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Anatomy of an LDT file

Every line in the file, what it means, and where the value comes from.

What an LDT file is

An LDT file is a plain text file that describes how a luminaire sends out light.

Its layout is called EULUMDAT. Axel Stockmar proposed it in 1990. It is not a formal standard, but European lighting programs read it.

A photometric laboratory writes the file after it measures the luminaire. DIALux and Relux read it.

The file holds one value per line. The first 30 or so lines hold names, dimensions, and factors. The rest hold angles and measured intensities.

Anatomy

A complete example file. The values are invented. The notes on the right explain each block.

 1  Example Lighting
 2  2
 3  0
 4  4
 5  90.0
 6  3
 7  45.0
 8  EL-2026-0418
 9  Linear pendant 1200
10  EX-LN-1200
11  EX-LN-1200.ldt
12  2026-09-01 Example Laboratory
13  1200
14  60
15  80
16  1150
17  50
18  30
19  0
20  30
21  0
22  100
23  100
24  1.0
25  0.0
26  1
27  1
28  LED module 24 W 4000 K
29  2400
30  4000
31  80
32  24.0
33  0.55
34  0.60
35  0.65
36  0.70
37  0.75
38  0.80
39  0.85
40  0.90
41  0.95
42  1.00
43  0.0
44  90.0
45  180.0
46  270.0
47  0.0
48  45.0
49  90.0
50  350.0
51  210.0
52  0.0
53  340.0
54  200.0
55  0.0
56  350.0
57  210.0
58  0.0
59  340.0
60  200.0
61  0.0
  1. Line 1 names the company.
  2. Lines 2 to 7 give the luminaire type, the symmetry, and the angle grid: how many C-planes and gamma angles the file holds.
  3. Lines 8 to 12 hold the report number, the luminaire name and number, the file name, and the date.
  4. Lines 13 to 15 give the housing size. Lines 16 and 17 give the glowing part. Lines 18 to 21 give its height seen from C0, C90, C180, and C270.
  5. Lines 22 to 25 hold four values. They are the downward flux fraction, the light output ratio, a conversion factor, and the tilt during the test.
  6. Line 26 says how many lamp sets follow. Lines 27 to 32 describe one lamp set: count, type, flux, colour, colour rendering, watts.
  7. Lines 33 to 42 hold the ten direct ratios.
  8. Lines 43 to 46 list the C-plane angles. Lines 47 to 49 list the gamma angles.
  9. Lines 50 to 61 hold the intensities, one block of three per C-plane.

Where the numbers come from

SourceFields
Data sheetCompany, luminaire type, name, number, lamp type, colour, watts, all dimensions.
Test reportReport number, flux, flux fraction, output ratio, factors, direct ratios, angles, intensities.
Lab softwareSymmetry, C-plane and gamma counts and steps, file name, date, lamp set count.
Your choiceNo LDT line is free text. Use the luminaire name for a note if you must.

Each field below carries its source label.

Dimensions and planes

The C-planes

Picture the luminaire hanging with the light pointing down.

Cut it with vertical planes, like the segments of an orange. Each cut is a C-plane.

C0 and C180 are the two halves of one cut. C90 and C270 are the cut at right angles.

The gamma angle runs inside one plane. It starts at 0° straight down and ends at 180° straight up.

Plan view with C-planes and side view with gamma angles C0 C90 C180 C270 length Plan view, seen from above γ = 0°, straight down γ = 90° γ = 180° Side view, one C-plane
Left: the C-planes cut the luminaire like the segments of an orange. Right: the gamma angle runs from straight down to straight up inside one C-plane.

Which way is the length

LDT and IES put the front at different angles

By habit, the LDT length lies along C90–C270. The source is CIE 121, section 2.8, as the Viso Systems guide reads it. C0 and C180 look at the long side. C90 and C270 look at the ends. For a linear lamp, EN 13032-1 (clause 4.2.3) sets the same axis.

In Europe, the front of an outdoor luminaire is at C90 (EN 13032-1, Annex D). For a road luminaire, the 90° plane crosses the road. The LDT format has no standard of its own, and LDT files follow this rule in practice. In an IES file, the front is at 0° (LM-63-19, definition 3.6), and the length lies along 0°–180°.

So a correct IES file is turned by 90° against the LDT file of the same luminaire: IES 0° holds the light of C90. Both formats count the horizontal angles in the same direction (LM-75-19, section 5.4.3), so the turn is not a mirror. The two length axes are then the same axis. A plain conversion that keeps the angle numbers is not turned. See IES and LDT compared.

