IES and LDT Files: How to Read a Luminaire's Photometric Data

Open the download page of a professional luminaire and you will often find, next to the datasheet and the DWG, a small zip file marked "photometric data". Inside are two plain text files: one ending in .ies, one in .ldt. They are easy to overlook, but they are the documents in the package that describe what the fitting actually does. Not what it looks like, not what the marketing render promises: how much light it sends, and in which directions, measured in a laboratory.

This guide sets out how to open and read both files without any lighting software: what the numbers mean, where the beam angle on the datasheet comes from, which values you can take straight into a specification, and where the file ends and calculation software has to begin. It is written for architects and designers who specify luminaires, whether or not they ever run DIALux, Relux or AGi32 themselves.

What a Photometric File Is

A photometric file is the written result of a laboratory measurement. The luminaire is mounted on a goniophotometer, an instrument the IES standard LM-63 defines as a photometer for measuring the directional light distribution of sources and luminaires, and its luminous intensity is recorded direction by direction. The result is a table of candela values: the strength of the light in each direction.

Two text formats carry that table. The IES file, defined by the ANSI/IES LM-63 standard, is the common format in North America. The LDT file, in the EULUMDAT format described by Axel Stockmar in 1990 and extended in 1998, is its European counterpart. They hold the same physics in a different layout, and as our recessed lighting spacing guide notes, calculation software such as DIALux, Relux, AGi32, Visual or ElumTools imports either one.

Both formats are old, stable and plain text. LM-63's 2019 edition is the standard's final update; its own foreword says the format is entering long-term stability and will eventually be replaced by the XML-based ANSI/IES TM-33. Until that transition finishes, the IES file and the LDT file remain what manufacturers actually publish, so they are what a specifier actually receives.

The Coordinates: C-Planes and Gamma

Every number in the file is pinned to a direction, and both formats use the same coordinate system to name directions: the CIE's (C, γ) system.

Imagine the luminaire at the centre of a sphere. The gamma angle runs around a vertical slice of that sphere: 0° points straight down, 90° is horizontal, 180° points straight up. The C-planes are the vertical half-planes that fan out around the luminaire's axis, named by their horizontal angle: C0, C90, C180, C270 and everything between. A direction is a pair: one C-plane, one gamma angle on it.

Elevation view: gamma runs from 0° straight down to 180° straight up. Diagram: Archlior.

Plan view, seen from below: the C-planes run clockwise around the luminaire. Diagram: Archlior.

One European convention is worth knowing before reading a linear luminaire's data. EN 13032-1 places the axis of a linear lamp at right angles to the C0 plane, so a linear fitting's long axis lies along C90-C270. The C0-C180 curve of a linear pendant is therefore the slice across it, and the C90-C270 curve is the slice along it. The two usually look completely different.

A file rarely holds every direction. It holds a chosen set of planes and angles, and its symmetry declarations tell you how to read the gaps. In an IES file, the list of horizontal angles does this: a single 0 means the light is the same in every plane, a list ending at 90 means the fitting is symmetric in each quadrant, 180 means symmetric about one plane, 360 means no symmetry at all. In an LDT file a one-digit symmetry indicator carries the same information.

Opening an IES File

An IES file opens in any text editor. A complete example, with invented values, reads like this:

IESNA:LM-63-2002
[TEST] EL-2026-0417
[TESTLAB] Example Photometric Laboratory
[ISSUEDATE] 2026-09-01
[MANUFAC] Example Lighting
[LUMCAT] EX-DL-150
[LUMINAIRE] Round downlight, 150 mm lens
[LAMPCAT] LED-12W-3000K
[LAMP] LED module 12 W 3000 K
TILT=NONE
1 -1 1.0 3 1 1 2 -0.15 -0.15 0
1.0 1 12.5
0 45 90
0
850 420 0

The first line names the standard edition; software reads it to choose its parsing rules, and you read it before trusting anything else, because two editions still circulate and one field changes meaning between them. The keyword block follows, each label in square brackets: who tested the fitting and under which report number, who makes it, its catalogue number, a description, and what lamp or LED module was measured. Four keywords are required: TEST, TESTLAB, ISSUEDATE and MANUFAC.

TILT=NONE closes the keyword block and says the output does not change when the fitting tilts, which is true for most LED products. Then come ten numbers on one line, in a fixed order: number of lamps, rated lumens per lamp, a candela multiplier, the counts of vertical and horizontal angles, the photometric type, the units, and three dimensions of the luminous opening.

Two of those ten repay attention. Lumens per lamp of -1 signals absolute photometry: the candela values were measured on the luminaire as tested, not scaled to a rated lamp figure, which is how LED files are usually written. The photometric type is almost always 1, meaning Type C, the (C, γ) system described above. The next line gives the ballast factor, a version-dependent code, and the input watts, driver included. In an LM-63-19 file that middle value is a file generation type code saying whether the data came from an accredited laboratory, a simulation or a scaled result; in an older LM-63-02 file the same position is simply reserved. This is why the edition line comes first.

