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Exposure
and Readout Modes
Frame transfer cameras use frame transfer CCDs and
support Non-Overlap and Overlap exposure and readout
modes. These modes are further described in the sections
that follow.
Note:
A frame transfer CCD has both a
light-sensitive area (sensor area) and a storage area
(frame transfer area). Since the image can be quickly
transferred from the sensor area to the frame transfer
area, there is no need for a mechanical shutter.
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Non-Overlap Mode
The Non-Overlap
mode allows you to expose the array for
the exposure time specified in
the software
and is similar in performance to a normal,
full-frame device.
The operational
sequence for this mode is:
- Clearing the CCD
- Exposing for the specified exposure
time
- Shifting the image from the sensor
area to the frametransfer area
- Reading out the CCD
Steps 1-4 are repeated for each frame
in a sequence.
Steps 1 and 3, clearing
the CCD and shifting the image, are short
and do not impact the frame rate.
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Non-Overlap Mode Operation
Non-Overlap mode
is illustrated in the timing diagram below. In this
example, the exposure time is 10 ms and the readout
time is 34.8 ms. The total time to take 3 frames is
134.4 ms (3 × 10 ms + 3 × 34.8 ms), equivalent to a
frame rate of 22.3 fps (3 frames ÷ 0.134 seconds). Note:
The exposure and readout times listed are for illustration
purpose only. Actual values may vary. Refer to the product
data sheet for the actual readout times.
Referring to Figure 4, it can be seen that exposure
and readout are carried out in non-overlapped fashion.
As a result, each frame in the sequence is precisely
exposed for the time specified (i.e., 10 ms).
Overlap Mode (Simultaneous Exposure-Readout)
Overlap
mode is extremely useful in applications requiring continuous
imaging (100% duty cycle). Once a frame is exposed and
transferred into the frame transfer area, the next exposure
immediately starts and continues until the previous
frame is read out or until the exposure time is finished,
whichever is longer (so the minimum effective exposure
time in this mode is the readout time). This mode of
operation allows you to continuously image a specimen
to obtain better kinetic information about a process.
Note: In Overlap mode, the minimum effective exposure
time is the readout time.
The simultaneous exposure-readout mechanism is
illustrated with two examples.
Example 1: Overlap
Mode when Exposure Time < Readout Time
Consider
a situation where full frame readout is 34.8 ms, the
exposure time is 10 ms, and three frames are taken in
overlap mode. The first frame is exposed precisely for
the length of time entered into the software (10 ms)
and all subsequent frames are exposed for the readout
time. The total time to acquire 3 frames is then 114.4
ms (3 × 34.8 ms + 10 ms), equivalent to a frame rate
of 26.2 fps (3 frames ÷ 0.114 seconds).
Note:
Because
the first frame is exposed for 10 ms and the others
for 34.8 ms, the first frame may look less bright compared
to all other frames.
In Overlap mode when exposure time < readout time,
the total time (TN) taken to capture N frames is given
by:
TN = (tR × N) + texp
Where
TN = Total time taken to capture a sequence of N
frames'
tR = readout time for one frame
N = total
number of frames in a sequence
texp = exposure time
Example 2: Overlap Mode when Exposure Time >
Readout Time
If the exposure time is set to 50 ms with the readout
time remaining at 34.8 ms, the time taken to acquire
3 frames will be 184.8 ms (3 × 50 ms + 34.8 ms), which
is equivalent to a frame rate of 16.2 fps.In Overlap
mode when exposure time > readout time, the total
time (TN) taken to capture N frames is expressed as:
TN = (texp × N) + tR
Where
TN
= Total time taken to capture a sequence of N frames
texp
= exposure time
N = total number of frames in a sequence
tR
= readout time for one frame
From the timing
diagram, you can see that because the exposure time
is greater than the readout time, all frames are precisely
exposed for the duration entered into the software and
have similar intensities.
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