pyldt

A Python library for reading and writing EULUMDAT (.ldt) photometric files — the standard format for luminaire intensity distribution data used in lighting design and photometric testing.

Developed in an ISO 17025 accredited photometry laboratory.

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Features

• Parse any valid EULUMDAT file into a clean Python object model
• Full symmetry expansion (ISYM 0–4) into a complete [mc × ng] intensity matrix
• Edit header fields (luminaire name, lamp data, photometric factors, geometry, …)
• Save back to a valid EULUMDAT file with correct ISO-8859-1 encoding
• Optional symmetry compression on write
• Safe file naming: auto-increments filename (_1, _2, …) if destination exists
• No dependencies beyond the Python standard library

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Installation

pip install eulumdat-py

For development or to access the examples:

git clone https://github.com/123VincentB/pyldt.git
cd pyldt
pip install -e .

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Quick start

from pyldt import LdtReader, LdtWriter

# Read
ldt = LdtReader.read("luminaire.ldt")
print(ldt.header.luminaire_name)
print(f"I(C=0°, γ=0°) = {ldt.intensities[0][0]:.1f} cd/klm")

# Edit a header field
ldt.header.date_user = "2026-01-01 / modified"

# Save (adds _1 suffix if file already exists)
saved = LdtWriter.write(ldt, "luminaire_modified.ldt")
print(f"Saved to: {saved}")

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Examples

File	Description
examples/01_basic_usage.md	Accessing header fields, lamp data and intensity values
examples/01_basic_usage.py	Runnable script for the above
examples/02_polar_diagram.md	Plotting the polar intensity diagram (4 C-planes)
examples/02_polar_diagram.py	Runnable script for the above (requires matplotlib)

Run any example from the repository root:

python examples/01_basic_usage.py
python examples/02_polar_diagram.py

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Project structure

pyldt/
├── pyldt/
│   ├── __init__.py      ← Public API
│   ├── model.py         ← LdtHeader, Ldt dataclasses
│   ├── parser.py        ← LdtReader
│   └── writer.py        ← LdtWriter
├── examples/
│   ├── 01_basic_usage.md
│   ├── 01_basic_usage.py
│   ├── 02_polar_diagram.md
│   └── 02_polar_diagram.py
├── tests/
│   ├── test_parser.py
│   └── samples/         ← .ldt test files (10 real manufacturer files)
├── docs/
│   └── eulumdat_format.md
├── CHANGELOG.md
├── LICENSE
└── README.md

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EULUMDAT format

EULUMDAT is the standard file format for luminaire photometric data, defined by the LiTG (Deutsche Lichttechnische Gesellschaft, 1990). It is widely used in Europe alongside IES files (North America).

Key concepts:

• C-planes: vertical planes through the luminaire vertical axis (azimuth, 0–360°)
• γ-angles: elevation angles within each C-plane (0° = nadir, 90° = horizontal, 180° = zenith)
• ISYM: symmetry code that allows the file to store only a subset of C-planes
• Intensities: tabulated in cd/klm (candela per kilolumen of lamp flux)

See docs/eulumdat_format.md for a detailed field-by-field description.

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Symmetry types (ISYM)

ISYM	Description	C-planes stored
0	No symmetry	All mc planes
1	Full rotational symmetry	1 plane
2	Symmetry about C0–C180	mc/2 + 1 planes
3	Symmetry about C90–C270	mc/2 + 1 planes
4	Quadrant symmetry	mc/4 + 1 planes

LdtReader.read() always returns a fully expanded matrix regardless of ISYM.

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License

MIT — see LICENSE.

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Context

This library was developed as a practical tool in the context of ISO 17025 accredited photometric testing. It is shared as open-source in the hope that it will be useful to others working with EULUMDAT files in Python.