CCD Image Sensors Information
CCD (charge coupled device) image sensors are electronic devices that are capable of transforming a light pattern (image) into an electric charge pattern (an electronic image). The CCD consists of several individual elements that have the capability of collecting, storing, and transporting an electrical charge from one element to another. This, together with the photosensitive properties of silicon, is used to design image sensors. Each photosensitive element will then represent a picture element (pixel). With semiconductor technologies and design rules, structures are made that form lines, or matrices of pixels. One or more output amplifiers at the edge of the chip collect the signals from the CCD. An electronic image can be obtained by—after having exposed the sensor with a light pattern—applying a series of pulses that transfer the charge of one pixel after another to the output amplifier, line after line. The output amplifier converts the charge into a voltage. External electronics will transform this output signal into a form suitable for monitors or frame grabbers. CCDs have extremely low noise figures.
CCD image sensors can be a color sensor or a monochrome sensor. In a color image sensor, an integral RGB color filter array provides color responsivity and separation. A monochrome image sensor senses only in black and white. Choices for array type include:
- linear array
- frame transfer area array
- full frame area array
- interline transfer area array
Digital imaging optical format is a measure of the size of the imaging area. Optical format is used to determine what size lens is necessary for use with the imager. Optical format refers to the length of the diagonal of the imaging area. Optical format choices include 1/7 in., 1/6 in., 1/5 in., 1/4 in., 1/3 in., 1/2 in., 2/3 in., 3/4 in., and 1 in. The number of pixels and pixel size are important considerations. Horizontal pixels refer to the number of pixels in a row of the image sensor. Vertical pixels refer to the number of pixels in a column of the image sensor. The greater the number of pixels, the better the resolution. For example, VGA resolution is (640 x 480), this means the number of horizontal pixels is 640 and the number of vertical pixels is 480. Pixels are usually square but can sometimes be rectangular.
Important image sensor performance specifications to consider when searching for CCD image sensors include spectral response, data rate, quantum efficiency, dynamic range, and number of outputs. The spectral response is the spectral range (wavelength range) for which the detector is designed. The data rate is the speed of a data transfer process, normally expressed in MHz. Quantum efficiency is the ratio of photon-generated electrons that the pixel captures to the photons incident on the pixel area. This value is wavelength dependent so the given value for quantum efficiency is generally for the peak sensitivity wavelength for the CCD. Dynamic range is the logarithmic ratio of well depth to the readout noise in decibels, the higher the number, the better. Common features for CCD image sensors include antiblooming and cooled. Some arrays for CCD image sensors offer an optional anti-blooming gate designed to bleed off overflow from a saturated pixel. Without this feature, a bright spot, which has saturated the pixels, will cause a vertical streak. Some arrays are cooled for lower noise and higher sensitivity. An important environmental parameter to consider is the operating temperature.
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