What is the IES file format?
A .ies file contains photometric data saved in ASCII text format that conforms to the Illuminating Engineering Society (IES) LM-63 standard. Photometric data include:
- Light source - total quantity of visible light emitted by the source; measured in lumens,
- Light intensity - amount of lumens in a given direction across a solid angle; measured in candelas,
- Light source power - measured in watts,
- A candela matrix arranged by vertical and horizontal angles defining the luminous-intensity distribution.
.ies files may also store metadata such as the manufacturer name, accession number, lamp description, and test laboratory details as structured [KEYWORD] pairs. The IES standard was first developed in 1986, updated in 1995 and 2002, and most recently revised in 2019 as ANSI/IES LM-63-19. Files from LM-63-1995 onward begin with a version identifier line such as IESNA:LM-63-2002; older 1986-era files start directly with keyword lines or the TILT= directive. The format is widely used in North America and Europe. .ies files are generated by an automated instrument for measuring emitted light - a goniophotometer.
.ies files are used by lighting equipment manufacturers for architectural project simulations. Such files allow designers to assess the efficiency of lighting systems before they are installed. Similar simulations are conducted for roadway illumination assessments. The format supports three photometry coordinate systems: Type C (the most common, used for indoor and outdoor luminaires), Type B, and Type A (used for automotive and floodlight applications). .ies is the North American counterpart to the European EULUMDAT (.ldt) format; most professional lighting tools read both. Unlike .ldt, .ies files do not include CRI (colour rendering index) or luminaire geometry data. .ies files can be opened with dedicated viewers, imported into CAD and rendering tools, and converted to other file formats.
Security & safety
RISK: LOWAn IES file is plain ASCII text with no executable code, so it is safe to open and cannot carry malware in any meaningful way. The only practical caution is sourcing: download IES files from the fixture manufacturer or a reputable photometric library so your lighting calculations are based on real measured data rather than someone's edited or fabricated file.
Format details
in a nutshell- The Bat! Message Index - Mailbox index file (.ies) used by The Bat! e-mail client; unrelated to lighting.
- ArcView Import/Export Specification - Legacy Esri ArcView export-definition file; rare and unrelated to photometry.
Programs that open IES files
Windows5 apps
Dialux evo
Free Import the IES via the luminaire catalogue / 'import luminaire' to run lighting calculations. Free professional lighting-design tool.
Autodesk 3ds Max
Paid Create a Photometric/IES light and load the .ies as its distribution for realistic render lighting.
Autodesk Revit
Paid Assign the IES file as a light source's photometric web in a lighting fixture family.
IESviewer (Andrey Legotin)
Freemium Open the IES to visualise its 3D/polar light distribution and read its parameters; also converts between IES and EULUMDAT. 
macOS2 apps
Blender
Open-source Add a light, set type to Spot/Area, and load the .ies through the IES texture node (Cycles) for accurate distribution.
Dialux evo (via Windows VM / Wine)
Free Dialux is Windows-only; run it in a VM/Parallels on Mac to import IES files. 
Linux1 app
Blender
Open-source Use the IES texture node on a light to load the .ies distribution; works natively on Linux.
IES Viewer (online photometric viewers)
Free Upload the .ies to view its polar/3D light distribution in the browser without installing software. Technical details
deep spec| Encoding | 7-bit ASCII plain text; Unicode code points above 0x7F are undefined in the standard |
| File structure | Optional version line → `[KEYWORD]` label block → `TILT=` directive → numeric test parameters → vertical-angle list → horizontal-angle list → candela matrix |
| Photometry coordinate systems | Type C (azimuthal, most common - indoor and outdoor luminaires), Type B (lateral tilt, floodlights), Type A (automotive) |
| Candela normalization | Values are typically normalized to 1 000 lumens and scaled at render time by the multiplying factor stored in the numeric header |
| TILT directive | `TILT=NONE`, `TILT=INCLUDE`, or `TILT=<filename>`; marks the boundary between the keyword block and the photometric data; defines whether tilt data is embedded, external, or absent |
| Angle-grid resolution | Variable; defined by the count and list of vertical angles (0°-180°) and horizontal angles (0°-360°) declared in the numeric header; parser rejects files where declared counts do not match the data |
| Typical file size | 2 KB - 200 KB; size scales with the number of vertical and horizontal measurement angles |
| MIME type | `text/plain`; also recognized as `application/x-ies` |
| File identification | No binary magic bytes; LM-63-1995 and later files begin with a version line such as `IESNA:LM-63-2002`; LM-63-1986 files begin directly with keyword lines or `TILT=` |
| Standard revision history | LM-63-1986 (original), LM-63-1991, LM-63-1995, LM-63-2002, ANSI/IES LM-63-2019; each revision extended the keyword vocabulary |
| Metadata keywords | Structured `[KEYWORD]` pairs including `[MANUFAC]`, `[LUMCAT]`, `[TESTLAB]`, `[ISSUEDATE]`, `[LUMINAIRE]`, `[LAMPCAT]`; extended set standardized in LM-63-2002 and 2019 |
| Data capture instrument | Goniophotometer; an automated goniometric device that measures luminous-intensity distribution at defined angle increments |
| Compression and encryption | None; stored as uncompressed, unencrypted plain text |
| Geographic adoption | Dominant standard in North America; widely accepted internationally; the parallel European format is EULUMDAT (.ldt) |
| Cross-platform support | Platform-independent plain-text format; read by Windows, macOS, and Linux CAD, BIM, and rendering applications |
| Released | 1986 (IES LM-63-1986); current revisions LM-63-2002 and LM-63-2019 |
| Latest version | ANSI/IES LM-63-2019 |
| Open standard | Yes · royalty-free |
| Specification | store.ies.org |
IES conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about IES files.
