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CCD
and CMOS Imager Architectures
Introduction
Charge Coupled Device (CCD)
detectors come in three major readout architectures,
Full Frame (FF), Frame Transfer (FT) and Interline (IL).
Each of these formats has certain advantages as well
as limitations that will be considered here. This discussion
will focus on the most common forms of these CCD types
which are single output devices that can be run as low
noise CCD detectors.
The Full Frame CCD
The Full Frame CCDs
are devices in which the total area of the CCD is available
for sensing incoming photons during the exposure period.
During readout of the CCD, charge is shifted sequentially
across the array necessitating the use of a shutter
to prevent smearing for almost all exposure lengths.
(Technically,if the exposure time is much longer than
the actual readout rate, then the level of smearing
can be quite small.) This format has 100% fill factor,
which means that 100% of each pixel area is being utilized
to detect photons during the exposure.
The frame transfer CCD
The frame transfer
CCD imager has a parallel register divided into two
distinct areas. The upper area is the image array, where
images are focused and integrated. The other area, the
storage array, is identical in size and is covered with
an opaque mask to provide temporary storage for collected
charge. After the image array is exposed to light, the
entire image is rapidly shifted to the storage array.
While the masked storage array is read, the image array
integrates charge for the next image. A frame transfer
CCD imager can operate continuously without a shutter
at a high rate.
The Interline Transfer CCD
The interline
transfer CCD has a parallel register that has been subdivided
so that the masked storage area fits between columns
of exposed pixels. The electronic image accumulates
in the exposed area of the parallel register, just as
it does in the frame transfer CCD. At readout, the entire
image is shifted under the interline mask. The masked
pixels are read out in a fashion similar to the full
frame CCD.
Other CCD Architectures
Spectral
Framing
 In
a mode particularly suited for spectroscopy, the CCD
is masked so that only a single row of the parallel
register is exposed. In this mode, one dimensional line
images can be acquired at very high speed until the
parallel register is filled up. Spectral framing CCDs
are used in time resolved spectroscopy. An observation
could consist of hundreds of individual spectra, distributed
over time.
Fast Framing Mode
The frame transfer concept
can be extended to multiple frames by masking most of
the parallel register and using only a small region
as the image array. A scene is focused on the image
array and a high speed shutter or strobe light is used
to time the exposure. After each exposure, charge from
the image array is quickly shifted under the mask and
a new image can be acquired. Once the parallel register
is filled with images, it is read out. Because fewer
rows are clocked to shift the image array into storage,
this mode works much faster than standard frame transfer.
Time Delay Integration
Time delay integration
(TDI) is an integration and readout mode which allows
the acquisition of long swaths of a moving imaged. For
example, a moving image is focused on an unshuttered
CCD imager. The parallel register is clocked in step
with image motion, so that charge packets always correspond
to the same image region as they move across the parallel
register. Charge accumulates and signal strength increases
as the pixels approach the serial register. When pixels
reach the serial register, they are transferred out,
digitized, and stored in the normal fashion. The exposure
time for each pixel is exactly the length of a full
parallel shift sequence, which is determined by the
velocity of the scene. Compared to a simple exposure,
TDI increases sensitivity in proportion to the number
of rows in the parallel register.
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