Luminaire Mounting Height for a Target Illuminance
The mounting height that lands a target horizontal illuminance at a point.
Example
You enter
- Luminous intensity toward point (candela) 1000
- Target horizontal illuminance (fc) 10
You get
- Mounting height above work plane 10.00 ft
- Slant distance to point 10.00 ft
Checked against published worked examples from: IES Lighting Handbook (point method, solved for height).
Details, formula, and sources
Scopeheight = sqrt( candela x cos(angle)^3 / E ). A 1,000 cd source needs a 10 ft mount for 10 fc directly below. Solves for HEIGHT; solving for the candela is separate. A higher mount lowers the illuminance. Direct illuminance from one source, ignoring interreflection; the photometric file and the IES target govern.
Formula or tablemount_height = sqrt( intensity_cd x cos(angle)^3 / target_fc ), the inverse of E_fc = intensity_cd x cos(angle)^3 / mount_height^2; distance = mount_height / cos(angle).
Source and editionPoint-by-point (inverse-square and cosine) method for horizontal illuminance (IES Lighting Handbook / IESNA point method), solved for the mounting height, by name.
Free accessThe result is the mounting height for the direct horizontal illuminance from one source, ignoring interreflection. 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, target_fc, mount_height_ft, distance_ft
- Inverse-square + cosine mount_height = sqrt(I cos^3(angle) / E); a higher mount lowers the illuminanceIES point method
- Single source direct illuminance from one luminaire, ignoring interreflected lightIES Lighting Handbook