This documentation is for a legacy ScanImage version. The current documentation is ScanImage 2019.

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The POWER CONTROLS panel pertains to the (optional) control of a voltage-controlled light modulator (typically a Pockels cell), allowing the power level used during image acquisition to be controlled.


The POWER CONTROLS panel only appears on ScanImage startup if at least one beamChanID is specified in the Machine Data File.

Power Controls

Slider control of the power level ScanImage should use during subsequently initiated image acquisitions.

Slider adjustments to power can be made during FOCUS acquisitions. Acquisition will be briefly stopped and restarted with each adjustment.

Direct numeric input of the power level ScanImage should use during subsequently initiated image acquisitions.
When Live Adjust is active, changes to Power have an immediate effect when focusing.

Normally, changes to Power have no immediate effect when not acquiring. For optical measurement & testing purposes, the Direct Mode option can be selected. This causes changes to the Power level to take immediate effect, i.e. Beam has specified power even though no scanning/imaging is being done. This mode applies to all Beams.


Note that following any image acquisition (including with Direct Mode enabled), the power for each Beam is set to minimum value measured during its last Beam Calibration. The Power level must be changed, with slider or text entry, to re-enable transmission with a non-scanning Beam.

By default, the Power level is specified as % of the maximum power determined during Beam Calibration. Specifying power in mW is not currently supported. Selecting the mW radio button has no effect.
The Limit value simply determines the maximum value to allow for the Power level setting, for the specified Beam. This is useful for preventing the value from accidentally being set too high.

The Power level is stored/loaded as part of the ScanImage Configuration (.CFG) file.


  • Power is adjusted to specified level immediately upon each trigger initiating an acquisition.
    • Triggers in ScanImage arrive at start of FOCUS or GRAB acquisitions, each LOOP Repeat, or each Slice within a standard (stepping) Image Stack.
  • Between acquisitions (i.e. between triggers), the Beam power level output is adjusted to minimum power measured during Beam Calibration – i.e. the Beam is 'OFF'
  • The command signal generated by ScanImage for selectedBeam will alternate between the specified Power level and the minimum power (during scanner flyback or turnaround)

Beam Selection

This menu selects among the (up to 3) Beams that have been configured (and named) in the Machine Data File. The Power level and Limit value displayed and controlled in this panel pertain to the selected Beam. Actions initiated via the Calibration menu items also pertain to the selected Beam.

P/z Adjustment

When collecting Image Stacks, step-wise or sweeping (i.e.Volume Imaging), in scattering tissue, the power level required at each slice in the stack generally increases with depth in order to maintain a fixed signal. ScanImage can adjust the power automatically during stacks.

Checkbox enables/disables automatic 'P vs z' adjustment feature for currently selected Beam

Specifies exponential length constant for currently selected Beam. Power is increased with depth according to 

P = P0 * exp^((z-z0)/Lz)

A value of Inf implies no power adjustment should occur for the selected Beam.


Value of Lz must be positive

  • In general, the user will experiment to identify the Lz value suited for their preparation, which is typically fairly constant from acquisition to acquisition.
  • Note the power level only adjusts in integer increments of the percentage value.
  • The P/z Adjust and Lz controls are User Settings, i.e saved/loaded to User (.USR) files. Thus, once value is determined/entered by user for each Beam, these values are typically loaded when ScanImage starts and apply to all stack configurations/acquisitions during that session.

Measure Lz


ScanImage provides a facility to interactively measure, rather than enter, the Lz value for the current specimen/stack. See Motor Controls for more information.

Calibration Menu


Many light modulators, including Pockels Cells, do not follow a linear relationship between the control voltage and output power.  ScanImage thus allows Beam Calibration, where the power transmitted as a function of control voltage is measured, using a light meter (typically a photodiode). Calibration data stored for each Beam determines voltage signal output to generate a specified percentage of the maximum power.

Calibrate...Causes Beam Calibration for currently selected Beam to be remeasured
Show CalibrationDisplays Matlab figure showing result of most recent Beam Calibration obtained for currently selected Beam
Measure Calibration OffsetUsed to measure voltage offset on Beam Calibration input channel when no laser is present, i.e. offset of light meter and input A/D channel combined. User is prompted to block laser beam for the Beam Calibration offset measurement. All exiting and subsequent calibration curves are adjusted to reflect this input offset.

Calibration curve is not displayed on successful calibrations - use Calibration >> Show Calibration to see calibration results for a particular Beam


If calibration curve appears noisy, non-monotonic, or otherwise suspicious, the calibration curve or raw calibration data is shown.


For Beams without a calibration input (or if no acceptable calibration was maintained), a naive calibration is used where power is specified as a percentage of the beamVoltageRange entered in the Machine Data File.