astronomy are usually cooled to liquid nitrogen temperatures, because infrared black body radiation is emitted from room-temperature sources. One other consequence of their sensitivity to infrared is that infrared from remote controls will often appear on CCD-based digital cameras or camcorders, if they don't have infrared filters. Cooling also reduces the array's dark current, improving the sensitivity of the CCD to low light intensities, even for ultraviolet and visible wavelengths.

Thermal noise, dark current, and cosmic rays may alter the pixels in the CCD array. To counter such effects, astronomers take an exposure with the CCD shutter closed. This "dark frame" image is then subtracted from the original image to remove the thermal noise effects.

Color cameras

Digital color cameras generally use a Bayer mask over the CCD. Each square of four pixels has one filtered red, one blue, and two green. (The human eye is more sensitive to green than either red or blue.) The result of this is that luminance information is collected at every pixel, but the color resolution is lower than the luminance resolution.

Better color separation can be reached by three CCD devices (3CCD) and a dichroic beam splitter prism, that splits the image into red, green and blue components. Each of the three CCDs is arranged to respond to a particular color. Some semi-professional digital video camcorders (and all professionals) use this technique.

Since a high-resolution CCD chip is very expensive as of 2005, a 3CCD high-resolution still camera would be beyond the price range even of many professional photographers. There are some high-end still cameras that use a rotating color filter to achieve both color-fidelity and high-resolution. These multi-shot cameras are rare and can only photograph objects that are not moving.

Competing technologies

Recently it has become practical to create a Active Pixel Sensor (APS) using the CMOS manufacturing process. Since this is the dominant technology for all chip-making, CMOS image sensors are cheap to make and signal conditioning circuitry can be incorporated into the same device. The latter advantage helps mitigate their greater susceptibility to noise, which is still an issue, though a diminishing one. CMOS sensors also have the advantage of lower power consumption than CCDs.

See also

External links

de:Charge-coupled Device es:CCD fr:Capteur de photoscope id:CCD it:CCD (elettronica) nl:Charge Coupled Device ja:CCDイメージセンサ pl:Matryca CCD

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