Road Lighting Calculation: How to Verify Road Lighting Performance with DIALux

A professional road lighting calculation is not simply about determining how many lux an LED street light can deliver. A road lighting design must demonstrate that the proposed installation provides the required lighting performance under the actual project conditions.

DIALux is widely used for street lighting simulation because it allows designers to define road profiles, lighting arrangements, luminaire parameters, and applicable standards, then generate detailed calculation results. These results can include lighting performance values, isolux lines, value charts, and calculation tables.

However, receiving a DIALux report does not automatically mean that the lighting design is compliant.

DIALux calculates the performance of the model. Verification determines whether that performance is sufficient for the actual road project.

To review a DIALux road lighting report properly, you need to confirm that the model represents the real project, the correct luminaire photometric data has been used, and the calculated results meet the applicable performance requirements.

What Information Is Needed for a Reliable Road Lighting Calculation?

The accuracy of a DIALux street lighting calculation depends heavily on its input data. The road geometry should represent the actual project, including road width, lane arrangement, carriageway layout, and relevant road-surface characteristics. Installation parameters are equally important. Mounting height, pole spacing, overhang, luminaire arrangement, orientation, and tilt can all influence the final lighting distribution.

Another critical input is the LED street light photometric data. Photometric files, such as IES or LDT files, describe how a luminaire distributes light and allow lighting design software to calculate the expected performance of that specific configuration.

This is why wattage and nominal lumen output alone are not sufficient for evaluating a road lighting design.

For example, two 100 W LED street lights may produce very different road lighting results if they use different optical distributions. One optic may concentrate light over a narrower roadway, while another may distribute light over a wider area. The resulting luminance, uniformity, and glare performance can therefore differ significantly.

This is where product selection becomes directly connected to lighting calculation. Infralumin LED road lights, for example, are available with Type I, Type II, and Type III optical options across multiple product ranges. The ASL28 LED Road Light and other Infralumin road-lighting models are designed for applications including roads, residential streets, bridges, parking areas, airports, and ports.

The important point is not simply to choose a Type I, II, or III distribution based on a general product description. The selected luminaire and optical configuration should be evaluated using its corresponding photometric data under the actual project conditions.

For projects using the EN 13201 framework, the applicable lighting class should also be determined before evaluating the calculation results. It should not be selected afterwards simply because the DIALux result happens to meet a particular class.

How Do You Calculate Road Lighting Performance in DIALux?

A typical DIALux road lighting calculation follows a straightforward workflow.

First, select the applicable road-lighting standard and define the street profile. Enter the actual road width, number of lanes, sidewalks or other relevant areas, together with the appropriate road-surface conditions.

Next, select the LED street light and import the corresponding IES or LDT photometric data. The luminaire should then be arranged according to the proposed installation, including mounting height, pole spacing, overhang, tilt angle, and single-sided, opposite, staggered, or centre arrangements where applicable.

After the road geometry and luminaire arrangement are defined, DIALux calculates the required evaluation fields and generates the relevant performance results. Depending on the selected lighting class and road application, these may include luminance or illuminance, overall uniformity, longitudinal uniformity, glare-related values, and other required criteria.

The final step is verification. Compare the calculated results with the requirements of the selected lighting class and confirm that the calculation model uses the same luminaire, optical configuration, and installation conditions as the actual project.

In short: define the road → select the correct luminaire data → set the installation geometry → run the calculation → compare the results with the required criteria.

What Should You Check in a DIALux Road Lighting Report?

The first number you see in a DIALux report should not automatically determine whether the design passes.

Road lighting performance is evaluated using several indicators together.

Luminance or Illuminance

Illuminance is measured in lux and describes the amount of light falling on a surface. It is relevant for applications where illuminance-based requirements apply.

For many motorized-traffic roads, however, road-surface luminance is the more relevant metric. Luminance is measured in candela per square metre (cd/m²) and describes how bright the road surface appears to an observer.

Road-surface luminance depends on more than the total lumen output of the LED street light. The result is influenced by the luminaire's luminous intensity distribution, installation geometry, road-surface reflection characteristics, and maintenance assumptions.

This is why a simple calculation such as: Total lumens ÷ road area cannot accurately verify professional road lighting performance.

Road Lighting Uniformity

Uniformity indicates how consistently light is distributed across the road. For road-surface luminance calculations, overall uniformity (Uo) describes the relationship between minimum and average luminance across the calculation area. Longitudinal uniformity (Ul) evaluates luminance variation along the direction of traffic.

This is important because an acceptable average luminance does not necessarily mean that the road is evenly illuminated.

For example, two designs may both achieve an average luminance of 1.0 cd/m². If one design has a Uo of 0.45 and another has a Uo of 0.30, the second design has greater variation between bright and dark areas.

A road lighting design should therefore not be judged by average luminance or average lux alone.

Pole spacing and optical distribution are often important factors affecting uniformity. Increasing pole spacing may reduce the number of luminaires required, but excessive spacing can create darker areas between poles. A different optical distribution may improve the lighting pattern without simply increasing wattage.

DIALux itself supports optimization of parameters such as pole distance, mounting height, tilt angle, luminaire distribution, glare, and uniformity during road-lighting planning.

Street Light Glare

Glare is another independent part of road lighting performance.

For projects evaluated using the M lighting classes of EN 13201-2:2015, disability glare is expressed through the threshold increment factor (fTI). Different lighting classes specify different maximum values.

The correct glare metric should always be determined by the applicable road-lighting standard or project specification. It should not automatically be replaced with UGR, which is primarily used for other lighting environments.

