What is the LDT file format?
A .ldt file contains photometric data. .ldt files are generally created and supplied by lighting product manufacturers. Each file stores data for a given luminaire, for example:
- light distribution curve,
- luminous intensity,
- physical dimensions of the luminaire,
- lamp and ballast parameters.
.ldt files are used by specialized lighting installation design software such as DIALux evo and Relux Desktop to simulate how a fitting will illuminate a space before installation.
.ldt is a plain ASCII text file following the EULUMDAT specification - a de-facto European standard for luminaire photometric data exchange, proposed by Axel Stockmar (Light Consult Inc., Berlin) in 1990. Unlike the North American .IES format, which opens with a version keyword, EULUMDAT has no file signature: every line has a fixed positional role, and intensity values are stored in candela per 1000 lumens (cd/klm).
Security & safety
RISK: LOWAn LDT file is plain ASCII text with no executable code, so it is safe to open and cannot carry malware in any meaningful way. The only practical caution is sourcing: download LDT files from the fixture manufacturer or a reputable photometric library so your lighting calculations are based on real measured data rather than an edited or fabricated file.
Format details
in a nutshellFULL NAMEEULUMDAT Photometric Data Fileaka EULUMDAT file, LDT light file
DEVELOPERAxel Stockmar (Light Consult Inc., Berlin) - de-facto European industry standard, 1990since 1990 (EULUMDAT, proposed by Axel Stockmar as a European luminaire data exchange format)
CATEGORYData Files
MIME TYPEtext/plain
TYPEPhotometric (luminous-intensity distribution) data; plain ASCII text with fixed line order
STANDARDOpen · royalty-free
This extension is also used by…
- ZEMAX Source EULUMDAT object data - ZEMAX OpticStudio can read EULUMDAT (.ldt) as a non-sequential source's photometry, which is why the database mislabeled the format as a 'ZEMAX source file'.
Programs that open LDT files
Windows5 apps
EulumdatTools
Free Web/desktop EULUMDAT viewer and editor; inspect and edit .ldt fields. Verify the site is live before relying on it. 
Dialux evo
Free Import the LDT via the luminaire catalogue / 'import luminaire' to run lighting calculations. Free professional lighting-design tool.
Relux Desktop
Free Import the EULUMDAT file as a luminaire to compute illuminance, glare and energy. Free Swiss lighting-design suite.
IESviewer (Andrey Legotin)
Freemium Open the LDT to visualise its polar / 3D light distribution and read parameters; also converts between EULUMDAT and IES. 
macOS2 apps
Blender
Open-source Blender's IES texture node reads photometric data; for EULUMDAT convert to .ies first (IESviewer), then load it on a light. 
Linux1 app
Text editor (any)
Built-in Open the .ldt as plain text to read the EULUMDAT fields; for use as a light, convert to .ies and load it in Blender.
Web1 app
IES/LDT online viewer (ieslibrary.com)
Free Upload the .ldt to view its polar / 3D light distribution in the browser and download an IES copy. Technical details
deep spec| File encoding | ASCII (7-bit) plain text; CR/LF line endings required by most parsers |
| File structure | Strictly positional line sequence - no field labels or tags; every line's meaning is determined by its position in the file |
| MIME type | text/plain |
| File signature | No magic bytes; the first line is the manufacturer identification string (Ident), typically the company name |
| Intensity unit | Candela per 1000 lumens (cd/klm); parsers scale to actual lamp lumens at calculation time |
| C-plane (horizontal) angles | Count declared by the `Mc` field; step angle `Dc` in degrees; full distribution typically covers 0°-360° in 5° or 10° increments |
| G (vertical) angles | Count declared by the `Ng` field; step angle `Dg` in degrees; spans 0° (nadir) to 90° or 180° depending on luminaire type |
| Symmetry code (Isym) | `Isym` 0 = no symmetry; 1 = rotational; 2 = symmetric about C0-C180 plane; 3 = C90-C270; 4 = quadrant - reduces stored data for symmetric fittings |
| Luminaire type (Itype) | `Itype` 1 = point source; 2 = linear; 3 = rectangular area source - determines how software models near-field luminance |
| Physical dimensions | Luminaire length, width, and height stored in millimetres in dedicated positional fields |
| Lamp and ballast block | Records number of lamps, nominal luminous flux per lamp, colour temperature, wattage, and a free-text ballast description line |
| Tilt correction | Optional section storing tilt angle and per-angle correction factors for luminaires mounted at non-horizontal positions |
| Data integrity | No checksum; the declared `Mc` and `Ng` counts must exactly match the number of data rows present, or parsers reject the file |
| Typical file size | 2 KB - 150 KB, scaling with the number of C-planes and G-angle measurements |
| Released | 1990 (EULUMDAT, proposed by Axel Stockmar as a European luminaire data exchange format) |
| Latest version | EULUMDAT (single revision; an unofficial 'EULUMDAT/2' extension exists but is rarely used) |
| Open standard | Yes · royalty-free |
| Specification | en.wikipedia.org |
LDT conversions
Possible
.LDT .IESGUIDE
Yes - load the LDT file in IESviewer (free) and export it as an IES file in seconds.
How to convert
.LDT .JPGGUIDE
You cannot convert photometric numbers to a photo, but you can get a JPG of the light distribution diagram in…
How to convert
.LDT .PDFGUIDE
Yes, you can produce a PDF from an LDT file - open it in DIALux, Relux, or IESviewer, generate a photometric…
How to convert
.LDT .LDCGUIDE
LDT-to-LDC conversion is possible through IESviewer, though it is rarely needed since most lighting software…
How to convert
Community Q&A
asked by usersNo questions yet - be the first to ask about LDT files.
Frequently asked questions
What is an LDT file?
It is a EULUMDAT photometric data file - plain text that describes how one light fixture distributes its intensity in every direction. It is the European equivalent of the American IES (.ies) file, used in lighting design and 3D rendering.
How do I open an LDT file?
To just read it, any text editor works (it's plain text). To see the light distribution, use a free viewer like IESviewer or an online photometric viewer. To use it, import it as a luminaire into DIALux, Relux, AGi32 or a renderer.
What's the difference between LDT and IES?
Both store the same kind of photometric data. LDT (EULUMDAT) is the European standard; IES (ANSI/IES LM-63) is the North American one. Most professional lighting tools read both, and converters move data between them.
How do I convert LDT to IES?
Use a free tool like IESviewer or an online photometric converter - it reads the EULUMDAT file and writes an equivalent IES file (the physical data is the same, only the layout differs).
Where do I get LDT files?
Fixture manufacturers (especially European ones) publish them on product pages, and free libraries host thousands. Always prefer the manufacturer's own file for accurate calculations.
Can I edit an LDT file?
Yes - it's plain ASCII you can open in Notepad++/TextEdit, but EULUMDAT is strictly positional: keep the declared C-plane and angle counts consistent with the data rows or parsers will reject the file. A EULUMDAT editor (IESviewer/EulumdatTools) is safer.
References
1Wikipedia - EULUMDAT photometric data formaten.wikipedia.org
2DIALux - free lighting design software (reads EULUMDAT/IES)www.dialux.com
