IES Files vs EULUMDAT: You Are Probably Asking the Wrong Question

Here is the honest answer before we complicate it: use the photometric file that is native to the exact luminaire, measured properly, complete, correctly oriented and validated inside the software you are using. The extension comes after that.

Lighting teams often argue about IES versus EULUMDAT as though one format can rescue bad data. It cannot. A perfectly structured file can still belong to the wrong optic, the wrong lumen package or a laboratory test that no longer matches the product being supplied.

Files do not emit light. Luminaires do. The file is only a measured description, and a description is useful only when it belongs to the thing you are actually specifying.

What an IES file actually is

IES files follow the Illuminating Engineering Society’s LM-63 format for transferring photometric data and related information. They are text-based files that can describe the luminous intensity distribution of a luminaire across measured vertical and horizontal angles, along with supporting metadata.

The format is widely used in North America and has become common globally because many manufacturers, calculation tools, BIM libraries and visualisation platforms accept it.

Depending on the file and the LM-63 edition used, the data may include information such as luminaire dimensions, lamp or source data, input watts, candela multipliers, angle sets and photometric type. The calculation tool then uses the intensity distribution to predict how light will reach the modelled surfaces.

What EULUMDAT actually is

EULUMDAT, usually supplied as an .ldt file, is another text-based format for exchanging luminaire photometric data. It is strongly associated with European lighting workflows and is widely used in tools such as DIALux and Relux.

An LDT file can carry luminaire geometry, lamp or source configuration, luminous flux information, intensity values and symmetry information. DIALux’s own LDT Editor can open both LDT and IES files, but it saves edited files as EULUMDAT.

That tells us something important. In a practical calculation workflow, both formats can be useful. The software ecosystem and the integrity of the original data matter more than a format loyalty debate.

Which one is more accurate?

Neither format is automatically more accurate.

Accuracy begins in the photometric laboratory. Was the actual luminaire tested? Was it measured at the correct operating condition? Is the distribution sampled at enough angles? Is the lumen output current? Was the file created from a real test, or generated from a generic family curve?

The next risk is orientation. A photometric file can be technically valid and still be used incorrectly if the luminaire’s C-plane orientation, tilt, rotation or mounting direction is misunderstood. This is especially dangerous with asymmetric wall washers, linear grazing optics, street luminaires and directional systems.

Conversion is another weak point. Moving data from IES to LDT or from LDT to IES can be useful, but metadata, symmetry assumptions, dimensions or orientation can change if the conversion is not checked. A converted file should never be accepted only because it imports without an error message.

Use IES when

1. The manufacturer supplies a current LM-63 file from a trusted photometric test.
2. The rest of your workflow, such as BIM, rendering, North American specifications or a particular calculation platform, is already built around IES.
3. You need broad interoperability across tools and project teams.
4. The luminaire is directional or asymmetric and you have confirmed the orientation after import.

Use EULUMDAT when

1. The manufacturer’s verified native file is LDT and the project team is working mainly in DIALux, Relux or another European workflow.
2. You need the EULUMDAT metadata structure used by the manufacturer’s library or internal product database.
3. The project is being documented around European product data and calculation practices.
4. You have opened the file in the target software and confirmed dimensions, distribution, lumen output, wattage and orientation.

What to check before using either file

Do not stop at “the file imported”. Check the luminaire name, test date where available, input watts, luminous flux, dimensions, symmetry, intensity distribution, beam direction and orientation in plan and section.

Compare the file against the product datasheet. If a 24-degree spotlight imports as a broad flood, or a wall washer throws its peak intensity backwards, the software has not misunderstood the design. The project team has failed to validate the file.

For critical fixtures, view the polar curve, run a simple test room and compare the result with the manufacturer’s published values. This takes minutes and can prevent an entire calculation from being built on the wrong photometry.

The seven-minute photometric file audit

1. Match the exact catalogue number, optic, lumen package, CCT and driver condition.
2. Compare reported luminaire lumens and input wattage with the current datasheet and test report.
3. Check the luminous opening, luminaire dimensions and mounting orientation.
4. Open the distribution graph and confirm the beam makes sense for the product.
5. Place one luminaire in a simple test room and rotate it deliberately.
6. Confirm whether the file is native or converted and compare total output, peak intensity, symmetry and orientation.
7. Retain provenance: manufacturer, laboratory, report number, test date and source URL.

The future is not IES versus LDT

ANSI/IES TM-33-23 moves towards richer, structured optical data with more complete spectral, geometric and provenance information. That is the real conversation: product intelligence that travels from laboratory to design to procurement without losing identity.

Primary Sources and Fact-Check Notes

1. ANSI/IES LM-63-19(R25) Current IES standard file format listing, reaffirmed in 2025.
2. ANSI/IES TM-33-23 XML-based standard format for richer luminaire optical data.
3. DIALux LDT Editor Official utility that opens LDT and IES data.
4. ANSI/IES LM-79-24 Current method for optical and electrical measurements of SSL products.

Fact-checked 25 August 2026. Confirm the adopted standard and local requirements for each project.