An IES file is the only document in a lighting quotation that cannot lie comfortably. A brochure can claim any lumen figure; a datasheet can round in its own favour; a marketing page can quote chip output and call it fixture output. An IES file has to state a candela value for every angle, and those numbers integrate to a total. If the total does not match the claim, the file says so.
That is why “send me the IES file for the exact SKU you are quoting” is the most useful sentence in industrial lighting procurement — and why a supplier who cannot produce one is telling you something important before you have read a single number.
This guide opens the file and reads it. You do not need to be a lighting designer to do this; you need a text editor and about five minutes per file.
Key Takeaways
- An IES file is plain text. Open it in Notepad — the header tells you who tested it, when, and on what.
- The ten-number parameter line holds the whole geometry.
-1in the lumens-per-lamp field means absolute photometry, which is what you want from an LED fixture.- The multiplier and ballast factor should both be 1.0. Anything else scales every candela value in the file, and it is the quietest way to inflate a fixture.
- Integrate the candela values and compare to the claimed lumens. A mismatch is the single most reliable sign of a fabricated file.
- Identical distributions across different wattages mean one file was scaled to fill a catalogue, not measured per SKU.
What an IES File Actually Is
An IES file (.ies) follows IESNA LM-63, the North American standard format for exchanging photometric data. Its European equivalent is the EULUMDAT file (.ldt); the two carry the same information in different syntax, and design software reads both.
The file describes one thing: how much light the fixture emits in every direction, expressed as candela values on a grid of vertical and horizontal angles. Everything a lighting calculation produces — delivered lux at your mounting height, uniformity, glare, spacing — is computed from that grid. Without it, every performance claim about a fixture in your building is unverifiable.
Critically, an IES file is a description, not a certificate. Anyone can write one in a text editor. What makes a file trustworthy is the test data behind it, which is why the header matters as much as the numbers.
The Header: Who Tested It, and On What
Open the file and you will see keyword lines before any numbers:
IESNA:LM-63-2002
[TEST] LTL-24-0891
[TESTLAB] Accredited Photometric Laboratory
[ISSUEDATE] 2026-04-18
[MANUFAC] Sunjoylight
[LUMCAT] CDNFC9185
[LUMINAIRE] 200W LED Flood Light, die-cast aluminium, 60° optic
[LAMP] Integrated LED module
[BALLAST] Integrated constant-current driver
TILT=NONEWhat to check here, in order of usefulness:
[TESTLAB]and[TEST]— a named laboratory and a report number mean the file traces back to a physical measurement you can ask to see. A blank[TESTLAB], or a manufacturer’s own name with no report reference, means the file may be a simulation.[ISSUEDATE]— a file dated years before the product you are buying suggests it was inherited from an earlier generation.[LUMCAT]— the catalogue number. This must match the SKU on your quotation. A file for a neighbouring model is the most common substitution, and it is usually innocent carelessness rather than fraud — but it makes your calculation wrong either way.[LUMINAIRE]— should describe the optic and wattage. If it is generic, ask which optic was tested.TILT=NONE— normal for a fixture whose output does not change with tilt angle.TILT=INCLUDEintroduces a correction table and appears mainly on discharge sources.
The Parameter Line: Ten Numbers That Define Everything
Immediately after TILT, two lines carry the structure of the whole file:
1 -1 1 37 73 1 2 0.43 0.09 0.32
1.0 1.0 201.3The first line, field by field:
| # | Field | Typical | What it means |
|---|---|---|---|
| 1 | Number of lamps | 1 | One light source (an LED module counts as one) |
| 2 | Lumens per lamp | -1 | -1 = absolute photometry — candela values are the fixture’s own |
| 3 | Candela multiplier | 1 | Scales every candela value in the file. Should be 1 |
| 4 | Number of vertical angles | 37 | Rows in the candela grid (0–180° in 5° steps here) |
| 5 | Number of horizontal angles | 73 | Columns (0–360° in 5° steps here) |
| 6 | Photometric type | 1 | 1 = Type C, the normal type for architectural and area luminaires |
| 7 | Units type | 2 | 1 = feet, 2 = metres |
| 8–10 | Luminous opening W × L × H | 0.43 0.09 0.32 | The emitting aperture in metres — used for glare calculation |
The second line is ballast factor · future use · input watts:
- Ballast factor should be
1.0. It multiplies output, like the candela multiplier. - Input watts (
201.3here) is the measured power. Divide total lumens by this to get the fixture’s true efficacy — and compare it to what the datasheet claims.
