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42 PI-MAX
®
3 System Manual Version 1.A
Full Frame CCD Readout
The PI-MAX3:1024x256 camera
uses a full frame CCD for data
acquisition. The upper left drawing
in Figure 11 represents a CCD after
exposure but before the beginning
of readout. The capital letters
represent different amounts of
charge, including both signal and
dark charge. This section explains
readout at full resolution, where
every pixel is digitized separately.
Readout of the CCD begins with the
simultaneous shifting of all pixels
one row toward the "shift register,"
in this case the row on the bottom.
The shift register is a single line of
pixels along the edge of the CCD,
not sensitive to light and used for
readout only. Typically the shift
register pixels hold twice as much
charge as the pixels in the imaging
area of the CCD.
After the first row is moved into the
shift register, the charge now in the Figure 11. Full Frame CCD Readout
shift register is shifted toward the output node, located at one end of the shift register. As
each value is "emptied" into this node it is digitized. Only after all pixels in the first row
are digitized is the second row moved into the shift register. The order of shifting in our
example is therefore A1, B1, C1, D1, A2, B2, C2, D2, A3....
After charge is shifted out of each pixel the remaining charge is zero, meaning that the
array is immediately ready for the next exposure.
Below are the equations that determine the rate at which the CCD is read out.
The time needed to take a full frame at full resolution is:
c R
t t t
 
exp
(1)
where
t
R
is the CCD readout time,
t
exp
is the exposure time, and
t
c
is the phosphor decay time.
The readout time is approximately given by:
(2)
 
   
i x v sr
y x R
t N t t N N t
 
  