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Frequently Asked Question

Why is the measurement with my QSL-2100 higher than expected?

Here is an example of what can happen:

Our science group has looked at data provided by one of our customers. First, we confirmed that PAR was correctly calculated from the raw data: For a measurement of 1500 mV, PAR was determined to be 3064 μE/m2/s, as calculated by the customer.

We then calculated PAR with a radiative transfer model, which gave the following results:

(1) 1227 μE/m2/s for irradiance (instrument equipped with a cosine collector)

(2) 2326 μE/m2/s for downwelling actinic flux (instrument equipped with a spherical collector such as a QSL, looking at the upper hemisphere only)

(3) 2498 μE/m2/s for downwelling and upwelling actinic flux (instrument is equipped with a spherical collector looking at both hemispheres)

These calculations were based on the assumption that the surface albedo is 7% and that the sky is free of clouds. We note that measurements with a cosine-collector-equipped PAR sensor (i.e., (1)) at San Diego agree excellently with similar model calculations.

Depending on whether case (2) or (3) are considered, measurements by the customer are high by 31% and 22%, respectively. These difference may have a variety of reasons, e.g.:

– The surface albedo at the location of the customer was considerably larger than 7%.

– Clouds in the vicinity of the Sun during the measurement may have enhanced radiation levels.

– The calibration factor is too low.

– A combination of these reasons.

From these calculations, we concluded that it may be time to have the instrument calibrated.