Maintained Performance

It is also important to determine whether a DIALux report represents the required maintained condition. LED lighting output can decrease over time, while environmental conditions and maintenance practices can also influence the available lighting performance. A design that meets the target only under initial conditions may not necessarily maintain the required performance during its operating life.

When reviewing a report, check whether the maintenance assumptions used in the calculation are appropriate for the project.

How to Verify a DIALux Result Against EN 13201

For projects applying EN 13201, the calculated results should be compared with the requirements of the applicable lighting class.

For reference, the EN 13201-2:2015 M lighting classes include requirements for maintained average luminance, overall uniformity, longitudinal uniformity, and disability glare. The applicable class and requirements must be determined according to the road and project conditions.

The verification process should follow a clear order:

Determine the required lighting class → confirm the project and calculation assumptions → compare each required result with the applicable performance limit.

The lighting class should not be selected simply because the proposed design happens to meet its numerical values.

DIALux supports road-lighting planning under EN 13201:2015 and other road-lighting standards, while also allowing the road profile, lighting class, luminaire arrangement, and other planning parameters to be configured. DIALux also notes that requirements can vary by country, making it important to check the applicable national requirements for the project.

A DIALux Report Can Pass on Paper but Still Be Wrong

A DIALux calculation can produce technically correct numbers while still failing to represent the actual project.

For example, a simulation may use a mounting height of 10 m and pole spacing of 30 m, while the actual installation uses 8 m poles with 40 m spacing. The calculation itself may be mathematically correct, but it does not verify the proposed installation.

The same problem can occur when an incorrect IES or LDT file is used. A photometric file for a different optic, wattage configuration, or luminaire model may generate reasonable-looking results but does not represent the actual product being supplied.

This is particularly important when comparing LED street lights from different manufacturers. Two luminaires with similar wattage and claimed luminous efficacy may produce different road-lighting results because of differences in their optical systems.

A correct calculation based on incorrect assumptions is not a valid verification of the actual road lighting project.

Before accepting a DIALux report, confirm that the road geometry, mounting height, pole spacing, luminaire model, optical configuration, photometric data, and lighting class all match the proposed project.

Example: How to Read a DIALux Road Lighting Result

Consider an illustrative project where the applicable requirement is M3.

A DIALux report shows:

Based on these values, the design passes the four reference performance criteria.

However, the design should only be accepted after confirming that the model represents the actual project and that the correct luminaire photometric file was used.

For example, if the calculation was completed using a Type II optical distribution but the supplied product uses a different lens configuration, the original result may no longer represent the installed system.

This is the difference between simply reading a DIALux report and actually verifying a road lighting design.

What Should You Ask an LED Street Light Manufacturer For?

When requesting a DIALux calculation from an LED street light manufacturer, the PDF report should not be the only information provided.

Ask for the exact IES or LDT photometric file used in the simulation and confirm the luminaire model and optical configuration. The calculation should also identify the mounting height, pole spacing, road geometry, luminaire arrangement, and applicable lighting class.

For road-lighting projects, manufacturers should support product selection based on the actual installation conditions rather than recommending a luminaire based only on wattage.

Infralumin offers LED road-lighting products with multiple wattage ranges and optical distribution options, allowing different luminaire configurations to be considered for roads and other outdoor infrastructure applications. The final selection should then be verified using the photometric data for the specific product configuration in the lighting calculation.

A DIALux report without clear photometric and installation assumptions is difficult to verify independently.

Need Support Selecting an LED Road Light for Your Project?

A suitable road-lighting solution should be selected based on the complete project rather than wattage alone.

Road width, mounting height, pole spacing, luminaire arrangement, optical distribution, and required lighting performance should all be considered together. A Type I, Type II, or Type III distribution may produce different results depending on the road geometry and installation conditions.

For Infralumin LED road-lighting projects, providing the basic project information—including road dimensions, pole layout, mounting height, and target lighting requirements—can help identify a suitable luminaire configuration for further DIALux evaluation.

You can explore Infralumin LED Road Lights to review available road-lighting configurations and product options.

Conclusion

A professional road lighting calculation should evaluate the complete lighting system rather than focusing only on wattage, lumen output, or average lux.

DIALux can calculate road lighting performance based on the defined road geometry, luminaire photometric data, installation conditions, and selected planning standard. But the report itself is only the starting point.

To determine whether a design is actually suitable for the project, confirm that the model represents the real installation, verify the exact photometric data and optical configuration, identify the applicable lighting requirements, and compare luminance or illuminance, uniformity, glare, and maintained performance with the required criteria.

For LED street light buyers and project planners, this also means looking beyond basic product specifications. The optical distribution and photometric data of the selected luminaire can directly affect whether the proposed pole spacing and installation geometry achieve the required road-lighting performance.

DIALux tells you what the modeled lighting system calculates. Proper verification tells you whether that calculated performance is sufficient for the actual road project.

References

1. DIALux. Street Lighting with DIALux evo. Official documentation covering road profiles, lighting classes, luminaire arrangements, optimization, calculation results, and documentation.

2. CEN. EN 13201-2:2015 — Road lighting, Part 2: Performance requirements. Defines road-lighting performance requirements, including luminance, uniformity, disability glare, and related criteria.

3. CEN. EN 13201-3:2015 — Road lighting, Part 3: Calculation of performance. Covers calculation methods and performance evaluation for road lighting.

4. Rusu, A. V., Galatanu, C. D., Livint, G., & Lucache, D. D. (2021). Average Luminance Calculation in Street Lighting Design, Comparison between BS-EN 13201 and RP-08 Standards. Sustainability, 13(18), 10143.
   MDPI research article

5. DIALux. Supported Standards for Street Lighting. DIALux supports road-lighting standards and references including EN 13201, CIE 140, CIE 115:2010, and IES roadway-lighting guidance.