Directional Response

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The directional response of an instrument is its sensitivity to light sensor as a function of the light’s angle of incidence. For example, a cosine collector has a directional response that ideally varies in proportion with the cosine of the zenith angle of incidence. This means the signal will have a maximum responsivity to light from directly overhead (local zenith) and should have no responsivity to light coming directly from the side or below. Scalar irradiance sensors (Figure 1) ideally respond equally to light independent of direction. Directional Response Characterization measures the directional response of a sensor or instrument for comparison with the ideal response.

Directional test data for scalar collectors is typically identical when the instrument is used in either water or air. "Cosine" response collectors typically have different directional responses depending on if they are being used in air or fully submerged in water. For this reason, the design of in-air and in-water cosine collectors have structural differences, and their performance is verified by testing either in air or in water at our facilities.

Typically, Biospherical measures the directional response in-air of all scalar collectors before final instrument assembly to verify the characteristics of the collector. Individual scalar response directional test data of the finished sensor is available for a fee.

Cosine collectors are characterized and validated for the particular instrument design. For both in-air and in-water instruments the customer can request an individual instrument directional response characterization and report for a fee.

Scalar irradiance collector
Figure 1. In-air directional characterization apparatus: a scalar irradiance collector is mounted on a computer controlled rotary table illuminated by a 1000 tungsten halogen lamp. For in-water characterizations, a similar computer-controlled rotary table is inverted and mounted above a large water tank and the optical collector is illuminated through a quartz window on one side of the tank.
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