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
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
The probe and BFS Total readings may be within
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.
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