[8.4.7] An instrument for measuring illuminance on a plane. Instruments that accurately respond to more than one spectral distribution are referred to as color corrected; i.e., the spectral response is balanced to V(λ) or V′(λ). Instruments that accurately respond to more than one spatial distribution of incident flux are referred to as cosine corrected; i.e., the response to a source of unit luminous intensity that is illuminating the detector from a fixed distance and from different directions decreases as the cosine of the angle between the incident direction and the normal to the detector surface.The instrument consists of some form of photodetector with or without a filter driving a digital or analog readout through appropriate circuitry.

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