Luminaire Spacing-to-Mounting-Height Ratio
The maximum center-to-center spacing between luminaires for reasonably uniform light, from 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 and source
max_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.
IES 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.
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