IES and LDT compared

Two formats, one luminaire

An IES file is the common format in North America. An LDT file, also called EULUMDAT, is the common format in Europe.

Most lighting design programs read both formats. A manufacturer usually publishes both files for each product.

The two files must describe the same luminaire. A designer who opens the IES file must get the same result as a designer who opens the LDT file.

This page shows where the formats agree, where they store the same idea differently, and what only one format can hold.

The same luminaire in both files

A pole-top luminaire that sends its light to one side. The values are invented. The same note number marks the same idea in both files.

As an IES file

 1  IESNA:LM-63-2002
 2  [TEST] EL-2026-0533
 3  [TESTLAB] Example Photometric Laboratory
 4  [ISSUEDATE] 2026-09-01
 5  [MANUFAC] Example Lighting
 6  [LUMCAT] EX-PT-400
 7  [LUMINAIRE] Pole-top luminaire, asymmetric, 3000 K
 8  [LAMP] LED module 18 W 3000 K
 9  [OTHER] Example file for the IESDesk Learn pages.
10  [MORE] Values are invented. Do not use them for design.
11  TILT=NONE
12  1 2400 1.0 7 7 1 2 0.2 0.16 0
13  1.0 1 18
14  0 15 30 45 60 75 90
15  0 30 60 90 120 150 180
16  158.6 159.8 330.5 1317.1 1910.9 643.4 0
17  158.6 159.1 305.5 1161.1 1673.8 565.4 0
18  158.6 157 246 790.1 1109.5 379.9 0
19  158.6 154.8 185.5 413.5 537.1 191.8 0
20  158.6 153.6 149.5 187.4 193.7 78.7 0
21  158.6 153.1 138.2 117.6 87.6 43.7 0
22  158.6 153.1 137.3 112.1 79.2 41 0

Names and text: the keywords on lines 2 to 10.
Angle grid: line 12 says 7 vertical angles and 7 horizontal angles. Lines 14 and 15 list them. The last horizontal angle is 180°: the luminaire is symmetric about the 0°–180° plane.
Dimensions: the last three numbers on line 12. Width 0.2, length 0.16, height 0, in metres. IES holds the glowing part only.
Flux: the second number on line 12, 2400 lumens.
Power: the third number on line 13, 18 watts.
Light data: lines 16 to 22, in candela. Each line is one horizontal angle. Line 16 is 0°, the front, with the peak of 1910.9 cd.
Only in IES: the version on line 1, the TILT line, and the ballast factor and the reserved value on line 13.

As an LDT file

  1  Example Lighting
  2  3
  3  3
  4  12
  5  30.0
  6  7
  7  15.0
  8  EL-2026-0533 Example file. Values are invented. Not for design.
  9  Pole-top luminaire, asymmetric, 3000 K
 10  EX-PT-400
 11  EX-PT-400.ldt
 12  2026-09-01 Example Laboratory
 13  400
 14  250
 15  70
 16  160
 17  200
 18  0
 19  0
 20  0
 21  0
 22  100
 23  100
 24  1.0
 25  0.0
 26  1
 27  1
 28  LED module 18 W 3000 K
 29  2400
 30  3000
 31  80
 32  18.0
 33  0.35
 34  0.41
 35  0.46
 36  0.51
 37  0.55
 38  0.60
 39  0.64
 40  0.67
 41  0.71
 42  0.74
 43  0.0
 44  30.0
 45  60.0
 46  90.0
 47  120.0
 48  150.0
 49  180.0
 50  210.0
 51  240.0
 52  270.0
 53  300.0
 54  330.0
 55  0.0
 56  15.0
 57  30.0
 58  45.0
 59  60.0
 60  75.0
 61  90.0
 62  66.1
 63  63.8
 64  57.2
 65  46.7
 66  33.0
 67  17.1
 68  0.0
 69  66.1
 70  63.8
 71  57.6
 72  49.0
 73  36.5
 74  18.2
 75  0.0
 76  66.1
 77  64.0
 78  62.3
 79  78.1
 80  80.7
 81  32.8
 82  0.0
 83  66.1
 84  64.5
 85  77.3
 86  172.3
 87  223.8
 88  79.9
 89  0.0
 90  66.1
 91  65.4
 92  102.5
 93  329.2
 94  462.3
 95  158.3
 96  0.0
 97  66.1
 98  66.3
 99  127.3
100  483.8
101  697.4
102  235.6
103  0.0
104  66.1
105  66.6
106  137.7
107  548.8
108  796.2
109  268.1
110  0.0

Names and text: lines 1, 8 to 12, and 28.
Angle grid: lines 2 to 7. Symmetry 3 means the left half mirrors the right half, so the file stores 7 of its 12 C-planes. Lines 43 to 54 list the C-planes. Lines 55 to 61 list the gamma angles.
Dimensions: lines 13 to 15 give the housing, 400 × 250 × 70 mm. Lines 16 to 21 give the glowing part: length 160, width 200, and four heights of 0.
Flux: line 29, 2400 lumens.
Power: line 32, 18 watts.
Light data: lines 62 to 110, in candela for each 1000 lumens. The blocks run from C270 through C0 to C90. The last block is C90, the front, with the peak of 796.2.
Only in LDT: the housing size, the two ratios on lines 22 and 23, the tilt angle on line 25, the colour on lines 30 and 31, and the direct ratios on lines 33 to 42.

