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PL-A780 Camera Specifications
Document No.: 04646-01
PixeLINK PL-A780
MACHINE VISION CAMERA
SYSTEM GUIDE
Copyright © 2004 PixeLINK
All Rights Reserved
3.2.4.2 Fast Reset Shutter
With Fast Reset Shutter, all rows in the ROI are reset, row-by-row, then exposed
simultaneously for a specified time. At the end of the exposure time the pixel values are read
out row-by-row. The row-by-row reset is faster than the readout process but it is not instant
or global.
It takes 3.5 µseconds per row to reset the array and rows at the top of the ROI
start integration as soon as they are reset.
For the full field of view, the reset operation will
take 11 milliseconds. Smaller ROIs or decimation will reduce the number or rows and hence,
the reset time required.
The integration time starts once the array has been fully reset. When the integration time is
complete, readout of the image data starts.
During the readout period, the pixels that
have not yet been read are still sensitive and will continue to integrate.
The readout
period can be determined from the equation in Section 3.2.3 on page 12. For the full ROI,
the readout time is 62.75 µs/row or 188.2 milliseconds. All combined, the frame interval from
trigger to completion of readout is 199.2 milliseconds plus the integration time.
Fast Reset Shutter requires a trigger event to reset the pixel data and start exposure for the
entire ROI (as opposed to Rolling Shutter, in which exposure is an on-going process). This
trigger event can be generated internally in the camera hardware, externally via the trigger
input MV connector (Section 4.6.2) or by software. The PL-A780 provides a number of
features to control the trigger timing and synchronize with external devices through General
Purpose Outputs (GPOs). See Sections 5.1.3.10 and Section 5.1.4.4 for more information.
Careful attention to the illumination is required for the best results. In Fast Reset Shutter
mode, strobe illumination should be used and the ambient light should be minimized. If using
the camera in a brightly lit environment, a mechanical shutter may be required to achieve
acceptable image quality.
To simplify the control of lighting when using the Fast Reset Shutter, the PL-A780 GPOs can
output a Flash Window that indicates the period of time following the completion of the array
reset and the start of readout. The duration of this signal is equivalent to the integration time.
This signal can be used to fire a flash and control the duration of the flash to match the
exposure time. Hence, regardless of the number of rows in the ROI, the illumination will be
correctly applied. See Section 8.2.4 on page 55 for more information
If using an external system of the GPOs in Strobe mode, the delay from the start of
integration (the trigger event with zero delay) and the start of the flash should be set to 3.5 µs
x (N + 10), where N is the number of rows in the image. The flash duration should not
exceed the exposure time. This will ensure the flash fires after the reset sequence is
complete and completes firing before readout commences.
For example, assume the PL-A780 is being used to capture full resolution images of a
moving object. The object creates a trigger as it passes a detector and the detector is placed
such that the object will be in front of the camera 20 milliseconds after passing the detector.
To freeze the action, a strobe light is set to fire with a 20 microsecond pulse when
commanded by the camera. The camera is configured for a 4 millisecond exposure time and
a hardware trigger with a trigger delay (see Section 5.1.3.11 on page 34) of 8 milliseconds.
The GPO 1 control is set to strobe mode with a delay of 20 milliseconds to allow the sensor
to fully reset before the flash fires. When the trigger arrives at the camera, the camera will
wait for 8 milliseconds before commencing the fast reset of the sensor, which will be
completed approximately 11 milliseconds later (19 milliseconds after the trigger event). GPO
1 will wait for 20 milliseconds after the trigger before it fires the flash. 4 milliseconds after the
fast reset is complete, the readout begins and takes approximately 188 milliseconds to
complete. The whole process from trigger to end of readout takes 211 milliseconds.
