SunScan User Manual v 1.05
Measurement options
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51
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 BFS 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 BFS calibration that is important.
Checking the probe/BFS 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 BFS horizontally in uniform sunlight. Place
the shade ring horizontal as well. Make sure the probe and BFS diffusers are
clean.
♦
In the SunData program, select the PAR display
(
∪
∪
∪
∪
+F
hot key), and take some
readings.
The display will show you the values of the SunScan probe, BFS Total and BFS
Diffuse sensors. All three values should be approximately the same. Store these
readings, and you will have complete results that you can refer back to later if need
be.
The BFS values should be within about 5% of each other.
Mismatches of this magnitude have only a minor effect on the beam fraction
calculations and hence the LAI. (There is no field method for re-matching the Total
and Diffuse sensors. Contact Delta-T if you have a problem).
The probe and BFS 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 BFS (you cannot recalibrate the probe if you
do not have a BFS).
♦
Set up the probe and BFS as for the previous test. On the Workabout, access the
Calibrate
procedure
(
∪
∪
∪
∪
+C
) and select
Recalibrate SunScan
, then follow the
instructions.
