IES Files Explained: What They Are and How Lighting Designers Use Them

Every time a photometric calculation produces accurate results — in AGi32, DIALux, Revit, or any web-based tool — it’s because of an IES file. IES files are the connective tissue between a physical luminaire and a simulated light model. They’re small text files, usually under 100 KB, but they encode everything a lighting calculation engine needs to predict where every lumen from a fixture will land.

Despite their central role in lighting design, IES files are frequently misunderstood, misused, and occasionally fabricated. This guide explains what they are, how they’re structured, how to get and validate good ones, and what can go wrong when you use bad ones.

What an IES File Is

An IES file is a plain-text data file formatted according to the ANSI/IES LM-63-19 standard, published by the Illuminating Engineering Society of North America. The LM-63-19 standard (2019 revision; earlier versions include ANSI/IES LM-63-02 and ANSI/IES LM-63-95) defines the exact syntax for encoding a luminaire’s photometric performance data.

The file encodes the candela distribution of a luminaire: luminous intensity (in candela) measured at a grid of angles covering the full sphere surrounding the fixture. This data is captured in a photometric test laboratory using a goniophotometer or an integrating sphere, then written into the ANSI/IES LM-63-19 format for distribution.

The .ies file extension is universal. Some manufacturers also provide files in the European EULUMDAT (.ldt) format, which encodes the same information in a different syntax and is more common with European design software like DIALux.

The Structure of an IES File

Opening an IES file in a text editor reveals its structure immediately. Here is a simplified but representative example of what the header and beginning of a data section look like:

IESNA:LM-63-2002
[TEST] ABC-2024-LED-150W-5700K
[MANUFAC] Example Luminaire Co.
[LUMCAT] EX-AREA-150-5K
[LUMINAIRE] 150W LED Area Light, 5700K, 80 CRI
[LAMP] LED Module
[WATTAGE] 150
[COMMENT] Tested per LM-79-19 at 25C ambient
TILT=NONE
1 16250 1.0 91 181 1 1 0.0 0.0 0.0
1.0 1.0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90...
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Header Keywords

Lines beginning with [KEYWORD] are metadata: manufacturer name, catalog number, lamp type, wattage, test report identifier, and comments. These are informational and are not used in the calculation engine, but they’re critical for maintaining a traceable documentation trail between a photometric report and the actual tested product.

Tilt Information

The TILT=NONE line (or TILT=INCLUDE or TILT=FILE) indicates whether the photometric data accounts for tilted mounting. Most area lights and floodlights are tested at the NADIR (straight down) orientation; tilt corrections, if needed, are handled in the design software rather than the IES file itself.

The Lamps/Luminaire Data Line

The critical single line of numbers immediately following TILT contains:

1. Number of lamps
2. Lumens per lamp (this is the total output used to scale the candela values)
3. Candela multiplier (usually 1.0)
4. Number of vertical angles
5. Number of horizontal angles
6. Photometric type (1 = C-plane, 2 = B-plane)
7. Units (1 = feet, 2 = meters)
8. Luminaire dimensions (width, length, height)

The lumen value on this line is paramount. It’s the number photometric software uses to scale calculated illuminance. If a manufacturer inflates this number to make their fixture appear brighter, every footcandle value in the resulting study will be over-reported.

Candela Distribution Table

After the angle lists, the file contains a table of candela values — one value for each combination of vertical angle (gamma, 0°–180°) and horizontal angle (C-plane, 0°–360° in increments). This is the actual measured light output data.

The C-Plane and Gamma Angle System

IES Type C photometry — by far the most common for architectural and area lighting — describes light distribution using two angular coordinates:

• C-planes are vertical planes through the fixture nadir, analogous to longitude lines on a globe. C0° is typically the “forward” or “along-road” direction for a streetlight, while C90° is perpendicular. For a symmetric fixture, C0 through C180 data is sufficient; for asymmetric fixtures, a full C0–C360 dataset is required.
• Gamma angles are measured from nadir (straight down = γ0°) toward the horizontal (γ90°) and up toward zenith (γ180°). Most outdoor fixtures only have meaningful output from γ0° to γ90°; a full cutoff fixture will have near-zero values above γ90°.

The candela value at any given (C, γ) coordinate tells you exactly how many candela the fixture emits in that direction. Integrating all values across the sphere gives total lumens (which should match the labeled lumen output within the tolerances specified in ANSI/IES LM-79-19).

How IES Files Are Created: ANSI/IES LM-79-19 Testing

IES files are produced as the output of standardized photometric testing per ANSI/IES LM-79-19 (Electrical and Photometric Measurements of Solid-State Lighting Products). ANSI/IES LM-79-19 is the metrology standard; ANSI/IES LM-63-19 is the file format standard. They work together.

An accredited test lab — certified under the National Voluntary Laboratory Accreditation Program (NVLAP) or DesignLights Consortium (DLC) Qualified Testing Laboratory (QTL) program — tests a production-representative sample of the luminaire on a goniophotometer. The instrument rotates the fixture (or a mirror system) through the full angular measurement grid while a detector measures luminous intensity at each position.

The test is conducted at a stabilized junction temperature, typically after the fixture has been operating for at least 30 minutes. All electrical measurements (input watts, input voltage, power factor) are recorded simultaneously.

