Photometric data files are the foundation of professional lighting design. They contain measured data describing exactly how a luminaire distributes light in space — enabling designers to create accurate simulations using software like DIALux, Relux, and AGi32. The two dominant formats are IES (.ies) and LDT/EULUMDAT (.ldt). Understanding the differences between these formats is essential for any lighting engineer working on international projects.
What Is an IES File?
IES (Illuminating Engineering Society) format, officially designated IESNA LM-63, is the photometric data standard developed by the Illuminating Engineering Society of North America. An IES file is a plain text file containing:
- Luminaire identification: Manufacturer name, catalogue number, luminaire description, and lamp type
- Photometric measurements: Candela intensity values at specified vertical and horizontal angles, measured using a goniophotometer
- Physical dimensions: Luminaire length, width, height, and luminous opening dimensions
- Electrical data: Input watts, ballast factor, and lamp lumens
- Classification data: Luminaire type, measurement units, and number of measurement planes
IES files use Type C goniophotometry (vertical axis), which is the standard measurement geometry for architectural and commercial luminaires. The data is organized in a structured text format that can be opened in any text editor for inspection.
IES File Structure
A typical IES file begins with header lines (marked with keywords like [MANUFAC], [LUMCAT], [LUMINAIRE]) followed by the TILT specification, then the photometric data array. The intensity values are arranged by vertical angles (typically 0° to 90° for downlights, 0° to 180° for indirect luminaires) across multiple horizontal planes (0° to 360°).
The resolution of the data — how many angles are measured — directly affects simulation accuracy. A high-quality IES file from a reputable manufacturer will include measurements at 2.5° or 5° intervals, while budget files may use 10° or even 15° intervals, leading to less accurate designs.
What Is an LDT (EULUMDAT) File?
LDT, formally known as EULUMDAT (European Lumen Data Format), is the photometric data standard predominantly used in Europe. Developed in Germany, it has been the default format for European lighting design for over 30 years. An LDT file contains similar information to IES but in a different structure:
- Header information: Company name, luminaire type, luminaire name, catalogue number
- Symmetry indicator: Defines the luminaire's photometric symmetry, which determines how much data needs to be stored
- Measurement planes: Number of C-planes and their angular spacing
- Intensity data: Candela values per kilolumen (cd/klm) — normalized, unlike IES which stores absolute candela values
- Luminous flux: Total luminaire lumens and lamp lumens
- Electrical data: Power consumption, power factor
- CIE flux codes: Standardized flux distribution percentages for quick comparison
Key Structural Difference
The most important technical difference is that LDT files store intensity in cd/klm (candela per kilolumen) — a normalized value — while IES files store absolute candela. This means LDT files can be easily scaled for different lumen outputs of the same optical design, while IES files represent one specific configuration.
IES vs LDT: Detailed Comparison
Geographic Usage
IES is the standard in North America, parts of Asia, and the Middle East. If you're working on projects in the USA, Canada, Australia, or the Gulf States, clients will typically request IES files.
LDT is the standard across Europe — Germany, the Netherlands, France, Scandinavia, and Eastern Europe. European lighting design specifications almost always require EULUMDAT files. A manufacturer exporting to Europe without LDT files is at a significant competitive disadvantage — many European specifiers will simply skip products without EULUMDAT data.
Software Compatibility
- DIALux (Evo and Classic): Supports both IES and LDT natively. Most widely used lighting design software globally (free, developed by DIAL in Germany)
- Relux: Supports both formats. Popular in German-speaking countries and Scandinavia
- AGi32: Primarily uses IES. Standard in North American lighting design
- Visual / Lighting Reality: IES primary, LDT supported
- CALCULUX (Signify): Both formats
How Photometric Files Are Created
Photometric data is generated through physical measurement of a luminaire sample using a goniophotometer in a dark room:
- Luminaire stabilization: The test sample is powered on and stabilized for at least 30 minutes until thermal equilibrium
- Mounting: The luminaire is mounted on the goniophotometer in its designed operating position
- Rotation and measurement: The goniophotometer rotates the luminaire through vertical and horizontal angles, recording light intensity at each point
- Data processing: Raw intensity data is calibrated and formatted into IES and/or LDT files
- Verification: Total luminous flux from the goniophotometer is cross-checked against integrating sphere measurements
At YNDLUX, photometric testing is performed using an EVERFINE goniophotometer system in a dedicated dark room facility. Our integrating sphere validates colour temperature (CCT) accuracy and colour rendering (CRI/Ra), while the goniophotometer measures luminous efficacy (lm/W), beam angle distribution, and generates both IES and LDT files. All data is traceable to calibrated laboratory equipment.
Reading and Interpreting Photometric Data
Polar Diagrams
Both IES and LDT data can be visualized as polar (candela distribution) diagrams showing light intensity at different angles:
- Beam angle: The angle where intensity drops to 50% of maximum. A 24° beam means tight spot, 60° provides wide flood
- Beam symmetry: Compare C0 and C90 planes. Asymmetric designs (common in retail wall washers) intentionally differ
- Cut-off: Sharp cut-off reduces glare — critical for offices meeting UGR<19 requirements
Room Calculations
When loaded into design software, photometric data calculates:
- Average illuminance (Em): Must meet EN 12464-1 minimums (500 lux offices, 300 lux retail)
- Uniformity (Uo): Ratio of minimum to average illuminance. Minimum 0.6 for most spaces
- UGR: Must be below limits for each room type (UGR≤19 offices, UGR≤22 retail)
- Specific connected load (W/m²/100lux): High-efficacy luminaires (>140 lm/W) significantly improve this value
Common Problems with Photometric Files
Simulated vs Measured Data
Some manufacturers provide files generated from optical simulation software rather than actual goniophotometer measurements. Discrepancies of 10-20% between simulated and measured data are common. Look for laboratory attribution and test report numbers to identify genuine measured data.
Outdated LED Data
LED technology evolves rapidly. Always verify the test date and ensure photometric data corresponds to the current production model. At YNDLUX, we re-test data whenever we update LED chips or optics and clearly version our IES/LDT files.
Incomplete Angular Resolution
Files with coarse angular resolution (10° or 15° intervals) produce inaccurate designs, particularly for narrow-beam luminaires. Always request high-resolution files (2.5° or 5° intervals) for precision applications.
How to Request Photometric Files
- Both formats: Request IES and LDT. If a manufacturer can only provide one, it may indicate limited testing
- Specific configuration: Wattage, colour temperature, beam angle, and optic type
- Laboratory info: Which lab performed the measurement and the test report number
- Test date: Confirm data represents the current production model
- Spectral data (optional): For critical colour applications, request SPD data
YNDLUX provides free IES and LDT photometric files for all commercial luminaires. Files are available for download on our product pages or by contacting our technical team. All data is measured on our EVERFINE goniophotometer system and updated with each production revision.
Converting Between IES and LDT
- DIALux: Can import either format and export to the other
- Photometric Toolbox: Professional conversion and editing tool
- LDT Editor (DIAL): Free tool for viewing and basic editing of LDT files
Note that converting from LDT to IES requires multiplying the normalized cd/klm values by the actual lumen output and dividing by 1000. Verify conversion accuracy before using in production designs.
Conclusion
IES and LDT files serve the same purpose — providing measured photometric data for accurate lighting design — but differ in format, geographic preference, and data normalization. For manufacturers targeting global markets, providing both formats is essential. For lighting engineers, understanding both formats enables better evaluation of luminaire quality across international projects.
Download IES and LDT files for YNDLUX commercial LED luminaires from our product pages, or contact our technical team for project-specific photometric support.
