Understanding IES Files: What They Are, Why They Matter, and How to Get Them

An IES file (.ies) is a standardized text file that contains a luminaire's measured photometric data — the three-dimensional pattern of how light leaves the fixture at every angle. The format is defined by ANSI/IES LM-63-19 (Standard File Format for the Electronic Transfer of Photometric Data and Related Information), the current edition of a standard first published in 1986. Photometric design software — AGi32, DIALux EVO, and others — uses IES files to calculate accurate foot-candle values, uniformity ratios, and light distribution patterns for any fixture placement scenario. Without an IES file for the specific fixture being specified, a photometric plan is an estimate, not a calculation.

If you've worked on a commercial lighting project, you've encountered IES files — even if you didn't know it. They're the invisible foundation of every legitimate photometric plan, every lighting specification, and every code-compliant calculation submitted for permit review.

What's actually inside an IES file

You can open an IES file in any text editor — Notepad, TextEdit, VS Code. It's plain ASCII text, typically 5–50 KB, readable by humans (barely) and by software (easily). Here's what you'll find:

Header information. The first section identifies the fixture: manufacturer name, catalog number, luminaire description, test report number, date of test, and optionally the testing laboratory. This is the metadata that links the file to a specific, real-world product.

Lamp and electrical data. Input wattage, total lumen output, number of lamps (or LED arrays), and ballast/driver information. For LED fixtures tested under IES LM-79, this captures the complete luminaire system performance — not just the bare LED chip output, but the actual lumens leaving the fixture after optical and driver losses.

The photometric data matrix. This is the core of the file: a grid of candela intensity values measured at specific vertical and horizontal angles around the fixture. Think of it as a three-dimensional map of light intensity — how many candelas the fixture produces at 0° (straight down), 15°, 30°, 45°, all the way to 90° (horizontal) and beyond, measured at multiple horizontal rotation angles.

A typical IES file contains intensity data at 37 vertical angles and 1–72 horizontal angles, depending on the fixture's symmetry. A fully symmetric downlight might have data at just one horizontal angle (since it's the same in every direction). An asymmetric area light might have data at 37 vertical angles × 25 horizontal angles = 925 individual intensity measurements.

When photometric software reads this file, it reconstructs the fixture's complete three-dimensional light distribution and uses it to calculate how much light lands on any surface at any distance and angle. That's how a photometric plan can predict, with ±2–3% accuracy, the foot-candle value at a specific grid point 80 feet from a pole-mounted fixture at 25 feet.

Why IES files matter for your project

Without one, the calculation is a guess. Every photometric plan depends on the accuracy of the fixture's photometric data. If the software uses a generic "representative" light distribution instead of the actual IES file for the specified fixture, the calculated foot-candle values could be off by 20–40%. That's the difference between a plan that passes review and a parking lot with dark spots between poles.

Plan reviewers know the difference. In jurisdictions with experienced plan reviewers, the fixture schedule on the photometric plan is cross-referenced against the photometric data used in the calculation. If the plan says "Manufacturer X, Model Y" but the IES file is from a different product, the reviewer may flag it. Some jurisdictions explicitly require that IES file documentation accompany the photometric plan submittal.

Missing IES files are a red flag about the fixture. Every reputable luminaire manufacturer photometrically tests their products and publishes IES files. If a manufacturer can't provide an IES file for a specific fixture, it likely means the product hasn't been independently tested — which means you can't verify its performance claims, and no photometric designer can produce an accurate plan using it. This is particularly common with low-cost imported fixtures sold through online marketplaces.

How IES files are created

The data in an IES file comes from physical testing in a photometric laboratory. Here's the process:

Step 1: Goniophotometer testing. A sample fixture is mounted in a goniophotometer — a specialized instrument that measures light intensity at precise angular intervals around the luminaire. The fixture rotates (or the sensor rotates around the fixture) through a matrix of vertical and horizontal angles while a calibrated photosensor records the intensity at each position.

Step 2: For LED products, testing follows IES LM-79. ANSI/IES LM-79 (Approved Method: Optical and Electrical Measurements of Solid-State Lighting Products) is the standard test method for LED luminaires. It specifies how to measure total luminous flux, electrical power, luminous efficacy, chromaticity, and luminous intensity distribution. The LM-79 test produces the raw data that gets formatted into the IES file.

Step 3: Data formatting per LM-63. The raw goniophotometer data is processed and formatted according to ANSI/IES LM-63-19 into the standard file structure. The file is published by the manufacturer as the official photometric data for that product.

Laboratory accreditation matters. NVLAP-accredited laboratories (National Voluntary Laboratory Accreditation Program, administered by NIST) provide the highest credibility for photometric testing. Major manufacturers test at NVLAP labs. Some also test at in-house labs with published uncertainty budgets. If a project requires the highest confidence in photometric data — a critical infrastructure project, a sports facility, a DOT roadway — specifying NVLAP-tested IES files adds a layer of verification.

How to get IES files

For most commercial projects, sourcing IES files is straightforward:

Manufacturer product pages. The fastest route. Major manufacturers — Acuity Brands (Lithonia, Holophane), Signify (Philips), Hubbell, Eaton, Cree Lighting, RAB Lighting, and many others — host IES files directly on their product web pages. Look for links labeled "Downloads," "Resources," "Photometric Data," or "IES File." The file downloads as a .ies text file.

