Free online IES to LDT converter

Lighting-design software such as DIALux, Relux, or a luminaire database may require EULUMDAT (.ldt) instead of an IES (.ies) file. Converting the file makes the same photometric distribution available to workflows built around the LDT format.

What the IES format is

IES is the Illuminating Engineering Society’s text-based photometric format, specified by LM-63. An .ies file contains luminaire and lamp descriptions, units, mounting dimensions, photometric type, horizontal and vertical angle arrays, and candela values used by lighting-calculation software.

Lighting laboratories and manufacturers publish IES files for use in DIALux, Relux, AGi32, Visual, CAD/BIM workflows, energy models, and other design applications. Files may use different LM-63 revisions and may include manufacturer-specific keywords that are not interpreted by every importer.

What the LDT format is

LDT is the usual filename extension for EULUMDAT, a text-based photometric format widely used by European lighting manufacturers and software vendors. Its header fields, angle conventions, symmetry declarations, and data layout differ from LM-63, so changing the extension does not perform a conversion.

EULUMDAT is useful when a catalog, calculation program, or data-management system accepts LDT but rejects IES. It can store the luminaire dimensions, lamp and flux information, measurement details, symmetry, and an intensity matrix, but it does not provide a universal container for every field found in modern IES files.

How to convert the file

Use a photometric editor that explicitly supports IES import and EULUMDAT export. DIAL’s LDT Editor is a suitable Windows option for opening, checking, editing, and saving photometric data. Lighting Analysts Photometric Toolbox is a commercial alternative with IES and LDT handling. Confirm the installed version’s import and export capabilities before starting; support can vary by product edition.

1. Make a working copy of the original .ies file. Keep the source unchanged for comparison and archiving.
2. Open the copy in the editor and inspect the photometric type, units, luminaire dimensions, horizontal angles, vertical angles, symmetry, luminous flux, and candela table.
3. Use File → Export or File → Save as, choose EULUMDAT or .ldt, and complete any missing manufacturer, luminaire, lamp, measurement, or electrical fields requested by the program.
4. Save with a new name such as fixture-name.ldt. Do not merely rename fixture-name.ies to fixture-name.ldt.
5. Reopen the exported file in the editor and, preferably, import it into the target application. Compare its polar curve, total luminous flux, dimensions, and orientation with the source.

DIALux evo and Relux are useful for validating the converted file in an actual lighting calculation. Import the source IES and the resulting LDT as separate luminaires, then compare the photometric display and a representative calculation. Their import functions should not be assumed to provide a general-purpose IES-to-LDT export workflow; use a dedicated editor when an LDT file must be produced.

Online conversion

Reputable general-purpose online converters for IES and EULUMDAT are uncommon. Use an online service only when its documentation identifies the photometric parser and output format, and only for files that contain no confidential product or project data. Avoid generic file-conversion sites that simply change the extension or do not show the imported angle grid and candela data. For manufacturer files, regulated projects, or batch conversion, a desktop photometric editor is safer because the files remain local and can be inspected before export.

Quality and compatibility checks

The result is a data translation, not a byte-for-byte copy. Header wording, field order, rounding precision, units, symmetry declarations, and descriptive metadata may change. Some IES keywords, multiple lamp records, electrical details, and vendor-specific information have no direct EULUMDAT field.

• Photometric type: Confirm that Type A, Type B, or Type C remains correct; the type controls how the angle data is interpreted.
• Angular grid: Check the number of horizontal and vertical planes, angle endpoints, intervals, and whether the first and last planes are duplicated.
• Units and dimensions: Verify feet versus metres and the luminaire length, width, and height.
• Flux and intensity: Compare total lumens and several candela values, not only the displayed filename or product description.
• Orientation and symmetry: Confirm the declared symmetry and mounting orientation. An incorrect symmetry setting can replace missing planes with invalid mirrored data.
• Target software: Open the LDT in the application that will use it and check a sample illuminance result against the original IES.

Retain the original IES beside the converted LDT when the file supports certification, tender documentation, product publication, or later recalculation. Recreate or manually verify metadata that the target format cannot represent.