How to read a dimension drawing

  1. Open the product drawing. Find the plan view (from below), the front view, and the end view.
  2. Length runs along C90–C270. For a linear luminaire, that is the long side. Width is the other side. For a round housing, length is the diameter and width is 0.
  3. Height is the housing height. Do not count suspension wires, the canopy, or a mounting bracket.
  4. Luminous area is the part that glows: the lens, the diffuser, or the opening. Measure it in the plan view. For a round area, width is 0.
  5. Height of luminous area C0 and C180: look at the long side in the front view. If the glowing part shows a vertical height, enter it in mm. If the glowing part is flat, enter 0.
  6. Height of luminous area C90 and C270: do the same in the end view.
  7. Check the symmetry. A symmetric body gives four equal values. A flat panel, a downlight, or a recessed troffer gives four zeros.
Plan, front, and end views of a linear luminaire Plan view: housing 1200 × 60, luminous area 1150 × 50 length 1200 mm width 60 Front view, seen from C0 and C180: the long side glows height 30 End view, seen from C90 and C270 height 0 Not to scale. The end caps are opaque, so the end view shows no luminous height.
The same pendant seen from three sides. The front view gives the luminous-area height for the long-side planes. The end view gives the height for the end planes.
Section and plan view of a round downlight housing 200 mm lens 150 mm Section view Plan view, seen from below
The housing is 200 mm across. Only the 150 mm lens glows. The lens is flat, so its height is 0.

When the four heights are not zero

A flat panel, a downlight, or a recessed troffer: all four are 0.

An opal globe pendant: all four equal the globe height.

A linear pendant with a side diffuser and opaque end caps: C0 and C180 hold the diffuser height, C90 and C270 hold 0.

A wall cylinder or a bollard: all four equal the luminous cylinder height.

Design programs use these values to draw the glowing body and to compute glare. A wrong orientation gives wrong glare numbers.

Mounting codes

EN 13032-1, Annex D, sets a mounting code for each luminaire type. The code says where the maker sets the 0° plane.

  • C1, a road luminaire. The 0°–180° plane runs parallel to the kerb. The 90° plane crosses the road.
  • C2, a wall, a track spotlight, or a floodlight. The 270° plane runs from the light centre through the mounting.
  • C3, a luminaire with rotational symmetry. Any plane can be the 0° plane.
  • C4 to C7 cover other symmetries. The maker marks the 0° plane on the drawing.

See Dimensions and orientation for drawings of ten luminaire types and the mounting code of each one.

Colour

Line 26d holds the colour appearance or colour temperature. Line 26e holds the colour rendering group or index. Both lines are free text for each lamp set. A program cannot always read them as numbers.

EN 13032-2, clauses 4.2.13 and 4.2.14, list the colour rendering index and the colour temperature as essential luminaire data where they apply.

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.

Value: line 26d, digits only, in kelvin. Example: 4000. Text such as 4000K also occurs. Common practice No document sets the form

Value: line 26e, the Ra value as a whole number. Example: 80. A group code such as 1A also occurs. Format guide AGI32

See the colour section of IES and LDT compared.

How to write values

  • One value on each line. Format guide
  • A number uses a decimal point. The format guides do not name the separator. Write a point, not a comma: a decimal comma can make a program misread the file. Common practice
  • Dimensions are in millimetres. Percentages run from 0 to 100. Format guide
  • The character counts in the field reference below are a recommended maximum, not a hard limit. Format guide
  • No document sets a date form for line 12. Write an unambiguous date, such as 2026-09-01, then the name. Common practice

Field reference

Every line in file order. Each entry gives the meaning, an example, and a common mistake.

