How to Read an IES or LDT Photometric File: A B2B Buyer's Guide to LED Light Distribution Data

1. Why the Photometric File Matters More Than the Datasheet

If you buy LED panel lights, high bays, tunnel lights or roadway luminaires, you have probably compared suppliers on "4000 lm, 120° beam angle, 120 lm/W." Those three numbers decide nothing, because they say nothing about where the light actually goes.

A photometric file is the measured light distribution of one specific luminaire. It comes off a goniophotometer, a rig that rotates the fixture through hundreds of angle combinations and records luminous intensity in candela at each point. Software such as DIALux evo, Relux and AGi32 reads that file and predicts illuminance, uniformity and glare in your actual room or road.

Two standards do different jobs here. ANSI/IES LM-79 is the measurement standard: how you test a solid-state luminaire. ANSI/IES LM-63 is the file format standard: how you write the result into a text file other software can read. Ask for both.

2. IES vs. LDT (EULUMDAT): Ask for Both

Both encode the same physical data: a candela matrix at defined vertical and horizontal angles. Neither is "more accurate." The practical rule: European consultants ask for .ldt, North American consultants for .ies, international tenders want both. A factory that can only supply one format has likely never measured the rest of its catalog. Note that ANSI/IES TM-33-18 defines an XML-based successor to LM-63; both formats are valid during the transition.

3. Anatomy of an IES File, Line by Line

An .ies file is plain ASCII. Open it in Notepad and you will see four blocks.

The Ten-Number Parameter Line

This single line breaks more imports than anything else. The ten fields, in order:

Photometric Type: A, B or C

Type C is the default for downlights, troffers, panels and most architectural luminaires: the luminaire stays still and the measurement planes rotate around the vertical axis. Type B is for floodlights and adjustable heads; Type A is for automotive and some roadway geometries. If a panel light file declares Type B, the export is wrong, not your software.

Absolute vs. Relative Photometry

When field 2 shows -1, the file uses absolute (direct) photometry: the candela values are as measured, with no scaling to a rated lamp flux. That is correct modern practice and what an LM-79 report reflects. A positive number means relative photometry, a legacy habit from the fluorescent era. For LED, insist on absolute.

4. Five Numbers to Check Before You Trust a File

1. Version, lab and date. IESNA:LM-63-2002 or newer, a named [TESTLAB], and an issue date within roughly 24 months. Then ask for the LM-79 report with a matching report number.
2. Header lumens vs. the candela matrix. The zonal lumen sum should reconcile with the stated total. If the header claims 4,000 lm but integrating the candela data gives 3,300, the header was edited.
3. Efficacy sanity check. Divide lumens by input watts. A quality 600×600 backlit panel sits around 100-130 lm/W; a good LED high bay around 130-160 lm/W. A 4000K CRI 80 product claiming 190 lm/W is a claim, not a measurement.
4. Which beam angle definition? IES defines beam angle at 50% of maximum intensity; CIE and NEMA define it at 50% of center-beam intensity. For a symmetric beam they match; for a batwing distribution they can differ a lot. Also do not confuse beam angle with field angle, which uses a 10% threshold.
5. Luminaire dimensions and units. A file in meters imported into a feet-based project produces a luminaire of the wrong physical size, and near-field calculations silently drift.

Photometry tells you how a fixture performs on day one, not how long it holds that output. Pair these checks with the lifetime verification steps in our guide to LED L70 lumen maintenance and lifetime claims.

5. Reading the Distribution Curve

The polar plot is the fastest read. Concentric rings are vertical angle from straight down (0°) to horizontal (90°); radiating lines are intensity. The further the curve reaches, the more light goes that way.

Most software plots two planes: C0-C180 and C90-C270. Nearly identical curves mean a rotationally symmetric distribution, typical of a round downlight or square panel. Diverging curves mean an asymmetric optic, exactly what roadway and aisle lighting need. Our guide to IESNA Type I-V roadway distributions and pole spacing covers that case.

Then check the zonal lumen table: how much flux falls in 0-30°, 30-60°, 60-80°, 80-90° and 90-180°. For an office, downward flux should dominate and uplight should be near zero. For outdoor fixtures, those zones produce the IES TM-15 BUG rating (backlight, uplight, glare) that most North American municipal specs require.

A photometric file also lets you predict uniformity before a single carton ships. If you are still working out how many fixtures a space needs, start with lumens, lux and watts explained, refine with the simulation, then cross-check glare with our plain-language UGR guide.

6. Seven Red Flags in a Supplier's Photometric File

1. Candela values that are suspiciously smooth. Real goniophotometer data has small jitter. A perfectly mathematical curve usually means someone generated the file in a spreadsheet.
2. No lab name, no report number, no date. Untraceable data cannot be defended when the project fails acceptance.
3. One file for a whole family. 30W, 60W and 100W versions of the same housing use different LED counts and often different optics. One distribution cannot describe all three.
4. Photometry of the LED package or module, not the complete luminaire. Chip data ignores lens, diffuser and housing losses. Only whole-luminaire LM-79 data counts.
5. "LM-79 tested" with no report attached. The file is not the report. You need the document behind it.
6. Output above the physical ceiling. A 3,000 lm LED board cannot deliver 3,600 lm out of a diffuser. Optical losses only go one way.
7. Wrong units type or photometric type. These do not throw errors; they quietly produce a wrong lighting design, which is far worse.

7. What to Write Into Your RFQ

Make photometric data a pass-or-fail line item rather than a "please provide if available" footnote.

• .ies and .ldt files for every SKU: each wattage, each CCT (color temperature), each optic and beam option
• Absolute photometry, LM-63-2002 or LM-63-19 format, with the version line present
• An LM-79 test report naming the laboratory, with report number and date, issued within 24 months
• Zonal lumen table and luminaire efficacy, so you can reconcile lumens against candela data yourself
• For outdoor fixtures: IES TM-15 BUG rating and the IESNA distribution type
• A written statement that the file matches the production sample, plus a golden sample for verification

That last point matters most with new suppliers. Photometric files, samples and factory audits decide whether a first order goes smoothly; our sourcing checklist for buying LED lighting from China shows how to combine them.

8. Free Tools to Open a File

Notepad or Notepad++ shows the raw text in seconds. For the visual read, DIALux evo and ReluxDesktop are free and import both formats; AGi32 offers a free Photometric Toolbox; standalone IES viewers plot 2D and 3D polar diagrams. Files from public photometric libraries must match your exact SKU, including CCT and optic, before you trust the simulation.

9. The Takeaway

Photometric files are cheap to ask for and expensive to fake convincingly. Put them in the RFQ as a mandatory line, spend five minutes on the checks above, and most suppliers who only have a datasheet will filter themselves out.

Solarix Lighting supplies measured .ies and .ldt files with matching LM-79 reports for every panel light, high bay, downlight, tunnel light and roadway luminaire we build, and our team runs DIALux layouts for your project geometry before you commit to a container. Our fixtures are installed across 20+ metro and rail transit projects, Baiyun Airport T3 and Baiyun Railway Station, where measured photometry was a contract requirement. Send us your room or road geometry and target lux level, and we will return a layout with the photometric files attached.