Cosine Irradiance (E_d or E_u)

Used for measuring upwelling or downwelling sunlight, a properly designed cosine collector, will report irradiances that are proportional to the cosine of the angle of incidence.  This diagram is a side view of a cosine collector, showing the relationship between the collector, occluding rings, and photodetectors.  For designs using multiple photodetectors such as this, wavelength detuning as the view angle of the photodetector moves off-axis must be controlled.