What Are LDT & IES? Photometric File Guide for DIALux & Relux

In commercial, retail, hospitality and office lighting projects, choosing the right luminaire is only the first step. To simulate illuminance, uniformity, UGR and energy consumption in DIALux or similar design software, engineers need reliable photometric data. The two most widely used formats are LDT and IES.

YNDLUX (Yndlux Lighting Co., Ltd) is a commercial LED lighting manufacturer based in Zhongshan, Guangdong, China, supplying directly from the factory to lighting designers, C3 main contractors, M&E contractors, system integrators, importers and OEM customers. The product range includes DC48V magnetic track systems, AC track lights, LED downlights and LED linear/trunking luminaires. All YNDLUX product lines come with free IES/LDT files to support design and lighting verification.

This article explains what LDT is, what IES is, the differences between the two formats, how to use LDT in DIALux and IES in DIALux, as well as the process to follow when requesting a manufacturer to send DIALux IES files for a real project.

What Is LDT?

LDT is the file extension for photometric data in the EULUMDAT format. This format is widely used in Europe and in many lighting design software packages, especially DIALux. When searching for "what is LDT", the short answer is that LDT is a technical file describing how a luminaire distributes light in space.

An LDT file is not a catalogue, a CAD drawing or a standard product datasheet. It contains measured photometric data that allows software to calculate the light output at various angles. Depending on the file structure and measuring equipment, the data may include:

Luminous flux of the luminaire or light source;
Luminous intensity distribution across C-planes and gamma angles;
Electrical power of the luminaire under test conditions;
Luminous efficacy or values related to the measurement;
Geometric dimensions and reference position of the luminaire;
Manufacturer information, product code and version name;
Data needed to generate light distribution curves and calculate illuminance.

In practice, an LDT file is usually linked to a specific configuration: wattage, colour temperature CCT, beam angle, lens type, drive current, driver or product version. Therefore, an LDT file for a product with the same shape but different wattage or beam angle should not be used as a substitute for the product specified in the technical documentation.

What Information Can Be Read from an LDT File?

When opening a file in specialist software or importing it into DIALux, users can view the light distribution curve, luminous flux, power and some identification data. However, the displayed content depends on the reading software and how the testing laboratory generated the data. Some information such as CRI, SDCM, L70 lifetime, IP rating or warranty may not be fully included in the LDT file. This information should be cross-referenced with the official luminaire datasheet.

The key point is that LDT describes the photometric performance of the luminaire in a specific test configuration, and does not automatically prove that the product is suitable for every project requirement. Engineers must still verify voltage, control system, ambient temperature, installation conditions and applicable lighting standards.

What Is IES?

IES is a photometric data format developed and widely used within the Illuminating Engineering Society standard ecosystem, commonly identified by the .ies file extension. When searching for "what is IES", the practical answer is: IES is a text file containing luminous intensity distribution data for a luminaire, so that simulation software can calculate how light affects a space.

The IES format is widely used in North America, Asia and in international projects. Many programs such as DIALux, Relux, AGi32, Autodesk and other simulation tools can read IES files, although the ability to display information fields depends on the software version.

An IES file typically includes the following main data groups:

Description and header lines of the file;
Number of lamps or light sources in the luminaire;
Rated luminous flux of the light source in the measurement;
Power and multiplying factors related to the measurement data;
Luminaire dimensions along geometric axes;
Measurement angles and corresponding luminous intensity values;
Light distribution data for software interpolation and calculation.

Not all IES files have the same level of detail. Some files are generated from full-luminaire measurements, while others may describe just the light source or be processed with correction factors. For projects requiring high accuracy, confirm with the supplier that the file was measured for the complete luminaire, with the correct product code and optical configuration.

Differences Between LDT and IES

LDT and IES are both photometric data formats, but they are not two different types of luminaire. The differences are mainly in file structure, reference system, data representation and usage conventions by market.

LDT/EULUMDAT: common in European projects and design workflows using DIALux;
IES: common in projects following the IES ecosystem and many international software tools;
Compatibility: DIALux can usually import both LDT and IES, but display results may differ if the metadata or coordinate system in the file is not set up correctly;
Photometric content: both can contain luminous intensity distribution data, but the data encoding and information fields are not the same;
Technical value: reliability depends on the measurement, equipment, test conditions and whether the file matches the actual product configuration — not just the file extension.

In a project, engineers should not choose between LDT and IES based on preference alone. The practical approach is to use whichever format the software, documentation standard or reviewing body requires. If the consultant requests LDT for working in DIALux, request the LDT file. If the design documentation or internal software requires IES, request the IES file. For international projects, it is best to request both formats to reduce compatibility risk.

How Are LDT and IES Used in DIALux?

