charge coupled device (CCD)
Charge Coupled Devices(CCD) are charge-coupled semiconductor arrays that convert light signals into electrical signals. CCD components are used as image sensors in digital cameras, digital video cameras, camcorders and other optical acquisition devices such as scanners and X-ray detectors.
Structure of a Charge Coupled Device
A CCD element is a semiconductor array of photodiodes, tiny MOS capacitors, field-effect transistors, control lines and conduction paths. When light falls on the photocell, it generates an electron charge that increases linearly with the amount of light, i.e., light intensity and exposure time. The electron charge is temporarily stored as an instantaneous charge image in a potential well. If this is charged, electrons can also flow off into the potential wells of the neighboring pixels. This disturbing effect is called blooming.
The total charge image of the CCD sensor thus corresponds to the light intensity at each individual pixel. Before the voltage image for the projected light is generated from the charge states of the pixels, the charge states are read out from the charge sinks according to certain schemes, transferred to the outside and processed there.
Charge Transfer Principle
The name Charge Coupled Device (CCD) indicates that it is a device with charge coupling. When the electrical charge is read out from the many potential wells, the charges are shifted from one to the next, each triggered by the pixel clock.
This method is also called chain memory or bucket brigade circuit. The charge of each individual smallest capacitance is thus transported to the end of the line one after the other and is available there as a charge image for further processing. The charge is transferred via field-effect transistors( FET), through which the potential wells, which act as transfer registers, are interconnected. During readout, no light may fall on the CCD sensor, as this would change the charges on the transfer registers. For this reason, there are different readout processes that address the charge problem differently: The Full Frame CCD( FF-CCD), Frame Transfer CCD( FT-CCD), Interline Transfer CCD( IT-CCD) and Frame Interline Transfer CCD( FIT-CCD). In general, during the readout process, the charges are shifted from pixel to pixel in rows and fed serially to the readout amplifier at the end of the row. This charge shift is subject to low losses, which are expressed in the Charge Transfer Efficiency( CTE).
The most important characteristic values of CCD sensors
CCD technology is characterized by enormous spectral light sensitivity and high resolution, allowing charge images to be produced even at low brightness. The image resolution is characterized by a dynamic development. Thus, there are already consumer cameras with 10 megapixels( MP) and more. Technically, CCD sensors with 100 megapixels have already been developed. Another aspect is the high dynamic range, which is in the range of 1:10,000, representing over 10,000 brightness levels. This corresponds to a quantization of `2^13` and higher. The chip sizes of CCD sensors range from a few millimeters to the 35 mm format familiar from 35 mm cameras.
Since CCD sensors are only sensitive to brightness, but not to color, the light signals for color capture must be filtered into the primary colors red, green and blue before being converted via color filters, a Bayer filter or an interference filter. With the Bayer filter, each individual CCD pixel is preceded by its own color filter. Since the color filters reduce the physical resolution of the image sensor, each pixel of the sensor is assigned its own RGB value by interpolating the brightness values of the neighboring pixels. The Foveon X3 sensor offers an alternative technique for color separation.
To increase the light sensitivity and improve the image quality of CCD sensors, there are various developments such as the Super CCD sensor, which has octagonal pixels, or the rotation pixels by 45 degrees. In the ClearVid sensor from Sony, the EXR sensor from Fujifilm and the Foveon X3 sensor, other pixel arrangements are realized. In the EXR sensor, the arrangement is such that two adjacent pixels can be combined using pixel binning.