Model vs Reality — What DIALux Does Not Calculate

In short

DIALux evo calculates illuminance and luminance for selected inputs. DIAL describes a dry road as the starting setting in its street-lighting tool, but the surface material and other parameters can be changed. Results apply to the documented geometry, photometry, reflection model and maintenance factor. DIALux evo

What the calculation model assumes

The basis for most lighting calculations is EN 13201-3, which defines how luminance and illuminance are to be calculated for road lighting. DIALux evo and Relux implement the same model. The assumptions are well-defined and documented — which is precisely why they are worth knowing.

The model works with four core assumptions:

What the calculation model assumes
Model assumption	What it means in practice
Point source	Photometry is represented from a reference point within the model’s application range; mounting height and light distribution are essential inputs.
Selected road surface	Select reflection data for the calculation case; rain, dirt and ageing are not added automatically.
Fixed observer	The prescribed observer positions provide comparable luminance results, not a simulation of every road user’s movement. M-class luminance calculations use a 1.5 m eye height and 60 m to the calculation field; the lane and calculated quantity determine which positions are assessed.
Street-calculation scope	A standard street field is not a complete model of facades, vehicles and every secondary reflection. This limits the scenario, not all DIALux functions.

None of these assumptions is wrong or careless — they are intentional constraints that make the calculation model replicable and comparable. The problem arises when results are presented as though real-world conditions always match the assumptions.

Water changes road-surface reflection

Even dry asphalt has diffuse and directional reflection. R-tables describe how reflection varies with incidence and observation direction; they do not describe a uniformly scattering sheet of matte paper.

When asphalt becomes wet, its reflection properties change. A thin water film transforms the surface from diffuse toward specular — sometimes almost like an uneven mirror. The physics shifts:

Light reflects in narrow streaks, not broadly. This produces high-luminance strips along the line of sight — and dark patches in between.
Uniformity can deteriorate; the change depends on geometry and wet-surface reflection data.
Object contrast can change when the background luminance distribution changes.
Bright reflections can create additional visual difficulties. Do not describe them as a separate perceived TI value; fTI is a defined calculation quantity.

The R-table system — and its limits

R1–R4 are standard references for dry road surfaces. Q₀ is the average luminance coefficient (sr⁻¹), while S₁ is a dimensionless specularity factor; S₁ is not a simple ratio of total specular and diffuse energy. Select the class and normalisation from documented or prescribed data. Appearance, colour and texture alone cannot establish the class reliably. CIE 2019

Wet-road analysis requires reflection data for a wet surface and a calculation method supporting those data. Selecting dry class R3 or R4 does not itself model water on the road. Document the software version, table and applicable wet-road requirement; distinguish Uo,w and national requirements from the dry-road result. CIE 066-1984

CIE TC 4-50 — Q₀, S₁, r.

What DIALux does not include

A single calculation case needs additional scenarios if the project is to assess other conditions:

Snow and ice. A snow-covered surface has entirely different reflection properties from dry asphalt, with high total reflectance but varying specular character depending on snow depth and texture.
Soiled and aged surfaces. Asphalt ages and becomes contaminated. A soiled surface behaves differently from new asphalt. The calculation model assumes a static reflection class.
Document the maintenance factor for the chosen period, lumen depreciation, soiling, cleaning and replacement strategy. ISO/CIE TS 22012:2019 provides a working method. No generic LMF or MF applies to every coastal environment, and a scalar factor does not model every change in light distribution. ISO/CIE TS 22012:2019
The prescribed observer positions provide comparable luminance results, not a simulation of every road user’s movement. M-class luminance calculations use a 1.5 m eye height and 60 m to the calculation field; the lane and calculated quantity determine which positions are assessed.
Compare photopic luminance and lux values with the corresponding requirements. Mesopic analysis can complement specific visual tasks and requires spectrum and adaptation; it does not show that a photopic calculation generally undervalues a particular luminaire.

Lux is not luminance

Lux is luminous flux per area (lm/m²), with photopic weighting. Luminance (cd/m²) describes light in a given observation direction. For a road surface, the relationship depends on incidence angles and reflection data; lux is not energy per area.

M-class compliance requires all applicable criteria, including average luminance, uniformity and glare. Equal average lux does not guarantee equal luminance or class compliance.

This means that watt/lux comparisons without optics data and reflection class tell you little about actual driver visibility. Raw energy efficiency (lm/W) is a measure of source efficiency, not of the installation's visual outcome.

What this means for procurement

A comparison requires the same road geometry, reflection data, observer positions, maintenance method and applicable requirements. Use each luminaire’s photometric file and documented maintenance data. R2 is one possible calculation case, not a general guarantee of comparable inputs.

But neither comparison gives a guaranteed answer to the question: "How well does luminaire A versus B meet the M-class requirement on a wet October road?"

This is not an argument against using DIALux — it is an argument for supplementing it with optics data, the correct road surface class, and in sensitive projects wet-surface calculations where these are available. A supplier that states this plainly is generally more credible, not less.

The role of optics on wet road surfaces

Optics affect light distribution, but a broad beam or an ME designation does not itself establish better wet-road performance. Compare luminaire photometry using the same geometry and relevant wet-surface reflection data, and check the applicable uniformity requirement.

More luminous flux does not automatically improve luminance uniformity. Assess optics, placement and surface reflection together. technical guide for optics.

What can actually be done

DIALux calculation on standard assumptions is a reasonable starting point. There are however additions that give better decision support:

R1–R4 are standard references for dry road surfaces. Q₀ is the average luminance coefficient (sr⁻¹), while S₁ is a dimensionless specularity factor; S₁ is not a simple ratio of total specular and diffuse energy. Select the class and normalisation from documented or prescribed data. Appearance, colour and texture alone cannot establish the class reliably.
Document the maintenance factor for the chosen period, lumen depreciation, soiling, cleaning and replacement strategy. ISO/CIE TS 22012:2019 provides a working method. No generic LMF or MF applies to every coastal environment, and a scalar factor does not model every change in light distribution.
Ask the supplier for optics data. Beam distribution class, IES file, and full-scale luminance distribution are more informative than mean lux comparisons.
Specify dry and wet scenarios where the project requires them. A passing dry-road case does not establish wet-road compliance.

Next level of understanding

Standard photometry does not describe every aspect of night vision

Compare photopic luminance and lux values with the corresponding requirements. Mesopic analysis can complement specific visual tasks and requires spectrum and adaptation; it does not show that a photopic calculation generally undervalues a particular luminaire.

Light quality & perception · 01

Mesopic photometry — how the eye sees at night

The S/P ratio, the Purkinje shift, and what lux does not capture.

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Frequently asked questions

Can you calculate for wet road surfaces in DIALux?

Which R-class should be used for Nordic roads?

Should DIALux results include the maintenance factor?

Document the maintenance factor for the chosen period, lumen depreciation, soiling, cleaning and replacement strategy. ISO/CIE TS 22012:2019 provides a working method. No generic LMF or MF applies to every coastal environment, and a scalar factor does not model every change in light distribution. ISO/CIE TS 22012:2019

VALDUR and real-world performance

We supply IES files, optics class, beam distribution data, and lifecycle documentation for VALDUR. We calculate on R2 as standard, but can on request run alternative reflection classes and document the actual maintenance factor for your climate and maintenance interval.

Summary

What raises the quality of project documentation is not replacing DIALux, but supplementing it with the correct R-class, a realistic maintenance factor, optics data, and — where warranted — a clear statement of what the calculation actually guarantees.

Model vs reality — what DIALux does not calculate