Identity, lines 1 to 12

1 Company Data sheet
The manufacturer or the data bank. Up to 78 characters. Example: Example Lighting. Value: text, up to 78 characters. Example: Example Lighting. Format guide
2 Type indicator, Ityp Data sheet
1 for a point source with symmetry about the vertical axis. 2 for a linear luminaire. 3 for a point source with another symmetry. Example: 2. Value: whole number, 1 to 3; AGI32 also lists 0, up to 1 character. Example: 2. Format guide
3 Symmetry indicator, Isym Lab software
0: no symmetry, the file holds all C-planes. 1: symmetry about the vertical axis, one plane. 2: symmetry to C0–C180, Mc/2+1 planes. 3: symmetry to C90–C270, Mc/2+1 planes. 4: symmetry to both, Mc/4+1 planes. Example: 0. Value: whole number, 0 to 4, up to 1 character. Example: 0. Format guide
4 Number of C-planes, Mc Lab software
The number of C-planes between 0° and 360°. Usually 24 for indoor and 36 for road luminaires. Example: 4. Value: whole number, 1 or more, up to 2 characters. Example: 4. Format guide
5 C-plane step, Dc Lab software
The angle between C-planes in degrees. 0 when the planes are not evenly spaced. Example: 90.0. Value: decimal, degrees; 0 means uneven steps, up to 5 characters. Example: 90.0. Format guide
6 Number of gamma angles, Ng Lab software
The number of gamma angles in each C-plane. Usually 19 or 37. Example: 3. Value: whole number, 1 or more, up to 2 characters. Example: 3. Format guide
7 Gamma step, Dg Lab software
The angle between gamma angles in degrees. 0 when not evenly spaced. Example: 45.0. Value: decimal, degrees; 0 means uneven steps, up to 5 characters. Example: 45.0. Format guide
8 Measurement report number Test report
The report number from the laboratory. Example: EL-2026-0418. Value: text, up to 78 characters. Example: EL-2026-0418. Format guide
9 Luminaire name Data sheet
The luminaire name. Example: Linear pendant 1200. Value: text, up to 78 characters. Example: Linear pendant 1200. Format guide
10 Luminaire number Data sheet
The article or catalog number. Example: EX-LN-1200. Value: text, up to 78 characters. Example: EX-LN-1200. Format guide
11 File name Lab software
The file name. The original layout allows eight characters; most programs accept more. Example: EX-LN-1200.ldt. Value: text, up to 8 characters (a recommended maximum). Example: EX-LN-1200.ldt. Format guide
12 Date and user Lab software
The date and the person or program that wrote the file. Example: 2026-09-01 Example Laboratory. Value: text, up to 78 characters; no date form set. Example: 2026-09-01 Example Laboratory. Common practice no document sets a date form for this line

Dimensions, lines 13 to 21

All values are in millimetres.

13 Length of luminaire Data sheet
The length of the housing, along C90–C270. For a round housing, the diameter. Example: 1200. Value: number, millimetres; width 0 means round, up to 4 characters. Example: 1200. Format guide
14 Width of luminaire Data sheet
The width of the housing. 0 for a round housing. Example: 60. Value: number, millimetres; 0 means round, up to 4 characters. Example: 60. Format guide
15 Height of luminaire Data sheet
The height of the housing. Do not count suspension wires or the canopy. Example: 80. Value: number, millimetres, up to 4 characters. Example: 80. Format guide
16 Length of luminous area Data sheet
The length of the glowing part. For a round area, the diameter. Example: 1150. Value: number, millimetres; width 0 means round, up to 4 characters. Example: 1150. Format guide
17 Width of luminous area Data sheet
The width of the glowing part. 0 for a round area. Example: 50. Value: number, millimetres; 0 means round, up to 4 characters. Example: 50. Format guide
18 Height of luminous area, C0 Data sheet
The height of the glowing part seen from C0. 0 when the glowing part is flat. Example: 30, a side diffuser. Value: number, millimetres; 0 for a flat glowing part, up to 4 characters. Example: 30. Format guide Which side each height means is common practice
19 Height of luminous area, C90 Data sheet
The height of the glowing part seen from C90. Example: 0, an opaque end cap. Value: number, millimetres; 0 for a flat glowing part, up to 4 characters. Example: 0. Format guide Which side each height means is common practice
20 Height of luminous area, C180 Data sheet
The height of the glowing part seen from C180. Equal to C0 for a symmetric body. Example: 30. Value: number, millimetres; 0 for a flat glowing part, up to 4 characters. Example: 30. Format guide Which side each height means is common practice
21 Height of luminous area, C270 Data sheet
The height of the glowing part seen from C270. Equal to C90 for a symmetric body. Example: 0. Value: number, millimetres; 0 for a flat glowing part, up to 4 characters. Example: 0. Format guide Which side each height means is common practice

