What Are IES and LDT Files? Using Them in DIALux and Relux
Short answer
IES (ANSI/IES LM-63) and LDT (EULUMDAT) files are photometric data files that describe in which direction and how strongly a luminaire emits light (the light distribution curve), together with its total luminous flux and power. Software such as DIALux and Relux calculates illuminance from these files; always check that the file matches the luminaire's actual power, lumens, optic and colour temperature.
In a lighting project, the calculation that sets the number and layout of luminaires cannot be done without knowing how the luminaire distributes its light. That information does not fit into a few numbers on a datasheet; it needs a file describing the intensity the luminaire emits in every direction. These are called photometric files, and their two most common formats are IES and LDT.
What is inside the file?
- Light distribution curve: the luminous intensity (candela) at every angle around the luminaire. The polar diagram is drawn from this data.
- Total luminous flux: the lumens leaving the luminaire.
- Power: the watts the luminaire draws; efficacy (lm/W) is calculated from this.
- Dimensions: the size of the luminous surface; the software draws the luminaire with it.
- Header information: manufacturer, model, measurement date and laboratory.
IES or LDT?
| IES | LDT (EULUMDAT) | |
|---|---|---|
| Standard | ANSI/IES LM-63 | EULUMDAT |
| Where it is common | North America, international | Europe |
| DIALux / Relux | Reads it | Reads it |
They carry the same data; the format read by the software used in the project is enough. DIALux and Relux read both.
How to add it to DIALux
- Download the file and note which product, optic and colour temperature it belongs to.
- In DIALux evo, import the file from the luminaire selection (dragging the file into the window also works in most versions).
- Compare the luminaire's lumens and power with the datasheet; if they differ, ask for the correct file.
- Place the luminaire on the plan, enter the mounting height and maintenance factor, and run the calculation.
Checklist before use
- Do the lumens and watts in the file match the quoted luminaire?
- Is the optic (beam angle) right? A narrow-beam spotlight file cannot be used for a wide-beam model.
- If colour temperature or CRI changed, was the luminous flux updated?
- Is the source of the file known (manufacturer, measurement date)?
Conclusion
IES and LDT files are the foundation of a lighting calculation: the wrong file means lighting that is right on paper but insufficient on site. You can request photometric files for Harmony ELS products from our contact page and find datasheets on the technical documents page.
Frequently Asked Questions
What is the difference between IES and LDT files?
Both carry the same kind of photometric data. IES (LM-63) is the North American format; LDT (EULUMDAT) is common in Europe. DIALux and Relux read both.
How do I open an IES file?
The file is imported into DIALux or Relux as a luminaire; free photometric file viewers can also show the light distribution curve. Opened in a text editor, the header information (manufacturer, model, lumens) can be read.
Is a separate IES file needed for every colour temperature and optic?
When the beam angle (optic) changes, the distribution curve changes, so each optic needs its own file. When colour temperature and CRI change, the distribution stays the same but the luminous flux may change; the correct lumen value must be used in the calculation.
Where can I get IES files for Harmony ELS luminaires?
You can request the IES files of the products you want to use through the contact form or by e-mail; files are shared per product and optic.