Lighting representative contacts. If the IES file isn't on the website — or if you need data for a specific lumen package, CCT, or optic configuration that isn't posted — contact the manufacturer's local lighting representative. Reps have access to the full product catalog's photometric data and can send IES files directly, usually same-day.

Software-integrated databases. AGi32 includes the Instabase — a searchable database of manufacturer photometric data accessible directly within the software. DIALux EVO supports manufacturer plugins that integrate entire product catalogs, including photometric data, into the design environment. Both approaches eliminate the manual download step.

Manufacturer specification tools. Many manufacturers offer online product configurators that let you select a specific fixture configuration (lumen package, distribution type, mounting) and download the corresponding IES file. This is useful for products with dozens of configuration options.

What about third-party databases? They exist, but exercise caution. Always verify that the IES file you're using is the manufacturer's current published data — not an outdated version from a previous product generation. Photometric performance changes when a manufacturer updates a fixture's LED array, optic, or driver. Using a 2019 IES file for a fixture that was revised in 2023 produces inaccurate calculations.

IES files vs. other photometric formats

The lighting industry uses three primary photometric data formats:

.ies (ANSI/IES LM-63-19): The North American standard. Used by virtually all US and Canadian photometric software, referenced by US building codes, and required by US plan reviewers. If your project is in the US, this is the format you need.

.ldt (EULUMDAT): The European standard, developed in Germany in 1990. Widely used internationally and natively supported by DIALux. Functionally similar to .ies but with a different file structure. European manufacturers typically publish both .ies and .ldt files.

.tm33 (ANSI/IES TM-33): The emerging next-generation format, based on XML rather than fixed-format ASCII text. TM-33 supports richer data than LM-63 — including spectral power distribution, color data, and extended luminaire information. It's designed to eventually replace LM-63, but adoption is still in early stages. For current US commercial projects, LM-63 (.ies) remains the standard.

For professionals using AGi32 or DIALux EVO: both support .ies natively. DIALux also reads .ldt files directly. If you receive a .ldt file from a European manufacturer and need to use it in AGi32, conversion tools exist (Lighting Analysts' Photometric Toolbox handles this).

Common IES file problems

Outdated files. Manufacturers update LED fixtures regularly — new LED arrays, revised optics, improved drivers. An IES file from 2019 for a product updated in 2024 won't match the current fixture's performance. Always request the most recent IES file, and verify that the file's test date and catalog number match the product you're specifying.

Generic vs. specific configurations. Some manufacturers publish one "representative" IES file for a fixture family that includes multiple lumen packages (say, 10,000 / 15,000 / 20,000 lumens) and multiple distribution types. The representative file won't match your specific configuration. Request the IES file for the exact lumen package and optic you're specifying — most manufacturers can provide configuration-specific data.

Files with no test report reference. A legitimate IES file includes a test report number and testing laboratory identification in the header. If this information is missing, the data's traceability is compromised. For critical projects, verify the test report number with the manufacturer.

No IES file available at all. This happens most often with low-cost fixtures from manufacturers without established photometric testing programs. If a fixture can't provide an IES file, it can't be used in a legitimate photometric plan. Specify a different product — the lighting market offers hundreds of alternatives at every price point, all with published photometric data.

Frequently asked questions

What is an IES file?

An IES file is a standardized text file containing a luminaire's measured photometric data — the intensity of light emitted at every angle around the fixture. It conforms to ANSI/IES LM-63-19 and is used by photometric software to calculate illuminance, uniformity, and light distribution for lighting designs.

How do I open an IES file?

You can open it in any text editor (Notepad, TextEdit) to view the raw data. To visualize the light distribution or use it for calculations, open it in photometric software like AGi32, DIALux EVO, or a free IES file viewer (Lighting Analysts' Photometric Toolbox includes a viewer).

Where can I download IES files for free?

From the luminaire manufacturer's website — go to the specific product page and look for a download link. Most major manufacturers (Acuity Brands, Signify, Hubbell, Eaton, Cree, RAB) provide IES files at no cost. If it's not posted online, contact the manufacturer's lighting representative.

What's the difference between an IES file and an LDT file?

Both contain photometric data. IES files (.ies) follow the North American standard (ANSI/IES LM-63). LDT files (.ldt) follow the European EULUMDAT format. Both are supported by DIALux. AGi32 natively reads .ies files; LDT files can be converted using Photometric Toolbox. For US projects, .ies is the standard format.

Do I need IES files to get a photometric plan?

You don't need to provide them yourself. If you've specified fixtures, the photometric designer will source the IES files from the manufacturer. If you haven't specified fixtures, the designer will select appropriate products and use their IES data for the calculations. The IES files are the designer's raw material — as essential to a photometric plan as structural loads are to a structural drawing.

What is IES LM-79?

ANSI/IES LM-79 is the approved test method for measuring the optical and electrical performance of solid-state (LED) lighting products. It specifies how a complete LED luminaire is tested in a goniophotometer to measure total lumens, input power, efficacy, color, and intensity distribution. The data produced by an LM-79 test is what gets formatted into the IES file.