Point-Method Illuminance (Inverse-Square + Cosine)
Horizontal illuminance at a point from a luminaire of known candlepower by the IES point method: E = candela x cos(angle)^3 / mounting-height^2 (a 1,000 cd source 10 ft up gives 10 fc directly below, 6.5 fc at 30 deg off nadir; lux = fc x 10.764). The direct illuminance from one source, ignoring interreflection, complementing the lumen/zonal average method. The photometric file and the IES target govern.
Formula and source
distance = mount_height / cos(angle); E_fc = intensity_cd x cos(angle) / distance^2 = intensity_cd x cos(angle)^3 / mount_height^2; E_lux = E_fc x 10.764.
Point-by-point (inverse-square and cosine) method for horizontal illuminance from a source of known candlepower, IES Lighting Handbook / IESNA point method, by name.
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