In short
A DIALux calculation describes what happens on a dry, flat road surface with a fixed observer position. On a wet road in November, none of that holds. The result is still useful — but it is a prediction, not a guarantee.
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:
| Model assumption | What it means in practice |
|---|---|
| Point source | The luminaire is treated as if all light comes from a single point. Size, shape, and mounting height do not affect inter-reflections. |
| Dry R-table | The road surface is assumed to have the reflection properties of dry asphalt or concrete. No adjustment is made for wet, soiled, or aged surfaces. |
| Fixed observer | Luminance is calculated from a single defined observer position (60 m ahead, 1.5 m height). Variations beyond that position are not calculated. |
| No inter-reflection | Light that bounces off facades, signs, vehicles, or a wet road surface and reflects again is not included. |
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.
Dry asphalt is diffuse. Wet asphalt is specular.
Dry asphalt is a largely diffuse reflector — roughly like a matte piece of paper. Light scatters broadly in all directions. This is the behaviour that R-tables (R1–R4) describe, and therefore what DIALux calculates against.
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 (Uo, Ul) falls. The uniformity shown in the calculation is not achieved on the wet surface.
- Contrast degrades. A dark object against a streaky, partially specular surface is harder to detect than against a uniform background.
- Mirror glare is added. The luminaire reflects in the wet surface. This creates a veil luminance and perceived TI (threshold increment) beyond the calculated value.
The R-table system — and its limits
R-tables (R1–R4, defined in CIE 66 and referenced in EN 13201) classify road surfaces by their reflection properties. The classification specifies both the average luminance coefficient (Q0) and the ratio of specular to diffuse component (C0).
| Class | Typical surface | Q0 (mean reflectance) | C0 (specular/diffuse) | Character |
|---|---|---|---|---|
| R1 | Light concrete, coarse asphalt | 0.10 | 0.25 | Near-fully diffuse |
| R2 | Typical asphalt (standard in calculations) | 0.07 | 0.58 | Predominantly diffuse |
| R3 | Dark textured asphalt | 0.07 | 1.11 | Mixed |
| R4 | Very dark asphalt, stone paving | 0.08 | 1.55 | More specular |
DIALux calculations are almost always run against R2 as default. That is a reasonable assumption for dry, new asphalt. A wet road surface, however, exhibits a specular component that has no direct equivalent in R1–R4. Classification attempts for wet surfaces (W-classes) exist in CIE documents, but they are neither standardised in EN 13201 nor implemented as standard in the most common calculation tools. A correct wet-surface analysis requires changing the reflection model — not simply applying a percentage adjustment to lux values.
What DIALux does not include
Beyond wet road surfaces, a number of real-world conditions are not captured by standard DIALux calculations:
- 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.
- Luminaire soiling loss (LMF). DIALux allows a maintenance factor, but the actual luminance loss from a dirty or moisture-affected lens cover is rarely modelled in procurement calculations.
- Variable observer position. A pedestrian, a cyclist, and a vehicle driver see the road surface from different heights and angles. The model calculates from a single fixed position.
- Spectral weighting. All measures in the calculation are photopically weighted. In mesopic conditions — which apply to all M-classes in EN 13201 — sources with a high S/P ratio are systematically underestimated (see mesopic photometry).
Lux is not luminance
Lux measures incident light on a surface — the energy per unit area arriving at a point. Luminance measures how much light is reflected toward the observer from a specific point on that surface. It is luminance — not lux — that the driver actually sees.
The relationship between lux and luminance runs through the road surface reflection factor. For an M-class installation, it is the mean luminance on the road surface — not mean lux — that determines whether the M-class requirement is met. Two installations with identical mean lux but different optics and road surface reflection classes can produce entirely different luminance levels — and entirely different visual experiences for the driver.
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
When two luminaires are compared in procurement on the basis of DIALux printouts calculated on R2 table, dry surface, and standard observer, they are compared under identical assumptions. That is correct and comparable.