Field 2: absolute vs relative photometry
This is the field that separates modern LED files from legacy ones, and it is worth understanding properly.
-1 means absolute photometry. The candela values are the actual measured output of the complete fixture. This is correct for LEDs, where the source is not replaceable and the fixture was measured as a unit under LM-79.
A positive number means relative photometry — the file describes a distribution for a lamp of that nominal output, and the software scales it. That was the right approach when a fixture took a replaceable 400 W metal halide lamp. On an integrated LED fixture it is a warning sign: it means the file may have been built by applying a distribution to an assumed lumen figure rather than by measuring the product.
The Angle Arrays and the Candela Grid
Next come the vertical angles, then the horizontal angles, then the candela values:
0 5 10 15 20 ... 175 180
0 5 10 15 ... 355 360
34444 34210 33580 32490 ...Reading this is simpler than it looks:
- Vertical angles run from 0° (straight down, for a downlight) to 180° (straight up). A file ending at 90° describes a fixture with no uplight at all.
- Horizontal angles describe rotation around the vertical axis. A symmetric fixture may list only
0 90or a single0; an asymmetric road or flood optic will list a full set. - The candela block is then read row by row: for each horizontal angle, the intensity at every vertical angle.
The first value in the block is normally the peak intensity on the beam axis, and it is the number you can immediately turn into delivered light:
lux = candela ÷ distance²
So 34,444 cd at 8 m mounting gives 34,444 ÷ 64 ≈ 538 lx directly beneath the fixture. That single arithmetic step, done in the quotation meeting, tells you more than the entire brochure. The relationship is explained in full in lumens vs lux vs candela.
The Five-Minute Verification
You do not need software to sanity-check a file. Four checks catch most problems:
1. Does the catalogue number match your quotation?
[LUMCAT] against the SKU line on the quote. If they differ, stop and ask.
2. Are the multiplier and ballast factor both 1.0? Fields 3 and the first value on the second line. A multiplier of, say, 1.15 silently raises every candela value by 15%, and the file will still integrate consistently with itself — which is exactly why it is the quietest available trick. There are legitimate uses (a tested fixture at a different drive current), but they should be disclosed, not discovered.
3. Does the total flux match the claim? Any lighting software reports total luminous flux when it loads a file; free IES viewers do the same. Compare it to the datasheet. A 10% shortfall is a rounding argument; a 30% shortfall is a different product.
4. Does peak candela ÷ mounting height² give a plausible lux? If a supplier claims 500 lx at 10 m, they are claiming 50,000 cd. If the file’s peak is 20,000 cd, the claim is arithmetically impossible.
Red Flags
Identical distributions across different wattages. Load the 100 W, 200 W and 400 W files for the same series and compare the shape of the curve and the ratio of peak to total. Real measurements of different builds differ in small, untidy ways. Files generated by scaling one measurement are suspiciously proportional.
Perfectly smooth candela values. Measured data has small irregularities. A curve that is mathematically clean often came from an optical simulation rather than a goniophotometer.
No test lab, no report number, no date. Any one of these missing is careless. All three missing means there is probably no underlying measurement to reference.
Chip lumens in the datasheet, fixture lumens absent. If the datasheet quotes an LED package figure multiplied by the number of chips, the fixture will typically deliver 10–20% less. The IES file is where that gap becomes visible, which is why some suppliers are reluctant to send one.
