If a Pockels Cell (Beams) (or other light modulator) is available and activated, the Beam command signals generated by ScanImage during an acquisition can be configured to blank the Pockels Cell throughput during those times within each acquisition that acquired data is ignored by ScanImage:
- For each Line scanned by the Fast (X) Dimension, data collected during time outside of the scanner's Acquisition Period is ignored. (see Scan Configuration Basics).
- For each Frame scanned, acquisition during the final line may be ignored – if Flyback on Final Line is selected. (see Slow (Y) Scan Dimension Basics).
This feature is termed Flyback Blanking in ScanImage, though the blanked time comprises not only the X and/or Y dimension flyback time but also turnaround time for bidirectional scans, and Scan Delay time used for scanner settling during each Line.
The Beam command signal(s) generated during a ScanImage acquisition – one for each active beam during the acquisition as specified in the Power Controls – are controlled by two sets of settings:
- Power setting(s) on the Power Controls panel
- The Blank Flyback and Fill Frac Adjust settings on the Configuration Controls panel
The Power Controls settings simply determine the 'ON' level that is used during the acquisition. By default, the signal is simply held at this level during the entire ScanImage acquisition.
If Blank Flyback is enabled, the Beam command signal then becomes a simple square wave, as illustrated below:
The Pockels command signals (Beams) are simple square waves whose duty cycle and phase are linked to the scan command settings
- Blanking the Beam signal adjusts the power to the Beam's 'OFF' level – the output value giving rise to minimum power transmission as measured during the last Beams for that beam – outside of the Acquisition Period.
- The duration of the Acquisition Period is determined by the Nominal Ms/Line, as described in Scan Configuration Basics The Fill Frac Adjust parameter is used to extend the Acquisition Period used to compute the Beam signal. The time specified is split evenly before and after the Acquisition Period.
The Fill Frac Adjust value is constrained to be values that are even multiples of the Analog Output sampling period (1/ Output Sample Rate).
- The added time specified by the Fill Frac Adjust parameter serves two purposes:
- Allows for the Pockels Cell (or other light modulator) physical modulation time, i.e. time to 'settle' to 'ON' level
- Provides margin of error for Acquisition Delay value
- When Flyback on Final Line is enabled (see Slow (Y) Scan Dimension Settings), the final Line of each Frame is used for the Slow (Y) dimension flyback. Then the Beam command signal is held at 'OFF' level for the duration of that final Line.