Illuminance (Lumen Method)

The lumen (zonal cavity) method for average maintained illuminance and luminaire count.

1. What this check does

Computes the room cavity ratio, the number of luminaires needed for the target average maintained illuminance, a near-square grid that fits the room, and the illuminance that grid delivers. Optionally checks spacing against the luminaire spacing criterion and reports lighting power density.

2. Inputs used

Room length, width and ceiling height; work plane height (default 0.76 m / 30 in) and suspension length.
Target maintained illuminance, chosen by you from IES recommendations or the owner standard.
Luminaire lumens and watts, coefficient of utilization (CU) from the photometric report at the computed RCR, light loss factor (LLF), and optionally the spacing criterion (SC).

3. Formulas and assumptions

hrc = ceiling height − work plane − suspension RCR = 5 · hrc · (L + W) / (L · W) N = E · A / (Φ · CU · LLF) rounded up Grid columns = round(√(N · L / W)) along the long side, rows = ⌈N / columns⌉ E_achieved = rows · columns · Φ · CU · LLF / A Spacing check: max(L / columns, W / rows) ≤ SC · hrc LPD = count · watts / A

Rectangular room with luminaires in a regular grid; the result is the average over the work plane.
CU already reflects the room surface reflectances; RokBench does not reproduce CU tables or recommended illuminance values.
LLF combines lamp lumen depreciation, luminaire dirt depreciation and ballast or driver factor as you choose.

4. What is NOT checked

Uniformity and point-by-point illuminance.
Glare and vertical illuminance.
Emergency lighting levels and duration.
Energy-code lighting power density limits.

5. Reference standards

Pointers only; consult the edition adopted by your authority having jurisdiction.

IES Lighting Handbook — lumen method and recommended illuminance
ASHRAE/IES Standard 90.1 — lighting power density
NFPA 101 Section 7.9 — emergency lighting
Manufacturer photometric reports (CU tables, spacing criterion)

6. Validation cases

Hand-calculated cases run on every change. See Validation.

7. Known issues and changes

See the Changelog. Try the tool: Illuminance (Lumen Method).