IES (.ies) is the North American photometric format defined by IESNA LM-63. EULUMDAT (.ldt) is the European format. Both encode the same core data: luminous intensity values at defined angles around a luminaire. IES uses a keyword-based header. EULUMDAT uses a fixed-line positional format. Most lighting design software reads both. European manufacturers typically provide LDT files. North American manufacturers provide IES files. Many provide both.
What photometric files contain
A photometric file describes how a luminaire distributes light in space. It encodes the luminous intensity (in candelas) at a grid of vertical and horizontal angles around the fixture. Lighting design software uses this data to calculate illuminance levels, uniformity, and glare for a given room or space.
Both IES and LDT files contain:
- Luminaire metadata: manufacturer name, luminaire name, catalog number
- Lamp data: number of lamps, lumens per lamp, wattage
- Measurement geometry: C-plane angles and gamma angles that define the measurement grid
- Intensity data: candela values at each angle combination
- Physical dimensions: luminaire length, width, height (for area source calculations)
The difference is how this data is structured in the file. IES uses a labeled keyword format. LDT uses a strict line-by-line positional format where each line number has a defined meaning.
Before uploading photometric files to your PIM or sharing them with lighting designers, validate your photometric files to check structure, completeness, and data consistency.
IES format (IESNA LM-63)
The IES file format is maintained by the Illuminating Engineering Society of North America (IESNA) in their standard LM-63. The current version is LM-63-02, though older LM-63-95 and LM-63-91 files are still widely encountered.
Structure
An IES file has three sections:
1. Label lines: Free-form text lines starting with keywords in square brackets. These carry metadata.
IESNA:LM-63-2002
[TEST] Lab report 12345
[MANUFAC] Acme Lighting
[LUMCAT] AL-400-LED
[LUMINAIRE] Recessed LED Panel 600x600
[LAMP] LED module 40W 4000lm
[OTHER] CRI 80, 4000K
2. TILT line: Indicates whether the luminaire has tilt-dependent photometry. Most files use TILT=NONE.
3. Data block: Numeric values in a fixed sequence:
- Line 1: Number of lamps, lumens per lamp, candela multiplier, number of vertical angles, number of horizontal angles, photometric type, units type, width, length, height
- Line 2: Ballast factor, ballast-lamp photometric factor, input watts
- Line 3+: Vertical angles
- Following: Horizontal angles
- Following: Candela values (one set per horizontal angle)
Example snippet
IESNA:LM-63-2002
[TEST] Report-2026-001
[MANUFAC] Acme Lighting
[LUMCAT] RLP-600
[LUMINAIRE] Recessed LED Panel
TILT=NONE
1 4000 1.0 37 5 1 2 0.595 0.595 0.068
1.0 1.0 40
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180
0 22.5 45 67.5 90
1520 1518 1510 1495 1472 ...
EULUMDAT format (LDT)
EULUMDAT was developed in Germany and became the de facto European standard for photometric data. Files use the .ldt extension. The format has no formal version numbering; it has remained largely stable since its introduction.
Structure
An EULUMDAT file is strictly positional. Each line number corresponds to a specific data field. There are no keywords or labels. The parser must know that line 1 is the company name, line 2 is the measurement type, line 3 is the symmetry indicator, and so on.
| Line | Content |
|---|---|
| 1 | Company name |
| 2 | Type indicator (measurement type: 1=point, 2=linear, 3=area) |
| 3 | Symmetry indicator (0=no symmetry, 1=C0 symmetry, 2=C0-C180, 3=C0-C270, 4=C0-C360) |
| 4 | Number of C-planes (Mc) |
| 5 | Distance between C-planes (Dc) |
| 6 | Number of intensities per C-plane (Ng) |
| 7 | Distance between gamma angles (Dg) |
| 8 | Measurement report number |
| 9 | Luminaire name |
| 10 | Luminaire article number |
| 11-13 | Additional description lines |
| 14 | Lamp/LED set identifier |
| 15 | Number of lamp sets |
| 16 | Lamp set data (lumens per set) |
| 17 | Color appearance (color temperature) |
| 18 | CRI |
| 19 | Input wattage |
| 20-22 | Luminaire dimensions (length, width, height in mm) |
| 23-25 | Luminous area dimensions |
| 26 | C-plane angles (one per line, Mc values) |
| 26+Mc | Gamma angles (one per line, Ng values) |
| After angles | Candela values (Mc x Ng total values) |
Example snippet
Acme Lighting
1
4
24
15.0
19
5.0
Report-2026-001
Recessed LED Panel 600x600
RLP-600
LED 40W
4000K CRI80
LED module
1
4000
4000
80
40.0
595
595
68
595
595
0
0.0
15.0
30.0
...
0.0
5.0
10.0
...
1520
1518
1510
...
Key differences
| Aspect | IES (.ies) | EULUMDAT (.ldt) |
|---|---|---|
| Origin | North America (IESNA) | Europe (Germany) |
| Standard | IESNA LM-63-02 | No formal standard number |
| Format | Keyword header + data block | Strict line-positional |
| Metadata | Flexible keyword labels | Fixed-position lines |
| Encoding | ASCII | ASCII (some files use ANSI/Latin-1) |
| Symmetry | Implicit (angle range determines symmetry) | Explicit symmetry flag (line 3) |
| Angle convention | Type C (vertical 0=nadir, horizontal 0=front) | Type C (same convention) |
| Candela data | Per horizontal plane, all vertical angles | Per C-plane, all gamma angles |
| Software support | Universal | Universal |
| File extension | .ies | .ldt |
Both formats use the C-gamma coordinate system where C-planes are horizontal rotations (0-360 degrees) and gamma angles are vertical tilts (0-180 degrees, where 0 is nadir/downward for a pendant luminaire).
Common validation issues
When photometric files are generated, exported, or converted, several issues can compromise data quality.
Angle count mismatch. The header declares 37 vertical angles and 5 horizontal angles, but the candela data block does not contain exactly 37 x 5 = 185 values. This is the most common structural error and causes lighting design software to misread the data.
Incomplete C-plane coverage. A luminaire with no symmetry should have data for the full 0-360 degree horizontal range. If the file only covers 0-90 degrees but does not declare symmetry, the software cannot correctly interpolate the missing quadrants.
Invalid tilt specification. IES files with TILT values other than NONE, INCLUDE, or a file reference are malformed. Most LED luminaires use TILT=NONE since the LED source orientation is fixed.
Luminous flux mismatch. The total lumens declared in the header should approximately match the flux calculated by integrating the candela distribution. A significant mismatch indicates either a wrong lumen value or incorrect candela data.
Encoding issues. EULUMDAT files from older German systems may use ANSI/Latin-1 encoding with umlauts in company or luminaire names. UTF-8 systems may display garbled characters. This does not affect the numerical data but can cause metadata parsing errors.
Validate your photometric files to catch these issues before they reach lighting designers or your PIM.
Which format to provide
If you manufacture luminaires and supply product data to distributors:
European market: Provide LDT files as the primary format. Most European lighting design tools (DIALux, RELUX) natively support EULUMDAT. Many European distributors and data pools expect LDT.
North American market: Provide IES files. AGi32, Visual, and US-based DIALux installations expect IES.
Both markets or global catalogs: Provide both formats. Generate them from the same goniophotometer measurement to ensure consistency. Many goniophotometer software packages export both formats simultaneously.
Data exchange platforms: Some platforms (like the DIAL LUMIDAT database or the RELUX product database) have their own upload requirements. Check whether they prefer LDT, IES, or their own format (ROLF for RELUX, ULD for DIALux evo).