DIALux uses photometric data to simulate a luminaire in a room, retail floor, corridor, hotel area, office or industrial space. From an LDT or IES file, the software can calculate many metrics such as average illuminance, minimum illuminance, uniformity, UGR, lighting power density and the contribution of each luminaire group.

Workflow for Using LDT Files in DIALux

Obtain the LDT file matching the correct product code, wattage, CCT and beam angle.
Open the project in DIALux and access the library or luminaire import function.
Select the LDT file from your computer or project folder.
Check the product name, dimensions, luminous flux, power and light distribution direction.
Place the luminaire on the floor plan and adjust mounting height, rotation and spacing.
Set the maintenance factor, room surfaces, ceiling/wall/floor reflectance and calculation criteria.
Run the simulation, check for dark or over-lit areas, then adjust the layout.

That is the basic workflow for LDT in DIALux. In a real project, save the original file together with the datasheet and configuration code in the project documentation folder. If the manufacturer updates the LDT version, do not silently replace the file in an approved project without re-checking the results.

Workflow for Using IES Files in DIALux

For IES in DIALux, the steps are similar: import the IES file, check the identification data, place the luminaire in the space and run the calculation. However, pay attention to axis orientation and luminaire length. Especially with linear luminaires, trunking luminaires or luminaires with asymmetric distribution, rotating the wrong axis can significantly alter the simulation results.

After importing the file, engineers should examine the light distribution diagram in DIALux. If a downlight expected to have a symmetric distribution shows an abnormally skewed diagram, re-check the mounting direction, coordinate system and the file itself. Do not rely solely on the luminous flux number while ignoring the shape of the light distribution.

Why Accurate Photometric Files Matter for C3 Main Contractors

In C3 projects, the lighting documentation package usually needs to be coordinated among the developer, design consultant, main contractor, M&E contractor, interior architect and equipment supplier. LDT/IES files bridge the gap between the product catalogue and simulation results. If incorrect data is used, the design may pass on paper but fail during on-site commissioning.

Common risks include:

Using a file for a higher-wattage version than the product actually ordered;
Using a file with a 24° beam angle for a luminaire that is actually 36°;
Using data for a luminaire without a diffuser cover but installing it with one;
Not distinguishing between LED chip luminous flux and total luminaire output flux;
Ignoring the effects of maintenance factor, dust and lumen depreciation;
Importing a file with a generic name that prevents identifying the configuration;
Setting the wrong orientation for linear luminaires or asymmetric-distribution luminaires.

Therefore, C3 main contractors should request photometric files at the material approval stage, not wait until the order has been placed or construction has begun. Early verification helps confirm luminaire quantities, layout spacing, installed power and the ability to meet illuminance requirements.

What Should You Request from the Manufacturer?

When sending a request, do not simply write "please send the IES file". A complete request helps the manufacturer select the correct data and reduces back-and-forth. Provide the following information:

Product code or specific product link;
Luminaire type: DC48V magnetic track, AC track light, downlight or linear/trunking;
Rated wattage or the wattage version required;
Colour temperature: e.g. 3000K, 4000K or 6500K if the project specifies;
Beam angle, lens type, reflector or anti-glare component;
Control version: on/off, DALI, DALI-2 or other control system;
Voltage and power supply configuration, especially for DC48V magnetic track systems;
Format required: LDT, IES or both;
Software used: DIALux, Relux or other;
Requirements for photometric test report, datasheet, mounting dimensions and certifications.

For a large project, request structured file names — for example, including product code, wattage, CCT, beam angle and control type. This helps the design team avoid confusion when the library contains dozens or hundreds of files.

How Does YNDLUX Provide IES/LDT Files?

YNDLUX provides free IES and LDT files for the entire product range, including DC48V magnetic track systems, AC track lights, LED downlights and LED linear/trunking luminaires. The files are intended to support lighting engineers, C3 main contractors and M&E contractors during simulation, equipment selection and technical documentation preparation.

To receive the correct file, customers can send the product code or configuration details to the YNDLUX team via the contact channel on yndled.com. If the product code has not been determined, send drawings, reference images, expected wattage, CCT and beam angle. The supplier should confirm the configuration before sending data, rather than providing a generic file with no clear identification.

For products with control options, specify the intended control system. YNDLUX is a member of the DALI Alliance and offers products with DALI-2 certification in matching configurations. However, photometric files and control certifications are two different types of documents: IES/LDT describe the light distribution, while DALI-2 certification relates to compatibility and control functionality for a specific product configuration.

Sample File Request Template

Customers may send a request using the following template:

"Please provide LDT and IES files for product code [product code], wattage [x W], CCT [x K], beam angle [x°], version [on/off or DALI/DALI-2]. The files are for a DIALux simulation of the [project name] project. Please also include the datasheet, mounting dimensions and confirmed total luminaire output flux."

