Free online IES and LDT viewer

What is an IES and LDT viewer?

An IES and LDT viewer reads a luminaire's photometric file, the candela table measured in the laboratory, and turns it into figures a designer can check: luminous flux, input power, efficacy, beam angle and the shape of the light distribution. This viewer accepts both IES (IESNA LM-63) and EULUMDAT (.ldt) files and reports on them in seconds.

On upload the file is parsed and validated by NR Lighting's cloud function, which returns the header (manufacturer, luminaire and catalogue reference, test data), the rated lamp or luminaire flux, the flux calculated by integrating the candela table, input watts, efficacy in lm/W and any errors or warnings. The warnings are there to catch corrupt or implausible data: a calculated flux higher than the rated lamp flux, a very low light output ratio, or an efficacy above 250 lm/W.

How to use the IES and LDT viewer

1. 01Drag an .ies or .ldt file onto the editor, or click Upload. Files must be under 5 MB; the format is detected from the extension and confirmed from the content, so a mislabelled file is still read correctly.
2. 02Wait for parsing. The file text is sent to NR Lighting's cloud function, which validates it and returns the header, rated and calculated lumens, watts and efficacy. Parse errors and data warnings appear as notifications.
3. 03Read the Quick Edit panel: manufacturer, luminaire name, system watts, total flux and luminous dimensions, with efficacy in lm/W recalculated as you type. Absolute-photometry IES files show the integrated luminaire flux in place of a lamp rating.
4. 04Switch to Advanced Edit for the full header: test lab, test number and dates, lamp description, CCT, CRI, mounting, ballast factor and candela multiplier for IES; luminaire type, symmetry, DFF, LORL, conversion factor, tilt and the C0 and C90 luminous areas for LDT.
5. 05Press Apply to regenerate the file text with your edits. The tool re-validates the result and warns if the typed lumens or watts no longer agree with the candela table. The Raw tab shows the resulting text and can be edited directly.
6. 06With a Pro subscription, open Analyze for the polar curve, beam and field angles, CIE classification, light output ratio and a PDF report, and use Download to save the edited file.

The method: zonal lumens, beam angles and CIE classification

Φ = Σ over zones of Ī_zone × 2π × (cos θ1 − cos θ2) × candela multiplier. Beam angle = full angle between the two directions where intensity falls to 50% of the peak; field angle uses 10%.

Luminous flux is verified by zonal integration on the file's own vertical-angle grid. For each zone between adjacent vertical angles the mean intensity across every C-plane is multiplied by the solid angle of that ring, and the zone lumens are summed. The IES candela multiplier is applied once; for LDT the conversion factor and the lamp flux are applied when the cd/klm values are read, so the multiplier is 1.

The result is cross-checked against the flux the file declares. Where a luminaire flux is stated (for LDT, lamp flux × LORL) the two are compared directly: a difference above 15% is reported as a warning and above 30% the file is flagged as possibly corrupt. Where only a lamp flux is given, a calculated flux above 115% of it is flagged as impossible and below 25% as a very low light output ratio. For display, when the integrated and declared figures agree within 3% the declared value is shown, because the file's grid carries about 1% numerical error and the laboratory's own integration is the better number.

Beam angles are taken on the C-plane holding the global peak. For Type C photometry the profile is mirrored through nadir using the opposing plane, so an asymmetric beam is measured edge to edge rather than doubled from one side, and the crossing points are found by linear interpolation between grid angles. The CIE classification splits the integrated flux into the lower hemisphere (0–90°) and the upper hemisphere (90–180°); it is not applicable to Type A or B photometry.

Worked example

Take a 3000 K LED accent downlight supplied as an LDT: lamp flux 1,000 lm, light output ratio (LORL) 87%, 11 W input, symmetric about the vertical axis, with intensities tabulated every 10° from nadir: 1,800, 1,650, 700, 250, 60, 15, 5, 1, 0 and 0 cd.

Worked example: reading the result

The zones sum to 857 lm. The file's implied luminaire flux is 1,000 × 0.87 = 870 lm, 1.5% away, so the viewer treats the two as agreeing and displays 870 lm and 870 ÷ 11 = 79.1 lm/W with no warning. Peak intensity is 1,800 cd at 0°. The curve passes through 900 cd (50%) at 17.9° and through 180 cd (10%) at 33.7°, so the beam angle is 36° and the field angle 67°. Every zone lies below the horizontal, so the classification is Direct with a downward fraction of 100%.

Limitations, and when to involve a lighting designer

The viewer reports what is in the file. It gives indicative design values, not compliance sign-off: a file that parses cleanly says nothing about glare, colour quality or whether the luminaire suits the room, and the flux figures are laboratory values before any maintenance factor. Zonal integration on a coarse angle grid carries around 1% error, so small differences from the declared flux are noise. Beam angles are read from the peak C-plane only, Type A and B results are approximate, and the CIE classification is a five-band summary rather than the full CIE flux code.

I check every IES and LDT file before it enters a Dialux model, because the calculation is only as good as the photometry behind it. Involve a designer when the data is feeding a BS EN 12464-1 or SLL Code calculation, when a substitution is being assessed against an equal-and-approved specification, or when the warnings above appear and cannot be explained. In accent-led retail schemes such as Molton Brown Regent Street or the Aston Martin Tokyo showroom, a few degrees of beam angle decide whether the product is lit or the floor is, and that judgement comes from reading the curve, not the catalogue.

IES vs LDT explained

IES and LDT are the two photometric file formats a lighting designer meets every day. Both carry the same underlying measurement, a table of luminous intensity in candelas against vertical and horizontal angles, but they package it differently.

IES files follow IESNA LM-63, the North American standard, and are plain text with a keyword header: [TEST], [MANUFAC], [LUMCAT], [LUMINAIRE], [LAMPCAT] and similar, then a TILT line and a numeric block. That block states the number of lamps, lumens per lamp (−1 for absolute photometry, where the candela values already represent the whole luminaire), the candela multiplier, the counts of vertical and horizontal angles, the photometric type (C, B or A), the units and luminous dimensions, the ballast factor and input watts, followed by the angle lists and the candela table. Type C, with vertical angles from 0° at nadir to 90° or 180°, is what almost every architectural luminaire uses.

LDT files follow EULUMDAT, the European convention used by DIALux and Relux. There is no keyword header: the file is strictly line-positional, with 26 fixed lines covering manufacturer, luminaire type, symmetry indicator (0 to 4), the number and spacing of C-planes and gamma angles, report number, luminaire and luminous dimensions, downward flux fraction (DFF), light output ratio (LORL), conversion factor and tilt, then one or more lamp sets with lamp count, flux, colour appearance, CRI and wattage. Intensities are stored in cd/klm, normalised to 1,000 lamp lumens, and decimals may use a comma. A stray blank line shifts every field, which is why this viewer re-anchors the header when the positional read looks wrong.

Both formats open in the same viewer, and the figures are reported the same way. If a manufacturer supplies only one and your software needs the other, the converter linked below handles the translation.