What remains are the angles and the candela values themselves: the list of vertical angles, the list of horizontal angles, then one block of measured candela per horizontal angle. Those numbers come only from the laboratory. Everything else in the file is annotation around them.

Opening an LDT File

The LDT file holds the same kind of measurement in a line-by-line layout, one fact per line, in a fixed order. Reading down the file you meet, in sequence:

• The company, databank or version identification.
• A type indicator: 1 for a point source symmetric about the vertical axis, 2 for a linear luminaire, 3 for any other symmetry.
• The symmetry indicator, then the number of C-planes and their spacing, then the number of gamma angles per plane and their spacing. Interior luminaires are typically measured on 24 planes, road lighting on 36.
• The measurement report number, the luminaire name and number, and the file date.
• The luminaire's dimensions and the dimensions of its luminous area, in millimetres.
• DFF, the downward flux fraction: the share of the light going below horizontal, as a percentage.
• LORL, the light output ratio of the luminaire, also a percentage.
• A conversion factor for the intensity values, then the lamp data: number of lamps, lamp type, total luminous flux in lumens, colour appearance and colour rendering group, and wattage including the ballast.
• Finally, the intensity values themselves, plane by plane, in cd/klm.

That last unit matters. The LDT file records candela per 1000 lumens of lamp flux, not raw candela. To turn any value into real candela for the product you are buying, multiply it by the lamp flux and divide by 1000. One measured shape then serves every lumen version of the same luminaire design, which is exactly why the unit exists. The IES world reaches the same place through the lumens-per-lamp figure; the LDT world builds it into every value.

Reading the Polar Diagram

Open a manufacturer's datasheet and the photometric file has usually been drawn for you, as a polar diagram. The drawing adds nothing: it plots the numbers the file already holds, connected point to point, so a file measured in 5° steps gives a curve with a corner every 5°. Reading the drawing well is the same skill as reading the file.

A round downlight: one curve for all C-planes, beam angle read at 50% of peak. Curve from illustrative values. Diagram: Archlior.

A linear pendant: two C-plane curves and an upward component. Curves from illustrative values. Diagram: Archlior.

The luminaire sits at the centre. Gamma runs around the diagram, 0° straight down. Each ring is an intensity value, and the curve shows the intensity in one pair of C-planes; a point far from the centre means strong light in that direction. Check the unit before comparing anything: diagrams are drawn either in candela or in cd/klm, and the same luminaire drawn in the two units has identical shapes and different numbers.

The beam angle falls straight out of the curve. It is the full angle between the two directions where the intensity drops to half its peak. On the example downlight: the peak is 1926 cd/klm straight down, half of that is 963, and the curve crosses 963 at 20° on each side, so the beam angle is 40°. Add the two sides; quoting one side is how a 40° downlight gets specified as an 80° one. A linear pendant has two beam angles, one across and one along, and the upper half of its diagram shows the share of light going to the ceiling.

To move from the diagram to the real product, convert cd/klm to candela: multiply by the lamp flux in lumens and divide by 1000. The example downlight at 1500 lumens peaks at 1926 × 1500 ÷ 1000, roughly 2900 cd straight down.

Two vendors can draw the same luminaire differently, so before trusting a comparison, check the drawing choices: candela versus cd/klm, the ring scale, which C-planes were drawn, whether the upper half is shown at all, whether the curve was smoothed, and whether the beam angle was measured from the peak or from the beam's centre. The CIE definition uses the beam centre, many programs use the peak, and the two disagree whenever the peak sits off axis.

What the File Tells You Without Any Software

You do not need DIALux to take real numbers from a photometric file. From a careful read you can lift:

• Input watts, driver included, and the total lumens, so the true efficacy in lumens per watt rather than the brochure's LED-only figure.
• The beam angle, in each C-plane that matters, at the 50% rule.
• The peak intensity and where it points, which decides what the fitting can do to a vertical surface or a workplane directly below.
• The symmetry, and with it how many curves you actually need to check.
• From an LDT file, the light output ratio and the downward flux fraction: how much of the lamp's light survives the fitting, and how much of that goes down.
• From an IES file, the maintenance category, a first hint of how quickly dirt will erode the output.
• Who measured it, when, and under which report, so the claim can be traced.

What the file cannot tell you is what the fitting does in a room. Illuminance on a desk depends on the room's geometry and every surface's reflectance, and that is what calculation software adds: as our recessed spacing guide puts it, the software imports the manufacturer's real photometric file and computes actual illuminance from the measured distribution, the room geometry and the reflectance of every surface. The file is the measurement; the software is the arithmetic. For a quick domestic layout a hand method is usually enough, but for anything specified, tendered or compliance-bound, the photometric calculation is the deliverable.

For Architects: Specifying With Photometric Files

Ask for the file, not just the diagram. The diagram is a rendering choice; the file is the measurement. A manufacturer who publishes photometric files is showing the laboratory's numbers, and the [TESTLAB] and report number let you trace them.

