Illuminance (Ev) refers to the luminous flux Φv per unit area impinging on a particular surface A from one or more sources of light. Illuminance is measured horizontally and vertically in the units of lux (lx) or footcandles (fc). In practice when incident light signal to be evaluated is not parallel (collimated) a cosine diffuser is used to correct the measurement geometry of illuminance detectors.
Ev = dΦv/dA (expressed in lx = lm / m² or fc = lm / ft²)
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