Simulated vs Actual Lighting: Why Dialux Designs Drift

Posted by the OSD team, factory engineers with 15 years of production and field service experience in Most Popular, Knowledge on Mar 4, 2026

Simulated vs actual lighting for solar street lights

Short answer: expect a 10–20% gap between your Dialux simulation and the installed result, and design for it. The largest single cause is not the software — it is the mounting-height reference. Most designs enter pole height as the light-source height, while the fixture actually sits 1–2 m lower at the end of the arm. Everything below is ranked by how much error each cause typically contributes, based on the design files customers send us for review.

The height reference error (worth up to ~30% of illuminance)

Illuminance on the road falls roughly with the square of the mounting height. An 8 m pole with a 1.5 m single arm puts the light source near 6.5–7 m; add a 0.3–0.5 m foundation projection above grade and the baseline moves again. Designing at 8 m and installing at 6.7 m cuts road-surface illuminance by about 30% on geometry alone — (6.7/8)² ≈ 0.70.

The fix is arithmetic. Fixture height = pole height − arm drop + foundation projection. We ask for the pole drawing, including arm length and setback, before we run any photometric calculation. A model built on pole height is wrong before the first fixture is placed.

The IES file doesn't describe the fixture that ships

A simulation is only as good as the photometric file behind it. Three mismatches explain nearly every "the design promised more light" dispute we handle:

Wattage drift. The design used a 4,000 lm file; the shipped unit carries a 3,200 lm optic. Same housing, different lumen package.
LED layout changes. If the emitter arrangement differs from the layout the IES file was measured on, the intensity distribution shifts even at identical total lumens.
Housing and lens revisions. The housing refracts and scatters, and the lens shapes the beam. Either revision alters the distribution, and the old IES file no longer represents the product.

Our rule: the IES file is issued per optic revision and per production batch, and the file's lumen output must match the luminaire's rated lumens within a few percent. If a supplier cannot produce an IES file for the exact optic being shipped, treat the simulation as an estimate rather than a specification.

Site conditions the simulation never sees

Four site factors move the measured result more than any software setting:

Map scale errors. A distance mistake when importing the site plan scales the entire layout. Spacing that is 5% long in the model is 5% short on the road.
Terrain. Dialux draws flat planes; real roads crown, slope, and undulate. On a 3% grade the near-lane reading shifts by more than the map error.
Obstructions. Trees, building faces, and signage block light the model assumed arrived unobstructed. Walk the alignment before finalizing.
Ambient light. Shop fronts and façade reflections raise the measured background. Contrast changes before absolute lux does.

Long-term losses the design should already include

The simulation models a brand-new fixture on day one. Real installations lose output over time: LED lumen depreciation (L70 figures near 50,000 hours are typical for mid-power packages — roughly 11 years at 12 h per night) and dirt on the lens. In DC-coupled fixtures, line losses stay small when the wire gauge is correct; long runs in split systems still cost a few percent.

We design to a maintenance factor of 0.7–0.8, which divides the initial design value so the road still meets target at mid-life. A simulation quoted at 100% of initial lumens with no maintenance factor is marketing, not engineering.

What closes the gap in practice

Three habits remove most of the error we see:

Verify the first unit with a lux meter. Measure average illuminance on the roadway at the first installed fixture, not the tenth. A meter reading settles arguments that drawings cannot.
Re-issue photometrics on any optic change. Lens, housing, or emitter revision means a new IES file, full stop.
State the tolerance in the contract. We commit to the design value with the maintenance factor applied; anything quoted without it is not comparable.

This page covers photometric verification for DC-coupled fixtures. It does not cover AC-coupled systems, tunnel or sports lighting, or controller-level dimming strategies — different physics, different rules. To run a project layout with corrected mounting heights, use our lighting design tool; for hardware, see the all-in-one solar street light line, where every unit ships with the IES file for its optic.

Next step: send us your pole drawing and road cross-section, and we will return a photometric report with mounting heights verified against the arm geometry — before you commit to a quantity.