Match the file to the exact variant. The keywords describe one measured configuration: one optic, one lamp or module, one catalogue number. A different beam optic or colour temperature is a different file. Our CRI vs TM-30 guide covers the colour side of the same discipline: keep the measured identity of what you specify attached to what gets ordered.

Keep units attached to numbers. A beam angle without its C-plane, a candela figure without cd or cd/klm, an efficacy without saying whether the driver watts are counted: each is a comparison waiting to go wrong.

File the file. Photometric files belong in the project folder next to the lighting plan, so that the calculation model, the tender and the as-built all point at the same measurement.

Checklist

Identity

• Edition line read first (LM-63-02 or LM-63-19 for IES; EULUMDAT version for LDT)
• Test laboratory, report number and issue date noted
• Catalogue number and lamp description matched to the exact variant being specified

Geometry

• Photometric type and units read before interpreting anything
• Symmetry established from the horizontal angle list (IES) or symmetry indicator (LDT)
• For linear fittings, the fitting's long axis placed along C90-C270 before reading curves

Performance

• Beam angle read as the full angle at 50% of peak, per relevant C-plane
• Peak intensity noted with its unit, cd or cd/klm
• Input watts including driver and total lumens noted; true lm/W derived
• LOR and downward flux fraction noted from LDT files

Use

• Vendor diagrams checked for unit, scale, planes shown and smoothing before comparing
• Files stored with the project documents
• Layouts for specified, tendered or compliance-bound work verified in photometric software

Five Mistakes

1. Comparing candela from one file with cd/klm from another. The shapes may match while the numbers differ by the lamp flux. Convert first: cd/klm multiplied by lamp lumens and divided by 1000 gives candela.
2. Quoting half the beam angle. The beam angle is the full angle between the two directions at 50% of peak; the two half-angles add. A 40° downlight is not an 80° one.
3. Reading lumens per lamp of -1 as missing data. It marks absolute photometry, the normal way LED luminaires are reported, and the candela values are already final.
4. Trusting the printed polar diagram over the file. The diagram is a drawing of the file's numbers, and vendors draw differently: units, ring scale, planes shown, smoothing, even whether the beam angle is measured from the peak or the beam's centre. When a diagram looks odd, open the file.
5. Mixing up the LM-63 editions. In LM-63-19 one field codes how the file was made, from accredited laboratory to computer simulation; in LM-63-02 the same position is reserved. Read the edition line first.

Frequently Asked Questions

What is the difference between an IES file and an LDT file? Both carry the same kind of laboratory measurement: luminous intensity by direction. The IES file follows the ANSI/IES LM-63 standard and is the common format in North America; the LDT file follows the EULUMDAT format and is the European counterpart. The layouts differ and LDT records intensities in cd/klm, but calculation software reads either.

Can I open a photometric file without lighting software? Yes. Both formats are plain text and open in any text editor. You can read the laboratory, the report number, the watts, the lumens, the symmetry and the candela table directly, which is what this guide does.

Why does my IES file show -1 for lumens? Because the file uses absolute photometry: the candela values were measured on the luminaire as tested rather than scaled to a rated lamp figure. LED files are usually written this way, and the values are ready to use as they are.

Is the beam angle on a datasheet inside the photometric file? Not as a number. The file holds the candela values, and the beam angle is derived from them: the full angle between the directions where intensity falls to 50% of the peak. Deriving it yourself takes a minute and tells you which C-plane the quoted figure came from.

What will replace IES and LDT files? The IES foreword to LM-63-19 names the XML-based ANSI/IES TM-33 as the format intended eventually to replace it, and describes LM-63 as entering long-term stability. Both current formats remain valid and are what manufacturers publish today.

The project is in Europe and the fixture is American. Which file do I need? Either. The measurement is the same; only the text format differs, and the usual calculation programs import both. If you receive only one format, free converters between the two exist.

Notice

Sources and further reading

• ANSI/IES LM-63-19, Standard File Format for the Electronic Transfer of Photometric Data (preview): scope, definitions including goniophotometer and photometric types, and the planned transition to TM-33.
• IESDesk. Anatomy of an IES file: the keyword block, the ten-number line, absolute photometry, angle lists and candela values, and the differences between LM-63-02 and LM-63-19.
• IESDesk. How to read a polar diagram: the (C, γ) system, cd versus cd/klm, the 50% beam angle rule with worked examples, and why two vendors' diagrams of the same luminaire can differ.
• Lighting Analysts (AGi32). EULUMDAT file format description: the LDT line list, symmetry indicators, DFF and LORL, lamp data and intensity values in cd/klm; referencing Stockmar (1990) and Stockmar, EULUMDAT/2 (1998).
• Archlior. Recessed Lighting Spacing: How Many Downlights and Where: how calculation software uses photometric files, and when hand methods are enough.