IES File Format Explained: How to Read & Use Photometric Data
What is an IES file? How to read photometric data, candela distributions, beam angles. Free IES viewer online. Guide for lighting designers and contractors.
Every professional lighting fixture comes with an IES file — a standardized photometric data file that describes exactly how the fixture distributes light. Understanding IES files is essential for accurate lighting design. Here's everything you need to know.
What Is an IES File?
IES stands for Illuminating Engineering Society. An IES file (extension: .ies) contains the measured light distribution pattern of a fixture in a standardized text format called IESNA LM-63.
Think of it as a fixture's "fingerprint" — it tells software exactly how much light goes in every direction.
What's Inside an IES File
An IES file contains:
Key Metrics You Can Extract
Beam Angle
The cone where light intensity is ≥50% of peak. Common values:
- 60° — Narrow, focused (spot lighting)
- 90° — Medium (general indoor)
- 120° — Wide (high bay, area lighting)
Field Angle
The cone where intensity is ≥10% of peak. Usually 20-40° wider than beam angle.
Total Lumens
Sum of light output in all directions. Listed in the header.
Efficacy
Lumens ÷ watts. Higher = more efficient. DLC requires ≥110 lm/W for most categories.
Distribution Type (IESNA)
- Type I — Two-way, narrow (pathways)
- Type II — Slightly wider (residential streets)
- Type III — Asymmetric forward (parking perimeters)
- Type IV — Wider asymmetric (building fronts)
- Type V — Symmetric circular (open areas)
How to View IES Files
Option 1: LightSpec AI (Free, Online)
Upload any IES file to [LightSpec AI](https://www.auvolar.com/tools/lightspec-ai) and get:
- Interactive 3D polar chart
- Beam/field angle extraction
- Photometric simulation on your room dimensions
- Comparison with other fixtures
Option 2: Desktop Software
- AGi32 — Professional, $1,500+ license
- DIALux — Free, German software
- Photometrics Pro — Budget option
Option 3: Text Editor
IES files are plain text. Open in Notepad to see raw data. Not visual, but useful for quick checks.
IES vs LDT Files
| Feature | IES (IESNA LM-63) | LDT (EULUMDAT) |
|---|---|---|
| Region | North America | Europe |
| Standard | LM-63-2002 | DIN 5040 |
| File ext | .ies | .ldt |
| Unit | Candela | Candela |
| Supported by | AGi32, LightSpec AI | DIALux, RELUX |
LightSpec AI supports both IES and LDT formats.
How IES Data Affects Lighting Design
The IES file determines:
- Fixture spacing — Wide beam = fewer fixtures, wider spacing
- Uniformity — Asymmetric distributions may cause hot/dark spots
- Mounting height — Narrow beams need lower mounting for coverage
- Fixture count — The #1 factor in project cost
Frequently Asked Questions
Q: Where do I get IES files?A: From the fixture manufacturer. Most provide downloads on their website. Auvolar IES files are available on each product page and pre-loaded in LightSpec AI.
Q: Can I use IES files from one fixture to simulate another?A: No. Every fixture model has a unique distribution pattern. Using the wrong IES file will give inaccurate results.
Q: What if a manufacturer doesn't provide IES files?A: Red flag. Reputable manufacturers always provide IES data. No IES file usually means the product hasn't been photometrically tested, which also means no DLC listing and no rebate eligibility.
Need Help With Your Lighting Project?
Auvolar provides free lighting design, photometric layouts, and rebate assistance for commercial projects.
Auvolar Engineering Team
City of Industry, California
Our engineering team has 15+ years of combined experience in commercial LED lighting design, photometric analysis, and energy-efficient building systems. We hold DLC QPL listing expertise and work directly with California utilities on rebate qualification. All technical content is reviewed by licensed electrical engineers.