How to Read IES and LDT Photometric Files Before Buying LED Luminaires

Short answer

Use IES or LDT files to understand where a luminaire sends light and to calculate its effect in a model. Do not accept a file merely because it opens. Check identity and test conditions, review output and intensity values in the relevant planes, look for unexpected symmetry or tilt assumptions, and compare the imported file with the supplier's datasheet. A photometric file is calculation input, not a complete product approval.

1. Review the decision fields first

File evidence	Question to answer before use	Why it matters to a buyer
Format and revision	Is it an IES LM-63 file or EULUMDAT LDT file that the selected software imports correctly?	Format conventions differ; successful import does not prove correct interpretation.
Product identity	Do manufacturer, catalogue number, test number, date, and description match the offered variant?	A related family file can produce a plausible but wrong calculation.
Dimensions and luminous area	Do physical and emitting dimensions agree with the drawing and visible aperture?	Dimensions affect placement, visualization, and glare-related interpretation.
Output and scaling	What luminaire output, lamp basis, multiplier, or normalization does the file use?	Incorrect scaling changes calculated illuminance even when the curve shape looks right.
Angles, planes, and symmetry	Are all necessary directions represented, and is declared symmetry valid for the optic?	Missing or misread planes can conceal an asymmetric, oval, wall-wash, or batwing distribution.
Intensity data	Where are peak values, cut-off changes, uplight, spill, and distribution differences?	Candela values determine how output is directed toward tasks, walls, ceilings, and observers.
Tilt or orientation	Does the file assume a lamp or luminaire tilt, and is orientation clear in the model?	A rotated or tilted distribution can place light on the wrong side of the project.

2. Know what IES and LDT files contain

IES files follow the Illuminating Engineering Society's LM-63 exchange format. LDT files follow the EULUMDAT convention commonly used by European lighting software and manufacturers. Both are text-based ways to transfer luminaire photometric information, especially luminous intensity values sampled at defined angles, with identifying and geometric data. They arrange fields differently, so do not compare raw line positions across formats.

The extension does not establish measurement quality. A file may come from laboratory measurement, transformed source data, a family approximation, or an incorrectly exported record. Read the keywords or identity fields and request the underlying photometric report when the decision is critical. Keep the original file unchanged in the submittal record; conversion between formats can introduce orientation, symmetry, rounding, or metadata errors that deserve a visual cross-check.

3. Connect intensity distribution to the drawing

Open the file in trusted photometric or lighting-calculation software and view the polar curves, candela table, three-dimensional distribution, zonal output, and relevant horizontal and vertical sections. A rotationally symmetric downlight may need only a compact representation, while a linear luminaire, elliptical spotlight, wall washer, aisle optic, or asymmetric floodlight needs clear plane orientation. Confirm which direction in the file corresponds to the luminaire's length, cross-axis, wall side, or aiming mark.

Beam and field angles are derived from intensity profiles and may differ by plane. One headline angle cannot fully describe non-symmetric distributions. Look at the full curve for shoulders, secondary peaks, abrupt cut-off, uplight, and spill. Peak candela indicates intensity in one direction; total lumens describe output integrated over directions. Both are needed to understand a narrow high-intensity beam versus a broad lower-intensity distribution.

4. Verify the exact variant and calculation model

Match model number, wattage or driver setting, LED option, CCT where output differs, optic, trim, diffuser, finish, emergency mode, mounting arrangement, and accessories. A louvre, honeycomb, snoot, lens, cover, or dark finish may change output or intensity. If the supplier proposes a substitute, require a new matching file rather than rescaling the old result without technical evidence.

Then calculate the real room. Set dimensions, mounting height, aiming, orientation, surface reflectances, working planes, vertical targets, calculation grids, maintenance assumptions, and control scenes. Review minimum, average, maximum, uniformity, vertical illumination, and glare-related outputs that fit the project. Inspect false-color views for clues, but use numerical results and geometry for decisions. Record software version and settings so the calculation can be reproduced.

Photometric file checklist

File format and revision are supported by the calculation software.
Manufacturer, model, test reference, description, and date are identifiable.
Wattage, driver setting, optic, CCT option, trim, and accessories match the offer.
Reported output and scaling agree with the product datasheet or test report.
Physical and luminous dimensions agree with the dimensional drawing.
Horizontal and vertical plane orientation is clear for the installed luminaire.
Declared symmetry matches the visible optic and intensity distribution.
Peak intensity, beam shape, spill, uplight, and cut-off are reviewed.
The project model uses correct mounting, aiming, reflectances, grids, and maintenance assumptions.
File, report, drawing, calculation, and approved sample share one controlled variant identity.

Common limitations

IES and LDT files do not by themselves prove electrical safety, input power, efficacy, color quality, flicker, dimming compatibility, thermal behavior, ingress protection, mechanical strength, lifetime, or market compliance. Metadata may be incomplete, and some format fields originated around lamp-based luminaires. Software may display defaults when information is absent; do not mistake a populated interface field for measured data.

Photometric measurements also represent stated operating and laboratory conditions. They do not automatically predict dirt, aging, voltage variation, installation tolerances, furniture changes, daylight, or surface finishes. Interpolation between measured angles and calculation points introduces another layer of modeling. A clean simulation can still be wrong when its source file, orientation, scale, or room assumptions are wrong.

What to include in a buyer inquiry

Send the luminaire schedule, model naming convention, plans, sections, mounting and aiming details, quantity, voltage, control method, CCT, optic, finish, accessories, and required project outputs. Request the original IES or LDT file for every proposed variant, plus the matching photometric report, datasheet, dimensional drawing, and explanation of orientation. Ask the supplier to identify any converted or calculated file. Browse AOCEN's commercial lighting products, review the catalogs, or contact AOCEN with the schedule. Exact files and project suitability require confirmation.

Related guides

Read the lumens, lux, candela, and luminance guide to interpret the quantities carried into a calculation. Use the beam angle guide when checking throw, field spread, and plane-specific distributions. The UGR and anti-glare guide explains why glare requires room and observer data beyond the file alone.

Source notes

The Illuminating Engineering Society publishes ANSI/IES LM-63 for electronic transfer of photometric data and identifies photometric data formats within its Computer Committee scope. DIALux provides a field-level description of the EULUMDAT LDT format. These references define data exchange; project acceptance still depends on exact-product evidence and the applicable design standard.