Note
It takes 3.55 µsec per row to reset the array and rows at the top of the ROI start integration as soon as they are reset.
On PL-A780 Machine Vision cameras, the Fast Reset Shutter mode resets all rows in the ROI, 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.
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It takes 3.55 µsec 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 10.65 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.
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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 calculated and for the full ROI, the readout time is 62.75 µsec per row or 188.2 milliseconds. All combined, the frame interval from trigger to completion of readout is 199.2 milliseconds plus the integration time.
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Fast Reset Shutter is activated when the trigger feature is turned on. |
Strobe illumination is desired when using the Fast Reset Shutter. To simplify the control of lighting, 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 GPO Parameters Flash Window Mode 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 µsec 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.
Assume the PL-A781 or PL-A782 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 of 8 milliseconds. See section on Trigger Delay.
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.
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 connector. See section Trigger and GPOs.
The PL-A780 provides a number of features to control the trigger timing and synchronize with external devices through General Purpose Outputs (GPOs). See Trigger and GPOs for more information.
If using strobe illumination or if your object is moving, the Fast Reset Shutter mode should be used.
With constant illumination and a static object the Fast Reset Shutter is not required. Using the Fast Reset Shutter mode changes the way exposure time is calculated and may result in overexposed images if constant lighting is used. Under these conditions use Rolling Shutter. Rolling Shutter can produce higher frame rates than that of Fast Reset Shutter since exposure time occurs during row read out.
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.
