LED Wall Washer Spacing Calculation — Uniformity, Mounting Distance and the DIALux Method

The Three Variables That Determine Everything

Wall washer spacing is controlled by three independent variables: mounting distance from the wall (D), spacing between fixtures (S), and beam angle of the fixture. You cannot optimise one without considering the other two. The goal is a combination of D and S that delivers acceptable uniformity across the facade — typically a uniformity ratio (Emin/Emax) of 0.3 or above — within the project’s illuminance target and fixture budget.

The IES Data Behind This Guide

All calculations in this guide are based on real IES photometric test data from TPK’s in-house laboratory, measured on the EVERFINE GO-2000B system. Two models are used as worked examples, representing the two most common facade scenarios: the LWW-PZT-W38A (15° beam) and the LWW-WZB-W55 (25° beam).

Step 1 — Calculate Beam Spread at Your Mounting Distance

The horizontal beam spread on the wall at a given mounting distance D can be estimated from the beam-angle geometry using half the beam angle. Wider beams spread over a larger area at the same distance; narrower beams concentrate the same output and reach further.

Step 2 — The Inverse Square Law and Illuminance at Distance

Illuminance falls with the square of the distance from the source. The IES test reports provide measured Eavg and Emax at a reference height of 10 m — the standard AAI (Average Aiming Illuminance) calculation. Scale these values to other throw distances using the inverse square law before comparing options.

Step 3 — Worked Examples: Two Facade Scenarios

Using the two models above: at a fixed throw distance, the 15° LWW-PZT-W38A concentrates its 221.7 lx baseline into a tighter area, suited to longer throws and narrow architectural features; the 25° LWW-WZB-W55 spreads 136.4 lx over a wider band, suited to shorter throws and broader surfaces. Choose the model whose beam matches the facade proportion, then set spacing to meet uniformity.

Step 4 — Uniformity Ratio: Reading the IES Data

The IES reports include Eavg and Emax at each throw distance; the ratio Eavg/Emax indicates the internal uniformity of a single fixture’s distribution. Inter-fixture uniformity — the dark zones between adjacent fixtures — is what spacing calculation addresses. The two are related but distinct: a fixture can have excellent internal uniformity and still produce dark bands if spacing is too wide.

Step 5 — Verify with DIALux

Hand calculations confirm the order of magnitude and give a starting spacing to test. DIALux (free) or AGi32 gives the actual result with real IES data, real facade geometry and real aiming angles. The workflow: model the facade, apply the IES file, set mounting height and aiming, run the calculation, read the uniformity, adjust spacing, repeat until Emin/Emax reaches 0.3 or above.