How to Read a Photometric Report: IES Files, UGR and DIALux for Buyers

"We'll send you the IES file" is standard language in project lighting. If your next question is "great — what's an IES file?", this guide is for you. Photometric documents look intimidating but reduce to a handful of numbers that decide whether a warehouse actually hits its lux target. Here is what each piece means and when to insist on a full DIALux simulation.

Step one: the IES file

An IES (or EULUMDAT) file is the measured light distribution of one specific luminaire, recorded in a goniophotometer. It is not a brochure figure — it is a text file containing thousands of candela measurements across angles, usually taken by an accredited lab per IES LM-79 (whole-fixture testing). Everything downstream — lux calculations, UGR, lighting designs — reads from this file.

Key point for buyers: the IES file must match the exact configuration you buy. A file generated for the 4000 K, clear-diffuser, 60° optic version tells you little about the 5000 K opal 90° version you actually ordered. Ask for the file that corresponds to your ordered variant, with the LM-79 report to back it.

The five numbers that matter

• Luminous flux (lm). Total light output. Compare like with like — same CCT, same drive current. A fixture "rated 18,000 lm" at 25 °C delivers less in a 45 °C ceiling void.
• Efficacy (lm/W). System efficiency, driver losses included (LM-79 measures the whole fixture). Anything above 130–150 lm/W is respectable for industrial gear; claims beyond ~190 lm/W deserve scrutiny of test conditions.
• Beam angle. The angle containing 50% of peak intensity. Narrow beams (60–90°) put lux on the floor between racks; wide beams (120°) suit open areas. Matching beam to mounting height beats adding wattage almost every time.
• UGR (Unified Glare Rating). A discomfort-glare index. Offices typically target UGR < 19; warehouses care less about glare and more about vertical illuminance on rack faces. If a datasheet shows UGR without stating the room geometry, treat it as marketing.
• CCT and CRI. Color temperature (K) and color rendering (Ra). Industrial default is 4000–6500 K at Ra ≥ 70–80; inspection, color-matching and retail zones raise CRI requirements.

What a DIALux simulation adds

A DIALux (or Relux/AGi32) calculation takes the IES files plus your room — dimensions, mounting height, reflectances, racking layout — and outputs the lux distribution your project will actually see. This is where theory meets the building:

• Whether you hit the lux target (and where you overspend on fixtures)
• Uniformity — average lux alone hides dark corners; min/avg ratios matter
• Energy check — installed W/m² against local codes or client ESG requirements

Rule of thumb: for simple retrofits with one fixture type at standard mounting heights, the supplier's spacing tables are adequate. For new builds, tenders, sports facilities, or anything where payment depends on hitting a lux figure — run the simulation before ordering. Changing a layout on paper costs minutes; relamping a warehouse costs a fortune.

Red flags in photometric paperwork

• Simulated IES files. Files generated by software from LED specs rather than measured. Legitimate for early design, but confirm final measurements before contract.
• Missing test lab identity. Serious reports name the lab, test standard and date.
• One IES file for an entire range. Different wattages and optics genuinely measure differently.
• Efficacy claims above physics. If a mid-power industrial fixture claims 220 lm/W system efficacy, ask for the LM-79.

How we support project calculations

For bid-stage work we supply measured IES files per model and run DIALux checks on your layouts — free with a project inquiry. Send the floor plan or rack layout with mounting heights, and you receive a calculation report you can attach to your own quotation.

Frequently asked

What's the difference between IES and LDT (EULUMDAT)?

Two file formats for the same data — IES is the North American convention, LDT/EULUMDAT the European one. Modern software reads both. Ask for whichever your designer prefers.

Do LED photometrics change over time?

Yes — output degrades and the IES reflects new-fixture performance (see our article on L70 lumen maintenance). For maintenance-factor planning, designers typically apply a 0.8–0.9 depreciation factor depending on environment and cleaning regime.

Can I verify a report myself?

Basic sanity checks, yes: open the IES in any free viewer (e.g., a photometric viewer online) and compare total lumens and beam shape with the datasheet. Deep verification — goniometer setup, temperature conditions — requires the original LM-79 report, which is why we supply it with the file.