A file that will not load, or loads with warnings. Malformed files are usually hand-edited files.
Using the File in a Calculation
Once the file passes inspection, it goes into DIALux evo, Relux or AGi32 together with your room or site geometry. What the software then returns is the only performance figure that matters: maintained lux on your working plane, at your mounting heights, with your spacings and your maintenance factor — plus uniformity and glare, which no catalogue can quote because they are properties of your building rather than of the product.
Two practical notes. Import the file for the exact configuration you will order — optic, wattage and CCT — since changing the lens changes everything the file describes. And state the maintenance factor you used in the output, because a study run at 0.8 in a dusty building will overpromise by more than 10%.
What to Ask For, in One Line
“Please send the IES file and the photometric test report for the exact SKU, optic and wattage on the quotation.”
Both, not either. The IES file lets your designer calculate; the test report shows where the file came from. We publish full goniophotometer reports for the units we have measured — including a 200 W flood at 35,309 lm and 175.4 lm/W with a peak intensity of 34,444 cd — as downloadable PDFs on the LED flood lights page, alongside the 150W and 200W pages that carry their own measured reports. Per-SKU IES files ship with every quotation.
The Bottom Line
The IES file is the point where a lighting claim becomes checkable. It costs nothing to request, takes five minutes to inspect, and turns a conversation about brochures into a conversation about measured performance. The four checks above — catalogue number, multiplier, total flux, peak candela — will filter out most of what is wrong with the fixtures on offer in this market.
Send us your geometry and target level and our engineers return the calculation with the IES files and test reports behind it. For how the underlying quantities relate, start with lumens vs lux vs candela; for applying them to a beam-driven product, see the flood light selection guide.
FAQ
What is an IES file? A plain-text photometric data file following IESNA LM-63 that describes how much light a luminaire emits in every direction, as candela values on a grid of vertical and horizontal angles. Lighting design software uses it to calculate delivered lux, uniformity and glare for a specific building. Its European equivalent is the EULUMDAT (.ldt) file; most software reads both.
How do I open and read an IES file? Any text editor opens it — it is plain text. The header keywords tell you the test lab, report number, date and catalogue number; the ten-number parameter line defines the grid and the scaling; then the vertical angles, horizontal angles and candela values follow. Free IES viewers add a polar plot and a total-lumens readout without needing full design software.
What does -1 mean in an IES file? It appears in the lumens-per-lamp field and signals absolute photometry: the candela values are the measured output of the complete fixture rather than a distribution to be scaled by a lamp’s rating. This is the correct form for an integrated LED luminaire. A positive value indicates relative photometry, which on an LED fixture suggests the file was constructed rather than measured.
What is the candela multiplier and why does it matter? It is a factor applied to every candela value in the file. It should be 1.0. A multiplier above 1 raises the whole distribution proportionally, so the file remains internally consistent while describing a brighter fixture than was tested. Legitimate uses exist — for example a fixture measured at a different drive current — but they should be stated openly in the quotation.
How can I tell if an IES file is fake? Four signs: the catalogue number does not match the quoted SKU; the multiplier or ballast factor is not 1.0 without explanation; the total flux does not match the datasheet; and the distribution is identical in shape across different wattages of the same series. A missing test lab, report number and issue date together mean there is probably no measurement behind the file at all.
Is an IES file the same as an LM-79 report? No. LM-79 is the test procedure and the report is the evidence — total flux, efficacy, power factor, colour data, measured on a specific sample. The IES file is the distribution data extracted from that measurement in a form software can read. Ask for both: the report shows the fixture was measured, the file lets your designer use the result.
Do I need an IES file for every wattage and optic? Yes, for anything you will actually install. Changing the optic changes the entire distribution, and changing the wattage changes the magnitude and often the thermal behaviour. A supplier offering one file for a whole series is offering one measurement stretched across products that were not measured.