If the project is at the tendering stage, additional files may be requested for two or three beam distribution options. For example, the same track light may require a narrow-beam version for accent lighting, a medium beam for display areas or a wide beam for general illumination. Comparing options on the same floor plan provides a more evidence-based selection.

How to Check LDT/IES File Quality Before Use

Before including a file in the design documentation, perform some basic checks. This is a simple task but can prevent many errors during construction.

Check name and product code: the name in the file must match the datasheet and quotation.
Check wattage: the wattage shown in the file should match the ordered version.
Check luminous flux: distinguish between light source flux and total luminaire output flux.
Check CCT and CRI: if this information is not shown in the file, cross-reference with the datasheet.
Check beam angle: verify the light distribution curve matches the product description.
Check dimensions: confirm length, diameter, height and optical centre point.
Check mounting orientation: especially important for linear and asymmetric-distribution luminaires.
Check in DIALux: import the file into a test project to see whether the results are reasonable.

If the file shows luminous flux or power significantly different from the catalogue, do not manually edit the values. Ask the manufacturer under which conditions the data was measured, whether it is a different version and which file is the official one for the project.

Photometric Files Do Not Replace Lighting Design

IES or LDT files are important inputs, but a file alone cannot determine the entire lighting scheme. DIALux results also depend on mounting height, luminaire spacing, surface colours, reflectance values, maintenance factor, room geometry and usage requirements.

For example, a downlight may produce high illuminance directly below but may be unsuitable if the spacing is too wide. A narrow-beam track light can highlight products in a retail store, but if used too densely it creates strong contrast and increases glare risk. With linear/trunking luminaires, the module length, suspension spacing and light distribution must be coordinated to avoid dark zones between luminaire sections.

In DC48V magnetic track systems, engineers also need to check system load, power supply arrangement, track run length and module layout. LDT/IES files help simulate the light from a module but do not replace the electrical drawing and load calculation for the track system. Similarly, for AC track lights, the adapter type, mounting position and compatibility with the chosen track must be confirmed.

Questions to Ask Your Supplier

To evaluate a set of photometric files, designers and contractors can ask:

Was the file measured for the complete luminaire or just the LED chip?
Which product code and configuration does the file correspond to?
Is the luminous flux in the file the luminaire output flux or the light source flux?
Does the file match the CCT, CRI, wattage and beam angle being quoted?
What measurement procedure or standard was followed?
Does the DALI/DALI-2 control version change the wattage or luminous flux?
Can the manufacturer provide both LDT and IES for cross-checking?
Does the file need to be updated if the lens, reflector or driver is changed?

These questions are especially important when the project has strict commissioning requirements or when the product is ordered as OEM. With OEM products, the product name on the file may need to show the customer's own brand, but the photometric data must still be linked to the actual hardware configuration being manufactured.

Quick Checklist Before Submitting Simulation Documentation

Has the correct file been used for the correct product code?
Does the file match the wattage, CCT, CRI and beam angle?
Can the luminaire name in DIALux be traced back to the datasheet?
Has the maintenance factor been set appropriately for the actual environment?
Have average illuminance, minimum illuminance and uniformity been checked at the correct calculation plane?
Has UGR or glare risk been considered in occupied areas?
Have total power and lighting power density been checked?
Have the original files, datasheets and version history been saved?
Has the simulation data been confirmed to match the final order configuration?

Completing all these steps helps close the gap between the software design and the product delivered to the job site. For projects requiring multiple options, request YNDLUX to send the corresponding DIALux IES files or LDT files for each option so the design team can compare under the same conditions.

Frequently Asked Questions

What is LDT and what is an LDT file used for?

LDT is the EULUMDAT photometric data format used to describe luminous intensity distribution, luminous flux, power and certain geometric information of a luminaire. LDT files are typically imported into DIALux or other simulation software to calculate illuminance, uniformity and lighting metrics.

What is IES? Can IES files be used in DIALux?

IES is a light distribution data file format widely used in the international lighting industry. DIALux can read IES files in many design workflows. However, users should still verify the correct product code, coordinate system, wattage, CCT and beam angle after importing the file.

Should I request LDT or IES files from the manufacturer?

Choose based on the software and documentation requirements of the project. If using DIALux or the project requires a European format, LDT is usually convenient. If the documentation or software requires IES format, use IES files. For C3 or international projects, request both for easier compatibility checking.

Does YNDLUX provide free IES/LDT files?

Yes. YNDLUX provides free IES/LDT files for the entire product range including DC48V magnetic track systems, AC track lights, LED downlights and LED linear/trunking luminaires. Customers should send the product code along with wattage, CCT, beam angle and control version to receive the correct file. YNDLUX can also provide related technical documentation, DALI-2 products by configuration and OEM support.

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