Photometric data is the foundation of every professional lighting design. Whether in retail, office, hospitality, or museum environments — without reliable measurement data for the luminaires used, no standards-compliant design is possible. This guide explains the two common photometric file formats, their differences, and what to look for when requesting manufacturer data.
What is an LDT file (EULUMDAT)?
The LDT format — officially EULUMDAT (European Lumen Data Format) — is the European standard for photometric measurement data. An LDT file contains the complete luminous intensity distribution of a luminaire, measured with a goniophotometer at defined angular steps (typically C0–C360 in 15° steps, γ 0°–180° in 5° steps).
The file contains, among other things:
- Luminous intensity distribution curve (LIDC) in all planes
- Total luminous flux in lumens
- Luminaire dimensions and luminous area
- Lamp power and operating power
- Manufacturer and type designation
LDT files are imported directly into DIALux and Relux — the two leading lighting design software programs in Europe.
What is an IES file?
The IES format (Illuminating Engineering Society) is the North American standard for photometric data. It essentially contains the same information as an LDT file, but in a different file format. IES files are primarily used in AGi32, Visual Lighting Design, and other North American design tools, but can also be imported into DIALux and Relux.
For European projects, the LDT file is the preferred format. International projects or tenders involving North American parties frequently require both formats.
LDT vs. IES: Technical Comparison
| Feature | LDT (EULUMDAT) | IES (IESNA) |
|---|---|---|
| Standard | European | North American |
| File extension | .ldt | .ies |
| Primary software | DIALux, Relux | AGi32, Visual |
| Compatibility | DIALux, Relux, AGi32 | DIALux, Relux, AGi32 |
| Measurement device | Goniophotometer | Goniophotometer |
| Preferred in Europe | ✅ Yes | Accepted |
Why the measurement source matters
An LDT or IES file is only as good as the measurement behind it. There are significant quality differences:
- Accredited laboratory with Type C goniophotometer: Gold standard. Measurements according to EN 13032-1/4 with calibrated instruments and documented measurement uncertainty. The data is reliable and accepted by lighting designers.
- In-house manufacturer measurement: Acceptable if performed with professional equipment. Critical: Without external validation, there is a risk of optimism in the stated values.
- Theoretical/simulated data: Not sufficient for professional projects. Simulation cannot accurately replicate the real light distribution.
YNDLUX measures all photometric data with an EVERFINE goniophotometer. The measurements include complete luminous intensity distributions, total luminous flux, colour temperature, and CRI values.
What data should you request from the manufacturer?
- LDT file (for DIALux/Relux) — per luminaire type and beam angle variant
- IES file — if international compatibility is required
- Data sheet with power, luminous flux, luminous efficacy (lm/W), CRI, CCT, beam angle
- SPD curve (Spectral Power Distribution) — for colour-critical applications
- Test report specifying the measurement laboratory and measurement method used
If a manufacturer cannot provide LDT files from goniophotometer measurements, this is a clear warning sign.
Importing LDT files into DIALux
- Open your DIALux project
- Go to „Luminaires“ → „Import luminaire“
- Select the .ldt file
- DIALux displays the luminous intensity distribution curve (LIDC) as a preview
- Place the luminaire in your project and start the calculation
Checklist: Photometric data for lighting design
- ✅ LDT file per luminaire type and optic variant
- ✅ IES file for international projects
- ✅ Measurement with goniophotometer (not simulated)
- ✅ SPD curve for colour-critical applications
- ✅ Test report with laboratory details
- ✅ Current data (no older than 2 years)
YNDLUX provides complete LDT and IES files for all luminaire types — based on EVERFINE goniophotometer measurements. Contact us for photometric data packages for your lighting design project.
