Point-Method Illuminance (Inverse-Square + Cosine)
Horizontal illuminance at a point from a luminaire of known candlepower by the IES point method.
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
Checked against published worked examples from: IES Lighting Handbook (point method).
Details, formula, and sources
ScopeE = 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 or tabledistance = 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.
Source and editionPoint-by-point (inverse-square and cosine) method for horizontal illuminance from a source of known candlepower, IES Lighting Handbook / IESNA point method, by name.
Free accessThe 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.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesintensity_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