دليل ملفات IES/LDT

Every lighting specification eventually reduces to a text file. The photometric file carries the one piece of information no datasheet can replace: the actual three-dimensional intensity distribution of the luminaire. Engineers who can read that file directly win arguments that engineers who cannot must settle by simulation trial and error. This white paper dissects the two formats in commercial use, IES NA LM-63 and EULUMDAT (LDT), explains every field that changes a calculation, and shows how to compare the distribution against the application before the design is frozen.

The intended reader is a specifying engineer, a luminaire designer, or a procurement engineer who has received a photometric file and needs to decide whether it is trustworthy and whether the luminaire it describes will solve the problem.

1. Why the photometric file is the controlling document

A datasheet reports scalar values: 150 lm/W, 5000 K, 150 W. A photometric file reports a function. Two luminaires with identical datasheet numbers can differ by a factor of three in the illuminance they deliver to a task surface, purely because one puts its flux directly under the fixture and the other spreads it to the ceiling and walls.

The economics follow directly. In a warehouse, changing from a narrow 60° distribution to a wide 120° distribution at the same flux changes the required fixture count, the electrical load, the installed cable, the ceiling structural loading and the maintenance labour. The file is therefore not a compliance attachment — it is the input that determines capital cost.

A useful discipline is to treat the first three documents as a single package. A file without a traceable LM-79 report behind it is an unverified distribution, and a stated efficacy without a photometric file is a number that cannot be designed with.

2. What is inside the file: the IES LM-63 structure

The IES format is ASCII and line-oriented. Its header block is fixed in order, which makes automated parsing straightforward and makes manual reading surprisingly pleasant once the sequence is known.

The equivalent LDT structure uses a keyword-value header with a numbered record list, followed by the same candela matrix. The angle arrays in LDT are not necessarily evenly spaced either, and the number of values per line is not fixed, so a parser must count rather than assume line boundaries.

3. Field-by-field interpretation

The table below lists the fields that actually change a downstream calculation, together with the failure mode when each is misread. The distinction between informational fields and computational fields is worth internalising: only the computational fields affect the result.

Two fields deserve elaboration. The candela multiplier exists because the test laboratory normalises the tabulated values to a convenient range; the true candela is the tabulated value multiplied by this factor. Dropping it produces a uniformly scaled result that looks plausible, and the error is invisible unless the file flux is reconciled against the datasheet. The photometric type determines the coordinate system: Type C luminaires are measured with the polar axis vertical, which is correct for general and industrial luminaires, while Types A and B are used for automotive and a few specialised applications. Reading a Type B file as Type C produces a distribution rotated by ninety degrees — a spectacular and sometimes expensive mistake in a road or tunnel design.

3.1 Reconciling file flux against datasheet flux

Before using any file, integrate the intensity distribution over the full sphere and compare the result with the stated lumen output. The integration is straightforward: sum the average intensity in each angular zone, multiplied by the solid angle of that zone.

If the integrated total differs from the datasheet lumen figure by more than about five percent, one of the following is true: the datasheet figure is nominal rather than measured, the file was measured on a different variant or drive current, or the multiplier is missing. Resolving this discrepancy before design is cheaper than resolving it after installation.

4. Polar curves and what the shape tells you

The polar intensity diagram is the file rendered visually. The blue curve below shows a typical industrial high-bay intensity lobe; the amber curve shows a symmetric wide distribution.

Three features of this plot carry design information. First, the peak intensity determines the maximum illuminance directly beneath the luminaire and therefore the worst-case glare contribution. Second, the half-peak angle defines the beam width conventionally quoted on datasheets. Third, the shape between the half-peak point and the 90° cut-off determines whether the distribution leaks flux toward the walls: a lobe that still carries significant intensity at 70–80° will glare observers and waste flux on vertical surfaces.

The beam angle quoted in marketing material is the full angle between the two directions at which intensity falls to fifty percent of peak. It is a single number extracted from a continuous function, and it is therefore a coarse descriptor. Two luminaires both sold as “90°” can differ by thirty percent in the illuminance they deliver at the edge of their specified coverage.

5. Distribution types I to V

Outdoor and industrial luminaires are classified by how the lateral intensity profile behaves, using a five-type system originally developed for roadway lighting and now applied wherever lateral throw matters.

Type V is the correct classification for a rotationally symmetric high-bay luminaire, and it is the reason a Z-16 high-bay installation can be calculated with a single azimuthal assumption. Type IV is common for Z-15 tri-proof luminaires mounted on a wall to light a loading apron, where the asymmetric throw keeps flux on the ground and off the building face. Selecting a Type III optic for a Type V task produces a characteristic symptom: adequate illuminance along the luminaire row and a dark band between rows, because the distribution never reaches sideways.

