Directional Response

To characterize an irradiance collector involves measuring its responsivity as a function of the direction of incident light.  Specialized test fixtures such as this immersion tank are used to quantify the directional response of submersible instruments. 

In the test fixture, the diffuser of the collector is located at the axis of rotation of the carriage to allow testing in zenith.  The instrument is rolled in the carriage to test for azimuthal differences.  

Example:  Cosine Collector

Typical results for PRR-800 cosine collector, in water, normalized to the response at q=0º.  Panel A (left) is the normalized response from –90o zenith angle to +90o.  Panel B (right) panel is the % deviation from ideal over the same range.  The bottom panel is a calculation of the percent integrated error due to departures from true cosine for a theoretical uniform diffuse light field rather than from the collimated beam of light used in testing.