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AN11Licor: Calculating Calibration Coefficients for Licor Underwater Type SA PAR Light Sensor without Built-In Log Amplifier

Document Number: 
Publication Date: 
Wednesday, July 26, 2017
PDF icon appnote11LicorJuly17.pdf226.89 KB


This application note applies to a Licor Instruments underwater type SA quantum light sensor. The current output of this sensor is measured through a log amplifier in your CTD (or through the PN 90310 Log Amp Module) to obtain adequate resolution over the measurement range.

This PAR sensor is compatible with the following Sea-Bird CTDs:

  • SBE 16, 16plus, 16plus-IM, 19, or 19plus CTD configured with an optional log amplifier and PAR sensor connector
    Note: Optional log amplifier and PAR sensor connector are not available on V2 SeaCATs (16plus V2, 16plus-IM V2, and 19plus V2).
  • SBE 25 CTD configured with a log amplifier and PAR sensor connector (optional on older versions)
  • SBE 9plus, 16, 16plus, 16plus-IM, 16plus V2, 16plus-IM V2, 19, 19plus, 19plus V2, 25, or 25plus CTD interfacing with a PN 90310 Log Amp Module. The Log Amp Module mounts on the CTD or cage, and connects to a single-ended or differential A/D voltage channel on the CTD.

Seasoft computes PAR using the following equation:

PAR = [multiplier * (10* 10(V-B) / M) / calibration constant] + offset

Make the following entries/selections in the CTD configuration (.con or .xmlcon) file:

M = slope of log amplifier     (Note 2)

B = offset of log amplifier     (Note 2)

calibration constant = from the Sea-Bird calibration certificate    (Note 2)

conversion units = appears in data file header; does not modify calculated values, which are controlled by multiplier entry

multiplier = 1.0 for output units of μEinsteins/m2•sec     (Note 3)

offset = 0, typically      (Note 4)


  1. In our Seasoft V2 suite of programs, edit the CTD configuration (.con or .xmlcon) file using the Configure Inputs menu in Seasave V7 (real-time data acquisition software) or the Configure menu in SBE Data Processing (data processing software).
  2. Sea-Bird provides two calibration sheets for the PAR sensor in the CTD manual:
  • Calibration sheet generated by Licor, showing Licor’s calibration data.
  • Calibration sheet generated by Sea-Bird, showing the parameters needed for entry in Sea-Bird software —
    Calibration constant — the in water Calibration Constant (μamps/1000 μmoles/m2•sec) from the Licor calibration sheet. 
    Note that 1 μmole/m2•sec = 1 μEinstein/m2•sec.
    M and B.
  1. Do not enter the LI-1000 Multiplier from the Licor calibration certificate as the multiplier.
    The multiplier can be used to calculate irradiance in units other than μEinsteins/m2•sec. See Application Note 11General for multiplier values for other units.
    The multiplier can also be used to scale the data, to compare the shape of data sets taken at disparate light levels. For example, a multiplier of 10 would make a 10 μEinsteins/m2•sec light level plot as 100 μEinsteins/m2•sec.
  2. Offset may be used to offset the data by a constant, if field data indicates a drift in the sensor. To calculate the offset: Enter M, B, calibration constant, and multiplier, and set offset = 0 in the configuration (.con or .xmlcon) file. With the sensor dark (covered), display the calculated PAR output in Seasave V7; then enter the negative of this reading as the offset in the configuration (.con or .xmlcon) file.


Application Note Revision History

Date Description
  Initial release.
September 2001 Previously referred to SEACON in discussions of .con files. Add references to modifying .con file using Configure menu in Seasave or SBE Data Processing in Windows software.
July 2004 Update to correspond to Sea-Bird calibration certificate that is currently shipped with sensor.
June 2005 Provide output in microEinsteins/m2 sec, and refer to Application Note 11 General for conversion to other units.
May 2007 Incorporate Seasave V7, and eliminate discussion of Seasoft-DOS.
March 2008 Update to include V2 SeaCATs (16plus V2, 16plus-IM V2, 19plus  V2).
February 2010 - Change Seasoft-Win32 to Seasoft V2.
- Add information on .xmlcon files.
- Update address.
October 2012 Update to include SBE 25plus.
July 2017 - Update for Seasave and SBE Data Processing versions 7.26.7 and later: Add selection of Conversion units in PAR calibration coefficients dialog.
- New template.