What differs by design

These differences are correct. They are not faults in a file.

One dimension block or two: An IES file holds the size of the glowing part only. An LDT file holds the size of the housing and the size of the glowing part.
The front: An IES file has the front of the luminaire at 0°. An LDT file has the front at C90 (EN 13032-1, Annex D). See the section below.
Units: IES uses feet or metres, and the file says which. LDT always uses millimetres.
Candela or candela for each 1000 lumens: IES holds candela. LDT usually holds candela for each 1000 lumens of lamp flux. In the example: 796.2 × 2400 ÷ 1000 = 1910.9 cd.
Symmetry: An LDT file states its symmetry and stores only the planes it needs. An IES file shows its symmetry by the last horizontal angle. The example is symmetric about the front-to-back plane. That is symmetry 3 (C90–C270) in the LDT file and a last angle of 180° in the IES file.
Keywords or positions: An IES file uses keywords, and a keyword can be absent. An LDT file uses fixed line positions, and a line can be empty but not absent.

The front: 0° in IES, usually C90 in LDT

A correct pair is turned by 90°

In an IES file, 0° is the direction in front of the luminaire (LM-63-19, definition 3.6). Forward light is the arc from 270° to 90° (TM-15-20, section 3.3).

In Europe, the front is at C90 (EN 13032-1, Annex D). For a road luminaire, the 90° plane crosses the road. For a wall luminaire or a floodlight, the 270° plane goes through the mounting. The LDT format has no standard of its own, and LDT files follow this rule in practice.

Both formats count the horizontal angles in the same direction (LM-75-19, sections 3.25 and 5.4.3). So the pair is turned, not mirrored.

So IES angle 0° holds the light of LDT plane C90. IES 30° holds C120. The example pair is made this way: the peak is at 0° in the IES file and at C90 in the LDT file.

The turn also decides the dimensions. The IES length lies along 0°–180°. The LDT length lies along C90–C270. After the turn these are the same axis. So the IES length equals the LDT length, and the IES width equals the LDT width. In the example: 0.16 m and 160 mm, 0.2 m and 200 mm.

A plain conversion program keeps the angle numbers. Its IES file has the front at 90°. A design program then shows the luminaire turned by 90°, and a calculation that uses the front, such as a BUG rating, reads the wrong side.

A luminaire that is symmetric about the vertical axis has no front. For it, the turn changes nothing.

A linear luminaire has no front, but it has a long axis. The turn still applies. In the LDT file, the long axis lies along C90–C270 (EN 13032-1, clause 4.2.3). In the IES file, it lies along 0°–180°.

See the dimensions page for a drawing of the turn.

Colour

An IES file has no colour field. A text keyword such as [LAMP] can name the colour only in words. An LDT file holds colour on line 26d, the colour appearance or colour temperature, and on line 26e, the colour rendering group or index. Both LDT lines are free text.

TM-33-23 is the IES format that holds colour for each emitter: colour temperature (section 4.5.9), CIE Ra and R9 (section 4.5.10.1), TM-30 Rf and Rg (section 4.5.10.2), Duv (section 4.5.11), spectral data (section 4.5.17), and chromaticity for each angle (section 4.5.18).

LM-79-24, section 11.1, states that a test report gives chromaticity, colour temperature, TM-30 values, and CRI for white light, and the spectrum where it applies. EN 13032-4, clause 7.1, states that an LED test measures chromaticity, colour temperature, the distance from the Planckian locus (Duv), colour rendering indices, and angular colour uniformity.

TM-30-24, section 3 and Table 3-1, scores colour against a reference light. Rf is a score from 0 to 100 for colour fidelity: 100 is a perfect match with the reference. Rg shows average saturation against the reference: 100 is the same, and above 100 is more saturated.

Field by field

Each name links to its full entry on the IES page or the LDT page. The values come from the example pair.

