IES file and DIALux: check a luminaire before you buy

An IES file is the only document about a luminaire that can be checked before payment: the designer calculates from it, DIALux gives the lux on the floor from it, and it shows when a supplier promises more than was measured. Below: what is inside the file, how to tell a sound one from a faulty one, and how to run someone else’s luminaire through your own facility in half an hour.

What is inside an IES file

The format is set by ANSI/IES LM-63: a text file that any editor can read. The first lines are keywords in square brackets: [TEST] the report number, [TESTLAB] the laboratory, [ISSUEDATE] or [DATA] the date, [MANUFAC] the manufacturer, [LUMCAT] and [LUMINAIRE] the article and description, [LAMP] the light source. Then the line TILT= (usually NONE) and ten numbers in a row: the number of lamps, the flux per lamp in lumens (or -1 for absolute photometry), the luminous intensity multiplier, the number of vertical and horizontal angles, the photometry type (1 is type C, for luminaires), the units of dimensions (1 feet, 2 metres), the width, length and height of the luminous area. The next line is the ballast factor, a reserved field and the input power in watts. Then the angles and the table of luminous intensity in candelas.

Our 2018 Prom 150W file looks like this:

IESNA:LM-63-1995
[DATA] 04.04.2018
[MANUFAC] "High Light Technology"
[LUMINAIRE] Prom 3x50 150W 19500lm 5700K
TILT=NONE
1 19500 3.00
19 19
1 2
0.750 0.140 0.125
1 1 150

It reads as follows: one “lamp” of 19,500 lm, multiplier 3.00, 19 angles in each plane from 0 to 90° in 5° steps, type C, metres, luminous area 0.75 × 0.14 × 0.125 m, ballast factor 1.0, 150 W. The first figure of the table, the intensity at nadir, is 3,944 cd; with the multiplier of 3 that is 11,832 cd for the luminaire.

Module file or luminaire file

The luminous intensity multiplier is where the honesty of a file most often breaks. If the photometry was taken from one module and the luminaire is assembled from three, the manufacturer puts a multiplier of 3.00 instead of measuring again. That is acceptable if the table really is for one module. But it also happens that the table is already for the whole luminaire and the multiplier equals the number of modules: then the file overstates the light several times over.

There is one check: calculate the flux from the curve itself (the integral of luminous intensity over solid angle) and compare it with the flux in the header. A luminaire does not give out more than is put into it, so a discrepancy above a few percent means that one of the figures is wrong. We ran our own seven files through this check. Prom 100/150/200 and Line give 8% to 12% more from the curve than the header states (Prom 150W: 21,833 against 19,500 lm): for a design we take the lower figure, and consistent files have been requested from the manufacturer. The two files for the bracket-mounted Classic 50W and 100W disagree by 1.75 and 3.49 times: we have removed their figures from the site and do not give them for designs. The same check of any third-party file is done by the IES viewer on the site: the file is parsed in the browser and is not sent anywhere.

The second sign is absolute photometry. A file under an LM-79 report contains -1 in the lamp flux field: the luminaire flux equals the integral of the curve, and there is nothing to compare it with, but the file has [TEST] and [TESTLAB]. A file with round figures in the header, no laboratory and a date seven years old is calculated or relative, and should be treated accordingly. Our files are exactly that, and we say so plainly.

How to load it into DIALux evo

1. Download the IES or LDT file for the exact article on offer: not the “series” but the model with that power and optics.
2. In DIALux evo drag the file into the project window or import it through the luminaire section: it appears in the project’s luminaire list.
3. Open the luminaire data sheet: it shows the flux, power, efficacy, polar curve and the share of flux into the lower hemisphere. Compare the three figures with the product data sheet and the quotation.
4. Build the room or street from the plan, place the luminaires, set the maintenance factor in the calculation parameters: under ShNK 2.01.05-24 (Annex 1, Table 3) 0.67 to 0.71 for clean rooms, or under CIE 97 with LLMF from the LM-80 report.
5. Run the calculation and read: the average Eav, the minimum Emin, uniformity Emin/Eav for each zone, the installed and specific power in W/m².

DIALux calculates from the luminous intensity table, not from the figure in the header: if the file is overstated, the calculation will be overstated too, and you find out only with a lux meter after installation. Hence the order: first check the file, then calculate.

What to look at in the result

• Flux, power, efficacy. The flux from the file divided by the power from the file is the efficacy that can be trusted within the class of the file. From our 2018 files it is about 130 lm/W; the model cards show higher calculated values from the module specification, and until the LM-79 report the figure from the file goes into the design.
• Light distribution curve. The half-intensity angle in the C0 and C90 planes tells which height the luminaire is for: Prom by its file 74° × 106°, the bracket-mounted Classic 108° × 114°. For a street the share of light above 90° matters: full cutoff means zero into the upper hemisphere.
• Utilisation factor. DIALux does not print it, but it follows from the result: UF = Eav × S / (N × F × MF). For a warehouse with high racks it comes to 0.4 to 0.5, for a light shop floor 0.6 to 0.7. If a supplier’s offer promises 200 lx with UF 0.9, the calculation does not agree with the geometry.
• Acceptance. The calculation points and plane must match the future measurement: ShNK 2.01.05-24 (clause 178, Table 26) accepts an installation at a measured illuminance not below 0.9 × Kz × En.

A calculation for your plan with our files is free of charge: request a calculation. The IES files themselves sit on the downloads page and download without a form.

Frequently asked questions

How does IES differ from LDT?

The same content in two formats: IES under ANSI/IES LM-63, LDT under EULUMDAT, the European format. DIALux reads both; LDT has fields for the luminous area dimensions and symmetry that old IES files lack. Ask for the one the laboratory issues, not a converted one.

Can an IES file be built from the luminaire data sheet?

No. A file is born on a goniophotometer: the luminous intensity table is measured, not derived from the flux. A file “made from the data sheet” gives a picture in DIALux, but not the illuminance at the facility.

Why is the flux in the file lower than in the model card?

Because they are figures of different classes. The card carries a calculation from the declared module efficacy; the file carries the photometry of the assembled luminaire, where part of the light is lost in the optics and to heat. The figure from the file goes into the design; when the LM-79 report appears, the card will show the measured value.

Are the 2018 files still fit for a design?

They are, as a lower estimate of the flux, as long as the optics and modules of the model are the same as in 2018; we say so for each model before the calculation. New files and an LM-79 report have been requested from the manufacturer. If the specification requires absolute photometry or a file no older than a certain date, say so before the calculation.