Importing IES Files into Design Software

AGi32

In AGi32, luminaire libraries are managed through the Luminaire Schedule. Add a new luminaire, click the photometric data field, and browse to the .ies file. AGi32 displays the resulting polar or Cartesian intensity distribution diagram immediately so you can visually sanity-check the data before using it in a calculation. The software supports ANSI/IES LM-63-19 files in all versions and EULUMDAT .ldt files as well.

DIALux and DIALux evo

DIALux supports direct import of IES files through the luminaire catalog import function. Many manufacturers maintain online portals that integrate directly with DIALux via the DIAL product portal. Drag-and-drop import of .ies files into a project is also supported in DIALux evo.

Autodesk Revit

Revit uses IES files to define the photometric web of a lighting fixture family. In the family editor, the photometric data is linked to the light source component. Once embedded in the family, the IES data is used by Revit’s rendering engine and by linked analysis tools like ElumTools (a popular Revit photometric add-in that uses AGi32’s calculation engine).

SketchUp with Rendering Plugins

SketchUp alone doesn’t perform photometric calculations, but plugins like LightUp or the Enscape rendering engine can import IES files to produce physically-based lighting renders. These are primarily visualization tools rather than code-compliance calculators.

Web-Based Tools

Modern web-based photometric tools — including the Illumination Pros calculator — accept IES file uploads directly. The file is parsed server-side, the candela distribution data is extracted, and calculations proceed using the same fundamental algorithms as desktop software. This makes accurate, portable photometric studies accessible without licensing expensive desktop software.

Checking IES File Quality

Not all IES files are equal. Before relying on a file for a code-compliance study or a competitive bid, check the following:

Verify the Lumen Value

Open the file in a text editor and find the lumens-per-lamp value on the data line following TILT. Compare it to the fixture’s published rated lumen output. If the IES file claims 20,000 lumens but the spec sheet says 18,500 lumens, the file will produce over-optimistic calculations. A variance of ±5% is typical; anything beyond 10% warrants investigation.

Check the Catalog Number Match

The [LUMCAT] keyword should match the exact catalog number of the fixture you’re specifying, including any options that affect optical performance (lens type, wattage, distribution). A “D” (diffuse wide) distribution file used for a project requiring “T3” (Type III roadway) distribution will produce completely wrong results.

Look for Physically Unreasonable Distributions

Plot the intensity distribution (most software does this automatically). A 150W area light should have a smooth, roughly symmetric distribution for a Type IV or Type V optic, with intensity peaking in the nadir and falling off toward the horizontal. Jagged, spiky distributions or anomalously high values at γ = 90° (horizontal) are red flags for a file that wasn’t generated from actual measured data.

Confirm the Wattage Keyword

Cross-reference the [WATTAGE] keyword value against the fixture’s input wattage from the spec sheet. Photometric software uses the efficacy (lumens/watt) of the fixture, so an incorrect wattage value corrupts any energy analysis.

Where to Find IES Files

Manufacturer Portals

Most commercial lighting manufacturers maintain downloadable IES file libraries on their websites, organized by product family and catalog number. Major manufacturers including Lithonia Lighting, Cree Lighting, Acuity Brands, Current (by GE), Signify (Philips), and others maintain well-organized portals. Search for the specific catalog number and look for “Photometric Data,” “IES File,” or “Downloads.”

DesignLights Consortium (DLC) Qualified Products List

The DLC QPL database (qpl.designlights.org) includes IES file downloads for every listed product. Since DLC listing requires ANSI/IES LM-79-19 testing at a QTL lab, files obtained via the DLC portal have a verified testing provenance. This is one of the most reliable sources for quality IES data.

IES File Libraries and Aggregators

Third-party photometric data libraries (some integrated into AGi32 and DIALux) aggregate IES files from multiple manufacturers. These are convenient but may contain outdated files. Always verify the date of the file against the manufacturer’s current product page.

Requesting Files Directly

If you can’t find a file online, contact the manufacturer’s technical support or regional sales team. Any reputable manufacturer should be able to provide current ANSI/IES LM-79-19-compliant IES files within one business day. If they can’t, that’s a meaningful signal about their testing and quality documentation practices.

Common Issues and Pitfalls

Outdated files for updated products. LED luminaires are frequently revised — driver upgrades, new LED arrays, revised optics — without a product number change. An IES file from 2019 may not represent a fixture shipped in 2024.

Mismatched wattage configurations. Many LED fixtures offer multiple wattage configurations via driver programming or integral wattage selectors. Each configuration should have its own IES file. Using a 200W file to represent a 150W configuration will over-predict output.

Generic or “representative” files. Some manufacturers provide a single IES file intended to represent an entire product family rather than a specific SKU. These can be acceptable for preliminary design but are not appropriate for permit-quality studies.

Missing or incorrect tilt data. Floodlights and sports lighting fixtures are typically aimed at steep angles (30°–60° from horizontal). If the IES file doesn’t include the appropriate tilt data, or if the design software doesn’t apply the correct aiming angle to the photometric data, the calculation results will be inaccurate at the extremes of the coverage zone.

Understanding IES files deeply — how they’re made, what’s in them, and how to validate them — is one of the highest-leverage skills a lighting professional can develop. Every photometric study you produce or review is only as accurate as the IES data underneath it.