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.