IES	LDT	Meaning	IES example	LDT example	Status
Names and text
Version line	—	The edition of the standard.	`IESNA:LM-63-2002`	`—`	Only in IES
[MANUFAC]	1 Company	The manufacturer.	`Example Lighting`	`Example Lighting`	Same idea
[TEST]	8 Measurement report number	The test report number.	`EL-2026-0533`	`EL-2026-0533 …`	Same idea
[TESTLAB]	—	The laboratory. An LDT file can name it on line 12.	`Example Photometric Laboratory`	`—`	Only in IES
[ISSUEDATE]	12 Date and user	The date of the file.	`2026-09-01`	`2026-09-01 Example Laboratory`	Same idea
[TESTDATE]	—	The date of the test report.	`not in the example`	`—`	Only in IES
[LUMINAIRE]	9 Luminaire name	The description of the luminaire.	`Pole-top luminaire, asymmetric, 3000 K`	`Pole-top luminaire, asymmetric, 3000 K`	Same idea
[LUMCAT]	10 Luminaire number	The catalog number.	`EX-PT-400`	`EX-PT-400`	Same idea
—	11 File name	The name of the file itself.	`—`	`EX-PT-400.ldt`	Only in LDT
[LAMP]	26b Type of lamps	The lamp or LED module.	`LED module 18 W 3000 K`	`LED module 18 W 3000 K`	Same idea
[LAMPCAT] [BALLAST] [BALLASTCAT]	—	Catalog numbers of the lamp and the driver, and the driver description.	`not in the example`	`—`	Only in IES
[MAINTCAT] [DISTRIBUTION] [FLASHAREA] [COLORCONSTANT] [LAMPPOSITION] [NEARFIELD]	—	Characteristic keywords from the test report.	`not in the example`	`—`	Only in IES
[OTHER] [MORE] [SEARCH] [FILEGENINFO] [_KEYWORD]	—	Free text, search words, and company keywords.	`[OTHER] and [MORE]`	`—`	Only in IES
Dimensions
Units type	—	Feet or metres in IES. LDT always uses millimetres.	`2 (metres)`	`mm`	Stored differently
—	13 Length of luminaire	The housing length. By habit along C90–C270 (CIE 121).	`—`	`400`	Only in LDT
—	14 Width of luminaire	The housing width. By habit along C0–C180.	`—`	`250`	Only in LDT
—	15 Height of luminaire	The housing height.	`—`	`70`	Only in LDT
Length	16 Length of luminous area	The glowing part, from front to back. IES length and LDT length are the same axis after the turn.	`0.16`	`160`	Stored differently
Width	17 Width of luminous area	The glowing part, from side to side.	`0.2`	`200`	Stored differently
Height	18 Height, C0 19 Height, C90 20 Height, C180 21 Height, C270	The height of the glowing part. IES has one value. LDT has one for each side.	`0`	`0, 0, 0, 0`	Stored differently
Lamps, flux, and power
Number of lamps	26 Number of lamp sets 26a Number of lamps	The lamp count. A negative LDT count means absolute photometry.	`1`	`1 set, 1 lamp`	Same idea
Lumens per lamp	26c Total luminous flux	IES holds the lumens for each lamp. LDT holds the total of the lamp set. -1 in IES means absolute photometry.	`2400`	`2400`	Stored differently
Candela multiplier	24 Conversion factor	A factor on every intensity value. The two factors can differ. The results must agree.	`1.0`	`1.0`	Stored differently
Ballast factor Reserved value or file generation type	—	The ballast factor, and a reserved value or the file generation type.	`1.0 and 1`	`—`	Only in IES
Input watts	26f Wattage including control gear	The power of the luminaire, driver included.	`18`	`18.0`	Same idea
—	26d Colour appearance	The colour temperature. An IES file can only name it in a text keyword.	`—`	`3000`	Only in LDT
—	26e Colour rendering group 26e Colour rendering index, Ra	The colour rendering, as a group or as an Ra value.	`—`	`80`	Only in LDT
—	22 Downward flux fraction, DFF	The share of the light that goes down.	`—`	`100`	Only in LDT
—	23 Light output ratio, LORL	The share of the lamp flux that leaves the luminaire.	`—`	`100`	Only in LDT
—	27 Direct ratios	Ten utilisation values for indoor room sizes.	`—`	`0.35 … 0.74`	Only in LDT
Angles and light data
Photometric type	—	Type C, B, or A. An LDT file is always Type C.	`1 (Type C)`	`—`	Only in IES
—	2 Type indicator, Ityp	Point source or linear luminaire.	`—`	`3`	Only in LDT
Horizontal angles	3 Symmetry indicator, Isym	The symmetry. LDT states it. IES shows it by the last horizontal angle. The 90° turn changes the plane: C90–C270 in LDT is 0°–180° in IES.	`0 to 180`	`3`	Stored differently
Number of horizontal angles	4 Number of C-planes, Mc 5 C-plane step, Dc 28 C-plane angles	The C-planes. LDT declares the full circle and can store a part of it. IES lists only the planes it stores.	`7`	`12, step 30, 7 stored`	Stored differently
Number of vertical angles Vertical angles	6 Number of gamma angles, Ng 7 Gamma step, Dg 29 Gamma angles	The angles from straight down.	`7, from 0 to 90`	`7, step 15`	Same idea
Candela values	30 Luminous intensities	The measured light. Candela in IES. Candela for each 1000 lumens in LDT. IES 0° holds LDT C90.	`1910.9`	`796.2`	Stored differently
TILT=	—	Says if the lamp output changes when the luminaire tilts.	`TILT=NONE`	`—`	Only in IES
—	25 Tilt during measurement	The tilt angle of the luminaire in the test.	`—`	`0.0`	Only in LDT

Same idea Both files hold the value, and the values must agree.

