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Chapter 6 Exposure and Readout 59
The time needed to take a full frame at full resolution is:
t
R
+ t
exp
+ t
c
(1)
where
t
R
is the CCD readout time,
t
exp
is the exposure time, and
t
c
is the shutter compensation time.
The readout time is approximately given by:
t
R
= [N
x
· N
y
· (t
sr
+ t
v
)] + (N
x
· t
i
) (2)
where
N
x
is the smaller dimension of the CCD
N
y
is the larger dimension of the CCD
t
sr
is the time needed to shift one pixel out of the shift register
t
v
is the time needed to digitize a pixel
t
i
is the time needed to shift one line into the shift register
t
s
is the time needed to discard a pixel
The readout times for a number of different CCD arrays are provided in Table 4 below.
CCD Array 1 MHz Readout Time (t
R
)
EEV CCD-37 512 x 512 0.28 sec.
Kodak KAF-0400 768 x 512 0.5 sec.
Kodak KAF-1400 1317 x 1035 1.5 sec.
Table 4. Approximate Readout Time of a Single Frame for Some CCD Arrays
A subsection of the CCD can be read out at full resolution, sometimes dramatically
increasing the readout rate while retaining the highest resolution in the region of interest
(ROI). To approximate the readout rate of an ROI, in Equation 2 substitute the x and y
dimensions of the ROI in place of the dimensions of the full CCD. Some overhead time,
however, is required to read out and discard the unwanted pixels.
Image Readout with Binning
Binning is the process of adding the data from adjacent pixels together to form a single
pixel (sometimes called a super-pixel), and it can be accomplished in either hardware or
software. Rectangular groups of pixels of any size may be binned together, subject to
some hardware and software limitations.
Hardware binning is performed before the signal is read out by the preamplifier. For
signal levels that are readout noise limited this method improves S/N ratio linearly with
the number of pixels grouped together. For signals large enough to render the camera