Causes of Cosine Reponse Error in Quantum Meter Sensors

Some of the radiation coming into a sensor at low angles is reflected, which causes the reading to be less than it should be. To partly correct for this problem, more expensive sensors are enclosed in a black cylinder with a small white plastic disk in the top. This is called a cosine corrected head. The plastic used in this sensor has a low reflectance at low angles so the cosine error is small.

Cosine Response of Apogee Instruments Quantum Meter Sensors

The cosine error for the Apogee Instruments Digital Quantum Meter is less than 1% if the radiation is ± 60 degrees from the zenith. The error is 3% when the radiation is ±70 degrees from the zenith. The error is 4% when the radiation is completely diffuse (equal intensity from all directions). A 30% error could occur if this meter was positioned vertically and used to measure radiation at very low angles (dawn or dusk).

Cosine Response Comparison

Spectral Response of Quantum Meter Apogee Sensor

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