Stored differently Both files hold the idea. The numbers differ because of units, the 90° turn, or structure.

Only in IES Only in LDT The other format has no place for the value.

How to check a pair

Open the IES file, the LDT file, and the data sheet of one product. Do these six checks.

Dimensions. The IES length in millimetres equals the LDT length of the luminous area. The IES width equals the LDT width. The housing size is in the LDT file only, and it matches the product drawing.
Flux. IES lumens per lamp × the number of lamps equals the LDT total flux. If the IES value is -1, the LDT lamp count is negative. The flux agrees with the data sheet.
Power. The watts are equal in both files, and they match the data sheet.
Symmetry and front. The front is at 0° in the IES file. It is at C90 in the LDT file (EN 13032-1, Annex D). LDT symmetry 3 (C90–C270) goes with a last IES angle of 180°. LDT symmetry 2 (C0–C180) has no short form in LM-63-19, so the IES file lists the full circle.
Light data. Take one LDT value. Multiply it by the flux and divide by 1000. The result equals the IES value at the same gamma angle and at the horizontal angle that is 90° less than the C-plane: C90 goes with 0°, C120 with 30°. Check the peak and one value to the side of it. If an angle is not in the file, use its mirror angle: 330° is 30° in the IES example, and C120 is C60 in the LDT example. If the values match at the same angle number, the IES file is a plain conversion and is not turned. If they match at no angle, the two files do not hold the same light data.
Text. The manufacturer, the catalog number, the description, and the date agree. Each one is in its correct field.

Common faults

The IES file has its front at 90°. It is a plain conversion of the LDT file, and it is not turned.
The light data is turned, but the width and the length are not.
The IES dimension fields hold the housing size. They must hold the glowing part only.
The two files give different watts for the same product.
The catalog number sits in the description field, and the catalog number field is empty.
[TEST] is empty. LM-63 requires it.
The IES horizontal angles run from 90° to 270°. LM-63-19 requires the first angle of a Type C file to be 0°. This layout is a sign of a plain conversion.

Sources

ANSI/IES LM-63-19. Definition 3.6: 0° is the direction in front of the luminaire. Clause 5.10: width and length follow the 0° photometric plane. Clause 5.16: the first horizontal angle of a Type C file is 0°.
ANSI/IES TM-15-20, section 3.3. Forward light is the arc from 270° to 90°.
ANSI/IES LM-75-19. Section 5.4.3: the IES Type C system counts horizontal angles clockwise seen from below a downward luminaire, so counterclockwise seen from above. Section 3.25: it is the same system as the CIE C-planes of an LDT file.
EN 13032-1:2004+A1:2012. Clause 4.2.3: the C-planes. The axis of a linear lamp is at right angles to the C0 plane, so it lies along C90–C270. Annex D: the 90° plane of a road luminaire crosses the road. The 270° plane of a wall luminaire or a floodlight goes through the mounting.
CIE 121:1996, section 2.8. The length along C90–C270. This page reads it through the Viso Systems guide.
DIALux evo, Description of the EULUMDAT format. The LDT line list, the symmetry table, and the order of the stored planes.
AGI32 Photometric Toolbox, EULUMDAT file format. The LDT line list, second reference. A negative lamp count means absolute photometry.
Viso Systems, Dimensions of lighting fixtures in .IES and .LDT files. The European and the North American laboratory style, and the 90° turn between them.
schorsch.com, Online converter for luminaire data files, help. A converter that turns the file, and what happens in a design program when a file is not turned.
IES LM-79-24. Section 11.1: a test report gives chromaticity, colour temperature, TM-30 values, and CRI for white light, and the spectrum where it applies.
IES TM-30-24. Section 3 and Table 3-1: Rf is a score for colour fidelity, and Rg shows saturation against the reference.
IES TM-33-23. The IES XML format that replaces LM-63-19 and holds colour for each emitter.
EN 13032-2. Clauses 4.2.13 and 4.2.14: Ra and colour temperature are essential luminaire data where they apply.
EN 13032-4. Clause 7.1: an LED test measures chromaticity, colour temperature, Duv, colour rendering indices, and angular colour uniformity.