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An irradiance collector has a "cosine response." An instrument equipped with such a collector measures "irradiance," or the radiative power received per area of flat surface. The angular response of such an instrument falls off with the cosine of the angle of incidence. For example, a light source mounted at 60° from the instrument's normal would only produce cos(60°) = 50% of the signal produced by the same light sources mounted overhead (cos(0°) = 1). One can look at it this way: imagine a receiver with an area of 1 cm^{2} lying on a table. When this receiver is illuminated with a parallel beam of light from overhead, the receiver will "cut out" an area of 1 cm^{2} from the beam. If the same beam illuminates the receiver from the side (e.g., incidence angle = 60°), the receiver will "cut out" a smaller area. Mathematically, the new "cut-out area" is the projection of the receiver area on a surface perpendicular to the beam of light, which is cos(60) x 1 cm^{2} = 0.5 cm^{2} . The number of photons received is proportional to the cut-out area, hence, the instrument's sensitivity varies with the cosine of the angle of incidence.

The angular response of a scalar (or actinic flux) sensor is independent of the angle of incidence. The response produced by an overhead light source is ideally the same as that for an irradiance collector, but the response for other angles is larger. Scalar geometry is useful to mimic the response of small particles, such as bacteria or air molecules. Bacteria suspended in either air or water "see" light from all directions equally well. Therefore, the directional response of an instrument to study the effect of light on bacteria should also be uniform.

In the real world, there is no isolated light source, but a complex light field, which may include the direct Sun, sky, and reflections of a ship's super structure. Model calculations for a PAR sensor equipped with either an irradiance or scaler collector can be found here. These calculations are for clear-sky conditions (i.e., no clouds, no other obstacles), surface albedo of 7% (e.g., water), and low aerosol loading. Both global irradiance and scalar (or actinic) flux depend on the solar zenith angle. Scalar flux is always higher. The change with zenith angle (SZA) is particularly large if the Sun is low (SZA > 70°).

For clear-sky conditions, the graph PAR for Various Geometries can be used to convert irradiance to scalar values and vice versa. For example, the solar zenith angle required for this conversion can be found here.

If there are clouds, the conversion of irradiance to scalar flux is not straight forward. This is one of the reasons why the instrument with the geometry best suited for the measurement task should be chosen.

For more information on PAR, you can go to the PAR category in the FAQ list and BSI's Application Note, "Light Quantities, Units, and Conversion Factors."