Convert LDT to IES (and IES to LDT) online

What is an LDT to IES converter?

An LDT to IES converter reads a EULUMDAT (.ldt) photometric file, rebuilds the full luminous intensity distribution from the C-planes actually stored in it, and writes the same luminaire in the IESNA LM-63 (.ies) layout. Run the other way, it rescales absolute candela to candela per 1,000 lumens and writes the EULUMDAT header block.

Both formats describe the same thing: how much light a luminaire sends in each direction, plus its size, lamp flux and power. EULUMDAT, the European format read natively by Dialux and Relux, stores intensities as cd/klm relative to the lamp flux and only the planes its symmetry code requires. LM-63, the North American format used by AGi32 and Revit, lists every plane explicitly, usually as absolute candela for LED products. Moving between them is bookkeeping rather than photometry, but the bookkeeping has to be exact or the calculated illuminance changes.

This converter runs inside NR Lighting's IES/LDT editor. Conversion is a Pro feature: the Convert tab shows a lock until you sign in with a Pro account, while viewing a file's header and photometric fields in the same editor is free.

How to use the LDT to IES converter

1. 01Sign in with an NR Lighting Pro account, open the editor and choose the Convert tab. It opens in Basic mode, which handles one file at a time.
2. 02Drag and drop, or click to choose, a .ies or .ldt file of up to 5 MB. The tool detects the format from the content, not the extension, and shows the source and target badges (LDT to IES, or IES to LDT).
3. 03Click Convert. The file is sent to NR Lighting's cloud functions over an encrypted connection, parsed, rewritten in the target format and returned to your browser; nothing is published.
4. 04Read the warnings before you download. Typical ones are 'File uses absolute photometry', 'Lumens calculated from photometry data' when an IES file gives no lamp lumens, and a relative-intensity warning when an LDT file has no flux.
5. 05Check the preview (the first 2,000 characters of the new file) and click Download. The file is saved under the original name with _converted and the new extension.
6. 06For a whole library, switch to Advanced: add up to 20 files, choose Auto (swap each format) or force IES or LDT, set find/replace, prefix and suffix rename rules and an optional manufacturer or catalogue stamp, then Convert All and Download ZIP.

The method: how the candela data is rescaled

LDT to IES: I (cd) = I (cd/klm) × k × Φ / 1000. IES to LDT: I (cd/klm) = I (cd) × m × 1000 / Φ. Φ = total lamp flux in lm, k = EULUMDAT conversion factor (line 24), m = LM-63 candela multiplier.

Reading an LDT file, the converter sums the flux of every lamp set (line 26c), applies the conversion factor and scales each stored intensity to absolute candela. It then expands the stored planes to the full set declared on line 4 using the symmetry code: Isym 1 copies the single plane to every C-angle, Isym 2 and 3 mirror about C0–C180 or C90–C270, and Isym 4 reflects the first quadrant. The IES file is written as IESNA:LM-63-2002 with [TEST], [MANUFAC], [LUMCAT] and [LUMINAIRE] keywords carried across, TILT=NONE, Type C photometry, dimensions in metres, and lumens per lamp set to −1 (absolute photometry) with a multiplier of 1, so the candela values are used exactly as written.

Reading an IES file, it takes lamp lumens × number of lamps as Φ, or integrates the distribution when the file is absolute (−1). Each candela value is multiplied by the LM-63 multiplier before dividing by Φ, which matters for the many files where the multiplier is not 1. It writes the downward flux fraction from a solid-angle-weighted integration of the distribution, converts feet or metres to millimetres, sets Isym 1 for a single-plane file and Isym 0 (all planes written) for everything else, and carries the CCT and CRI across when the IES header declares them.

Worked example: a 3000 K retail downlight

Take a EULUMDAT file for a recessed LED downlight of the kind used in a fashion store: Isym 1 (rotationally symmetric), 24 C-planes declared at 15° with one plane stored, 37 gamma angles at 5° from 0° to 180°, lamp flux 1,500 lm, 15 W, 3000 K, CRI 90, conversion factor 1.00, and a peak of 1,320 cd/klm at gamma 0°.

Scaling: 1,320 × 1.00 × 1,500 / 1,000 = 1,980 cd at nadir. At gamma 30° the stored 640 cd/klm becomes 960 cd, and at gamma 60° the stored 45 cd/klm becomes 67.5 cd. The single stored plane is copied to all 24 C-angles, so the IES file carries 24 identical columns of 37 values, 888 candela values in total.

The photometric line written is 1 -1 1 37 24 1 2 0.090 0.090 0.110 for a 90 mm diameter, 110 mm high fitting: one lamp, absolute photometry, multiplier 1, 37 vertical and 24 horizontal angles, Type C, metres. The ballast line reads 1 1 15. The tool reports one warning, 'File uses absolute photometry (1500 lumens)', and 1,500 lm at 15 W is 100 lm/W in either format. Converting the new IES file back to LDT returns 1,980 × 1 × 1,000 / 1,500 = 1,320 cd/klm, the original value, though now with Isym 0 and all 24 planes written out: a larger file, identical photometry.

Limitations, and when to involve a lighting designer

The converter moves data faithfully; it does not improve it. A file with a truncated distribution, a wrong multiplier or an implausible flux converts into an equally wrong file in the other format, so run the validator first and read every warning.

• IES to LDT writes the light output ratio as 100 % and the conversion factor as 1.00, sets the luminous area equal to the luminaire dimensions, and infers the luminaire type from the length-to-width ratio. Edit these in the output if the manufacturer's data says otherwise.
• IES to LDT inserts 4000 K and CRI 80 when the IES header has no colour keywords. For a 3000 K scheme, correct them before the file goes into a luminaire schedule.
• IES to LDT detects only rotational symmetry; bilateral and quadrant-symmetric luminaires are written with every plane (Isym 0). The photometry is the same, the file is just longer.
• LDT to IES always writes TILT=NONE, sets the test lab to 'Converted', sums multiple lamp sets into one, and cannot represent the EULUMDAT downward flux fraction or light output ratio because LM-63 has no fields for them.
• An LDT file with no lamp flux stays relative: the IES output contains cd/klm values labelled as candela, and the tool warns you. Fix the flux first.
• The output is a working file for calculations, not a manufacturer's certified photometry. Where a compliance submission, tender or BREEAM assessment relies on the numbers, ask the manufacturer for the native file in the software's own format and use the conversion to check it.

When conversion is not the real question

I convert photometry most weeks, usually because a European luminaire has to sit in a Revit family or an AGi32 model, or a North American product is going into a Dialux Evo calculation for a retail or hospitality scheme. The conversion is the easy part. Deciding whether a 1,500 lm downlight with a 30° beam is the right fitting for the merchandise, the wall-wash and the glare limits in BS EN 12464-1 is design, and if that decision carries cost or compliance risk, that is the point at which to involve a lighting designer rather than a converter.

IES vs LDT: which software needs which format?

Most current software reads both, but native support and defaults differ, and BIM tools are the strictest. Keep both versions of a file and record which one a calculation used.