6. Measured versus nominal flux

The gap between the nominal flux on a datasheet and the flux actually represented in the photometric file is one of the most common sources of an optimistic design. The curves below show the ratio of file-integrated flux to nominal flux for three product classes.

At nominal drive current the file flux sits below the datasheet figure for all three classes, because the datasheet is measured under controlled 25 °C conditions while the file usually represents the luminaire in its optical assembly with its own thermal load. The divergence widens at reduced drive current, where the datasheet figure is often a linear extrapolation that does not account for the falling efficacy of the LEDs.

The practical rule is to design against the file, not the datasheet, and to state the drive current at which the file was measured. A design built on datasheet flux and verified against a file will consistently fail commissioning measurement by eight to twelve percent — an error that is usually blamed on the meter.

7. Common errors in handling photometric files

The last row is administrative, and it is the cause of more rework than any technical error on the list. Photometric files are revised whenever an optic, diffuser, gasket or drive current changes, and a design report that does not identify the file revision cannot be defended six months later.

8. Where the file format affects the product design

The choice between an IES file and an LDT file is largely a market convention rather than a technical one, but it has consequences for the luminaire itself.

Neither format carries spectral data, which matters for horticulture and for colour-critical work: a PPFD calculation requires spectral power distribution in addition to the photometric file, and a colour-appearance specification requires the SPD and the binning data. For Z-13 cleanroom panels in a colour-critical inspection context, the photometric file must be requested together with the SPD and the LM-79 colour report; the file alone specifies the geometry of the light, not its spectrum.

9. Verification procedure before accepting a file

A five-minute check catches most defects. The steps below are ordered so that the cheap structural checks precede the expensive numerical ones.

1. Confirm the format ident and the software version tag at the top of the file.
2. Confirm the luminaire catalogue code matches the product ordered, including optic variant and drive current.
3. Check that the stated number of vertical and horizontal angles matches the count of values in the arrays.
4. Multiply the peak tabulated candela by the candela multiplier and compare with the expected peak from the datasheet photometric curve.
5. Integrate the distribution and reconcile against the stated lumens per lamp times the lamp count.
6. Inspect the polar plot for rotational symmetry and confirm the distribution type matches the intended application.
7. Check the upward hemisphere for unintended flux, particularly for a luminaire that will be used in an uplight-rated installation.
8. Record the file name, revision and date in the design report.

10. Conclusion

The photometric file is the only document that describes how a luminaire will actually light a space, and it is readable by anyone willing to spend an hour with the specification. For Z-16 high-bay, Z-13 cleanroom and Z-15 tri-proof luminaires, the working sequence is: obtain the file for the exact variant, reconcile its integrated flux against the datasheet, read the polar plot to confirm the distribution type suited to the application, and record the revision in the design report.

Two design habits follow from the analysis above. First, always design against file flux rather than nominal flux, and state the drive current at which the file was measured. Second, treat the distribution shape as a design variable of equal weight to flux: the difference between choosing a 60° and a 120° optic is frequently larger than the difference between two competing suppliers at the same beam angle.

11. Referenced standards

• IES LM-63 — Approved method: standard file format for the electronic transfer of photometric data and related information
• IES LM-79 — Approved method: electrical and photometric measurements of solid-state lighting products
• IES LM-80 — Approved method: measuring luminous flux and colour maintenance of LED light sources
• IES TM-21 — Projecting long term lumen maintenance of LED light sources
• ANSI C78.377 — American national standard for electric lamps: specifications for the chromaticity of solid state lighting products
• CIE 121 — The photometry and goniophotometry of luminaires
• EN 13032-1 — Light and lighting: measurement and presentation of photometric data of lamps and luminaires
• IEC 62471 — Photobiological safety of lamps and lamp systems

12. Contact us

QUEENDOM supplies IES and LDT photometric files for the industrial and cleanroom luminaire range, together with the LM-79 reports that back them. Send the intended application, mounting height and target illuminance, and the engineering group will return the matching file, the recommended optic variant, and a reference layout so that the distribution can be checked against the application before the order is placed.

Related products and applications

The luminaire families whose IES/LDT files are published are listed below.

• High-bay luminaire (Z-16)
• Industrial high-bay luminaire (Z-13)
• Tri-proof luminaire (Z-15)
• Application overview: Lighting application solutions
• More technical papers: Lighting knowledge resources