Photometric factors, lines 22 to 25

22 Downward flux fraction, DFF Test report
The share of light that goes downward, in percent. Example: 100 for a direct luminaire. Value: decimal, percent, 0 to 100, up to 4 characters. Example: 100. Format guide
23 Light output ratio, LORL Test report
The luminaire output divided by the lamp output, in percent. 100 for absolute photometry. Value: decimal, percent, 0 to 100, up to 4 characters. Example: 100. Format guide
24 Conversion factor Test report
A factor applied to the intensities. Usually 1.0. Value: decimal, up to 6 characters. Example: 1.0. Format guide
25 Tilt during measurement Test report
The tilt of the luminaire during the test, in degrees. Road luminaires use it. 0.0 for indoor luminaires. Value: decimal, degrees, up to 6 characters. Example: 0.0. Format guide

Lamp sets, line 26

26 Number of lamp sets Lab software
How many lamp sets follow. Usually 1. Absolute photometry allows only one. Value: whole number, 1 or more, up to 4 characters. Example: 1. Format guide
26a Number of lamps Test report
The number of lamps. A negative value marks absolute photometry. Example: 1. Value: whole number; negative means absolute photometry, up to 4 characters for each lamp set. Example: 1. Format guide
26b Type of lamps Data sheet
The lamp type. Example: LED module 24 W 4000 K. Value: text, up to 24 characters for each lamp set. Example: LED module 24 W 4000 K. Format guide
26c Total luminous flux Test report
The total flux of the lamps in lumens. In absolute photometry, the measured luminaire flux. Example: 2400. Value: decimal, lumens, up to 12 characters for each lamp set. Example: 2400. Format guide
26d Colour appearance Data sheet
The colour temperature or a colour name. Example: 4000. Value: text, up to 16 characters, kelvin as digits. Example: 4000. Common practice No document sets the form
26e Colour rendering group Data sheet
The colour rendering group. This line holds a group or the index below. Example: 1B. Value: a group code or the Ra value, up to 6 characters. Example: 1B. Format guide AGI32
26e Colour rendering index, Ra Data sheet
The colour rendering index. It shares line 26e with the group above. Example: 80. Value: Ra as a whole number, or a group code, up to 6 characters. Example: 80. Format guide AGI32
26f Wattage including control gear Data sheet
The power including the driver, in watts. Example: 24.0. Value: decimal, watts; control gear included, up to 8 characters for each lamp set. Example: 24.0. Format guide

Direct ratios, line 27

27 Direct ratios Test report
Ten values for room indices 0.6 to 5. Utilization-factor calculations use them. Example: 0.55 to 1.00. Value: 10 decimals, 0 to 1, up to 7 characters each. Example: 0.55. Format guide

Angles and intensities, lines 28 to 30

28 C-plane angles Test report
One line per C-plane, Mc lines, from 0°. Example: 0.0, 90.0, 180.0, 270.0. Value: decimals, degrees, from 0, up to 6 characters each. Example: 0.0, 90.0, 180.0, 270.0. Format guide
29 Gamma angles Test report
One line per gamma angle, Ng lines, from 0°. Example: 0.0, 45.0, 90.0. Value: decimals, degrees, from 0, up to 6 characters each. Example: 0.0, 45.0, 90.0. Format guide
30 Luminous intensities Test report
The intensity in cd/klm: candela per 1000 lumens. One block per stored C-plane, Ng values each. Example: 350.0, 210.0, 0.0. These numbers come from the laboratory only. Value: decimals, candela per 1000 lumens; count set by the symmetry indicator, up to 6 characters each. Example: 350.0, 210.0, 0.0. Format guide

A polar diagram draws these intensity values as a curve. A cone diagram and an isolux diagram show the light that they give on a surface.

Sources

  • DIALux evo, Description of the EULUMDAT format. The line list and the symmetry values.
  • AGI32 Photometric Toolbox, EULUMDAT file format. The line list, second reference.
  • 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.
  • EN 13032-2. Clauses 4.2.13 and 4.2.14: the colour rendering index and the colour temperature as 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.
  • CIE 121:1996, section 2.8. The rule: length along C90–C270.
  • Viso Systems, Dimensions of lighting fixtures in .IES and .LDT files. The orientation drawings, and the 90° turn between the two laboratory styles.
  • schorsch.com, Online converter for luminaire data files, help. The two habits for a long luminaire: C0 along the short axis in LDT, along the long axis in IES.
  • ANSI/IES LM-63-19. The IES standard, for the comparison. Figure 1 gives the IES axis rule.
  • IES, Learn about IES files. A plain-language overview of the other format.