Luminaire Spacing-to-Mounting-Height Ratio
The maximum center-to-center spacing between luminaires for reasonably uniform light.
Example
You enter
- Spacing-to-mounting-height ratio (from the photometric report) 1.3
- Mounting height above the work plane (ft) 8
- Proposed center-to-center spacing (ft) 9
You get
- Max spacing (ft) 10.40
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeFrom the fixture's spacing criterion (SC), historically the spacing-to-mounting-height ratio (S/MH): max spacing = SMH x mounting height ABOVE THE WORK PLANE (not floor-to-ceiling). A fixture with an SMH of 1.3 mounted 8 ft above the work plane can be spaced up to 1.3 x 8 = 10.4 ft; a 9 ft layout is fine, a 12 ft layout leaves dark scallops. Narrow-beam / high-bay optics have a lower SMH (0.5-1.0) and must be spaced tighter; wide troffers run higher (1.2-1.5). The perimeter row is set at about half this spacing off the wall. A layout screen; the fixture's actual photometric distribution, the room reflectances, and the target uniformity govern the final spacing.
Formula or tablemax_spacing_ft = smh_ratio x mounting_height_ft, where the mounting height is measured from the luminaire plane down to the WORK plane. Uniform if the proposed center-to-center spacing <= max_spacing_ft.
Source and editionIES luminaire spacing criterion (SC), historically the spacing-to-mounting-height ratio (S/MH), by name; the fixture's photometric distribution and the target uniformity govern the final layout.
Free accessThe spacing-criterion relation is public IES lighting practice; the SMH ratio itself is published on the fixture's photometric report.
What governsEstimate. AHJ and licensed professional govern.
API field namessmh_ratio, mounting_height_ft, actual_spacing_ft, max_spacing_ft
- IES spacing criterion max spacing = SMH x mounting height above the work plane; uniform if actual spacing <= maxIES Lighting Handbook (spacing criterion / S-MH ratio)
- SMH by optic narrow-beam/high-bay ~0.5-1.0 (tighter); wide troffers ~1.2-1.5; perimeter row ~half spacing off the wallmanufacturer photometric report