Editorial owner: MCL Solar Knowledge Center

Review scope: Roadway photometry, nominal beam angle, IES distribution, calculation inputs, and procurement verification.

Key conclusion

A single beam-angle number cannot determine whether a solar street light is suitable for a road. Nominal beam angle describes only part of the optical spread and may be defined differently by different suppliers. IES roadway distribution classifications describe where the luminaire sends light relative to the road geometry. They are not interchangeable with a statement such as “70 degrees equals Type II” or “90 degrees equals Type III.”

Road-lighting selection should use the photometric file for the exact luminaire, the proposed mounting geometry, and a calculation against the project’s lighting criteria.

Beam angle and roadway distribution answer different questions

Term	What it describes	What it does not prove
Nominal beam angle	An angular spread based on a stated intensity threshold or supplier convention.	Roadway uniformity, glare, backlight, spacing, or compliance with a road-lighting criterion.
IES Type II, III, IV or V	The shape and reach of the horizontal light distribution relative to the luminaire and roadway.	That a specific pole height, setback and spacing will meet the project target.
IES or LDT file	Measured photometric intensity data used by calculation software.	That the file belongs to the quoted model unless identity and test documentation are checked.
DIALux or equivalent calculation	Predicted illuminance or luminance for stated geometry, surface, maintenance and photometric inputs.	Actual installed performance if the inputs, product or installation differ.

Why the same beam angle can produce different road results

Two luminaires can carry the same nominal beam-angle label while having different peak intensity, forward throw, side distribution, backlight and glare. Lens orientation, LED position, optical chamber geometry and the definition used for the angle all affect the result.

A road also adds variables that a beam angle cannot represent:

road width and number of lanes;
pole height, overhang, tilt and setback;
single-sided, staggered, opposite or central-median arrangement;
pole spacing and curve or intersection geometry;
road-surface reflection characteristics;
required average level, uniformity, glare and surrounding-light limits;
maintenance factor and expected lumen depreciation.

What LM-79 and an IES file contribute

ANSI/IES LM-79 covers optical and electrical measurements of solid-state lighting products. A model-specific LM-79 report can support values such as input power, luminous flux, efficacy, color characteristics and photometric distribution under the stated test conditions. The electronic photometric file is then used in lighting calculations.

LM-79 should not be confused with LM-80. LM-80 concerns lumen and color maintenance of LED packages, arrays and modules; it is not a complete roadway luminaire distribution test.

A practical road-lighting selection workflow

Define the road: width, lanes, traffic and pedestrian context, pole locations, mounting height, setback and arrangement.
Define the criterion: state the applicable tender, authority or design requirement for illuminance or luminance, uniformity, glare and other limits.
Obtain exact photometry: request the IES or LDT file for the quoted model and optical option.
Verify identity: match model name, power, LED configuration and report information to the proposal.
Run calculations: use the exact geometry, maintenance factor and surface assumptions.
Review alternatives: compare distributions, mounting arrangements and spacing rather than selecting by wattage or beam angle alone.
Commission the installation: verify product identity, aiming, pole geometry and field measurements against the agreed method.

How to discuss Type II and Type III distributions

Type II distributions are often considered for narrower roadways or arrangements where the required lateral reach is more limited. Type III distributions generally provide broader lateral coverage. That general description is not a design result. A specific Type II file can outperform a specific Type III file for a given geometry, and the reverse can also be true.

The correct procurement question is therefore not “Which angle is Type III?” It is: “Which model-specific photometric distribution meets the stated criteria at the proposed geometry with the agreed maintenance factor?”

Red flags in a supplier proposal

A beam angle is presented as proof of roadway compliance.
The same IES file is used for multiple powers or optical options without explanation.
The report model does not match the quotation.
Only total lumens are supplied, with no distribution file.
DIALux screenshots are provided without the input report or calculation summary.
Pole spacing is promised before the road geometry and criterion are known.
LM-80 is presented as if it were a complete luminaire photometric report.

Information to send for a calculation

road cross-section and lane widths;
pole height, arm length, tilt, setback and arrangement;
target spacing or existing pole positions;
applicable lighting class or tender acceptance criteria;
surface assumptions and maintenance factor;
required operating and dimming schedule.

Sources and further reading

For the broader workflow, see the solar street light road design guide.

Solar Street Light Beam Angle vs IES Distribution: Road Design Guide