Cheetah KAC Camera User Manual | Camera Link (CLF) Interface
October 1, 2019
Page
98
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135
Rev 7.2
lines within the image sensor (the time it takes to read out one frame of the image
sensor). In other words, the minimum exposure time is equal to the frame time. Longer
exposure times are possible, but exposures shorter than one frame time are not
supported.
Upon receiving the active edge of the trigger signal, the camera resets image sensor line
#1 and exposure of line #1 begins. One line-time later, line #2 is reset and the integration
of line #2 begins. Each line is reset one-line time after the prior line and this process
continues until the entire image sensor is reset. When the trigger signal goes inactive
(must be one frame time min. later), the integration concludes for the first line and the
image sensor is readout one line at a time. The time between trigger pulses (start of
exposure) must be at least the exposure time plus the readout time of the image sensor.
It is possible to use strobed illumination in rolling shutter mode, if the scene is completely
dark. In RS mode, the flash must occur after all the lines within the image sensor have
been reset (one frame readout time) and the exposure time must therefore be set to the
sum of the image sensor readout time plus the exposure window for the flash. The
sequence is as follows: the exposure starts and all lines in the image sensor are reset (one
frame time), the strobe illumination flashes, the exposure ends and the readout begins.
Once the readout has completed, the next exposure can begin.
Trigger Mode -- Exposure and Readout Times:
In trigger mode, the exposure time and readout time are not overlapped and occur
sequentially in time. If the period of the trigger pulse is longer than the exposure time and
the readout time combined, the camera frame rate will be the trigger rate. However, if
the exposure time plus the readout time exceed the period of the trigger pulse, then the
camera ignores subsequent trigger pulses until the exposure time and readout times are
completed (See the following figures). After the exposure and readout times are
completed, the camera accepts the next trigger pulse and the sequence repeats.
Therefore, in the case where the exposure time and readout time exceed the trigger
period, the frame time increases in increments of the trigger period.
For example, if the exposure time and readout time are both 20mS and the trigger period
is 50mS, then the frame rate is 20Hz (1 / 0.050). If the exposure time is increased to 40mS
(readout time remains 20mS), then the camera skips one trigger pulse and the frame rate
is 1 / (2 x 0.050) = 10 Hz. If the exposure time increases to 90mS, then the camera skips
two trigger pulses, and the frame rate is 1 / (3 x 0.050) = 6.6 fps. As you increase the
exposure time, the frame period increases in increments of 50mS (the trigger period).
Figure 57: Standard Trigger Mode (Internal Exposure Control).