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 choice affects how light is distributed across the carriageway. An optic with broad, even beam distribution (such as the MEW/MEW3 class) can reduce the luminance streaks that specular wet surfaces otherwise create, by spreading light across lane boundaries and suppressing house-side striping. The magnitude of this effect in practice depends on road geometry, pole spacing, and the exact reflection class.
It is one of the clearest connections between optics choice and actual visibility outcome on Nordic road surfaces: optimised light distribution from the right optic contributes more to even luminance on a wet carriageway than increased lux values from a suboptimal optic. See the technical guide for optics for a review of beam distribution classes.
What can actually be done
DIALux calculation on standard assumptions is a reasonable starting point. There are however additions that give better decision support:
- Specify the correct R-class. If the road surface is dark textured asphalt, R3 is more accurate than R2. The consequence can be 10–20 % lower mean luminance compared with an R2 calculation.
- Use maintenance factor realistically. A luminaire in a coastal environment with soiling exposure should use LMF 0.80 or lower, not 0.90, which is the common default.
- Ask the supplier for optics data. Beam distribution class, IES file, and full-scale luminance distribution are more informative than mean lux comparisons.
- Factor in lifecycle cost. A luminaire that meets the M-class requirement on a dry surface but not on a wet surface is technically correct — but for a Nordic climate, a lifecycle-based procurement requirement for actual performance may be more accurate. See purchase price vs lifecycle cost.
Next level of understanding
DIALux assumes lux is the right measure. But lux is weighted for daylight — and street lighting is used in darkness.
All M-classes in EN 13201 fall within the mesopic range, where the eye's sensitivity shifts toward shorter wavelengths. Two luminaires with the same lux but different spectra do not deliver the same visual performance at night.
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.
Frequently asked questions
Can you calculate for wet road surfaces in DIALux?
DIALux evo supports switching the reflection table, so it is possible to use R3 or R4 to simulate a more specular surface. Dedicated wet-surface classes (W-classes) exist in CIE documents but are not standardised in EN 13201 or implemented as standard in the most common tools. A full wet-surface model currently requires specialist tools or manual adjustment of reflection data.
Which R-class should be used for Nordic roads?
R2 is standard in most calculations and a reasonable approximation for new medium-texture asphalt in good condition. For dark textured asphalt, R3 is more accurate. Concrete surfaces may be closer to R1. Road managers rarely measure reflection class on a project-by-project basis in normal procurement, but values are sometimes available from national road authority databases.
Should DIALux results include the maintenance factor?
Yes, and the requirement follows from EN 13201. The maintenance factor (MF) is a composite figure that includes lumen maintenance factor (LMF), luminaire soiling factor (LSF), and any luminaire survival factor. EN 13201-4 specifies how MF is to be calculated. A procurement calculation that does not report MF or sets it to 1.0 overstates real-world performance over the lifecycle.
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
DIALux is a well-defined and replicable tool — but it calculates under assumptions that rarely fully match Nordic operating conditions. A dry R2 surface, a fixed observer, and no inter-reflection are reasonable simplifications for comparison purposes, but they do not give a guaranteed answer about how an installation performs in rain, snow, or on aged asphalt.
The most important consequences: uniformity (Uo, Ul) drops on a wet surface compared with the calculation. Contrast degrades. Mirror glare is added. Lux measured at road level is not the same as luminance toward the observer. And all measures are photopically weighted — despite the fact that all street lighting operates in the mesopic range.
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.
Sources
- CEN. (2015). EN 13201-3: Road Lighting — Part 3: Calculation of Performance. European Committee for Standardization.
- CEN. (2015). EN 13201-4: Road Lighting — Part 4: Methods of Measuring Road Lighting Performance. European Committee for Standardization.
- CIE. (1984). CIE 66:1984: Road Surfaces and Lighting (jointly with PIARC). Commission Internationale de l'Éclairage.
- CIE. (2010). CIE 115:2010: Lighting of Roads for Motor and Pedestrian Traffic. Commission Internationale de l'Éclairage.
- Boyce, P. R. (2014). Human Factors in Lighting (3rd ed.). CRC Press.
- Rea, M. S. (ed.). (2000). IES Lighting Handbook (9th ed.). Illuminating Engineering Society of North America.