Point-Method Illuminance (Inverse-Square + Cosine)

Horizontal illuminance at a point from a luminaire of known candlepower by the IES point method.

Run the calculator

Example

You enter

Luminous intensity toward point (candela) 1000
Mounting height above work plane (ft) 10

You get

E fc 10
E (lux) 107.64
Slant distance to point 10.00 ft

Details, formula, and sources

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.

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.

The result is the direct horizontal illuminance from one source ignoring interreflection; lux = fc x 10.764. A design relies on the manufacturer's photometric file and the IES target level.

Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

Field names used by the API: intensity_cd, mount_height_ft, e_fc, e_lux, distance_ft

Inverse-square + cosine horizontal illuminance falls off with the square of the slant distance and the cosine of the incidence angleIES point method
Single source direct illuminance from one luminaire, ignoring interreflected lightIES Lighting Handbook