To carry out the read out from the CCD array, three architectures have been utilized:
1. Full Frame Read Out: The entire CCD array acts as an active area. This type of device uses mechanical shutter mechanism to stop light from reaching the element to avoid smearing when the charges are passed down the Vertical-CCDs in parallel and later serially moved out using the Horizontal-CCDs. This process is quite time consuming.
2. Frame Transfer: Half of the contiguous array area is used for exposure and the remaining half is opaque. The charges are transferred from the active area to the opaque area in very less time and then are read out from there. This process is faster than the full frame read out as during the time charges are being read out of the opaque area, the active area can be used for capturing new image. It also has an advantage of reduced smearing and light contamination while charge transfer. However, this has a disadvantage of using double the silicon area.
3. Interline Transfer: In this type of architecture, each pixel has an active area and an opaque area adjacent to it. The charge is quickly transferred from the light sensitive Photodiode to the adjacent Vertical-CCD unit. This uses only one transfer cycle to hide the entire image thus allowing very high shuttering speeds and minimum smear. This too has the disadvantage of increased silicon estate, but modern advancements have tried to increase the quantum efficiency of the array by using microlenses which redirect the light away from the opaque regions. The use of microlenses has increased the fill factor to about 90% of the other architectures without compromising on the speed.
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