Delta-T Devices SunScan SS1 User Manual page 46

PAR calibrations
This section describes the basis for the light calibrations used in the
SunScan system, and explains when and how you might want to
recalibrate the probe or restore its factory calibration.
Factory light calibration
The SunScan probe and Beam Fraction Sensor are calibrated to give
PAR readings which match those of a standard PAR quantum sensor in
typical bright daylight conditions.
This matching cannot be made completely reproducible because an ideal
PAR quantum sensor has perfect spectral and cosine responses whereas
the SunScan probe and BF5 can only approximate to the ideal. However,
for most normal usage, the SunScan calibration will be perfectly
satisfactory, but if you are working under strong artificial lights (for
example) you may need to contact Delta-T for advice.
SunScan readings of LAI and fractional interception depend for their
validity on the ratio of the transmitted light to the incident light rather than
their absolute values, so it is the matching between the probe and the BF5
calibration that is important.
Checking the probe/BF5 matching
It is good experimental practice to carry out this test in the field before
(and after) taking a lot of readings.

Mount the SunScan probe and your BF5 horizontally in uniform
sunlight. Make sure the probe and BF5 dome is clean.

In the SunData program, select the PAR display and take some
readings.
The display will show you the values of the SunScan probe, BF5 Total and
BF5 Diffuse readings. The SunScan and BF5 Total values should be
approximately the same. Store these readings, and you will have
complete results that you can refer back to later if needed.
The probe and BF5 Total readings may be within 5-10% of each other
without greatly contributing to errors in canopies where the transmission is
below 50%.
Errors from the mismatch are likely to be swamped by the magnitude of
the variation in the samples. However, if you want to improve the
matching, then proceed with the Recalibrate option.
The Recalibrate option
This option matches your probe to your BF5 (you cannot recalibrate the
probe if you do not have a BF5).

Set up the probe and BF5 as for the previous test.
46  What to Measure and How
SS1 User Manual v3.3