A photometric file isn't a brochure. It's a measured map of luminous intensity in candela at every angle around the fixture. A goniophotometer makes it, DIALux reads it, and together they are how a fixture becomes a design instead of a guess.
Four ideas to carry into the next specification conversation you have.
A two-axis goniometer sweeps the fixture against a calibrated detector. The detector reads illuminance and intensity is calculated, I = E × d². One common scan grid is 24 C planes × 181 γ steps = 4,344 points.
Type C for almost everything architectural (downlights, street lighting, panels); Type B for floodlights and sports projectors; Type A is the automotive convention. Any goniometer can produce any system and A and B convert to C, so the real failure is a file that DECLARES the wrong photometric type.
IES = North American (IESNA). LDT = European (EULUMDAT). DIALux reads both. Premium fixtures ship both files.
Free in its basic version, published by DIAL GmbH in Lüdenscheid. The industry standard. NLC is building its photometric library; IES and LDT files for specific products are available on request.
Fields per IES LM-63-2019; commercial DIALux/AGi32 software uses all of them.
| Field | What it carries | Why it matters |
|---|---|---|
| TILT | Mounting tilt (NONE / line / file) | Streetlight aim correction |
| Number of lamps | Count of light sources | Bulb/source modelling |
| Lumens per lamp | Source lumen output, or -1 for absolute photometry | On an LED file, expect -1 |
| Candela multiplier | Scaling factor (usually 1.0) | Re-scale for different flux |
| Vertical angles | Number of γ steps (this count comes first) | Resolution above/below |
| Horizontal angles | Number of C planes | Resolution around fixture |
| Photometric type | 1 (Type C) / 2 (Type B) / 3 (Type A) | Coordinate system |
| Candela grid | Intensity at every (C, γ) | The actual photometry |
| Units type | 1 = feet, 2 = metres | Wrong units, wrong geometry |
| Luminous opening | Width, length, height of the emitting area | Glare and near-field results |
| Ballast factor | Multiplier applied to the candela values | Scales the whole distribution |
| Input watts | Total fixture power | Efficacy calculation |
Five things to know about photometric files and how to read them.
The detector has to sit far enough away that the fixture behaves as a point, and the rule is relative: at least five times the largest light-emitting dimension (CIE 121, LM-79). A 0.6 m luminous opening needs 3 m; a 2 m linear needs 10 m. That is why photometric laboratories are long rooms and why mirror goniophotometers exist to fold the path. Ask for the photometric distance and the aperture together, and for an LM-79 report from a laboratory whose ISO/IEC 17025 accreditation scope covers the test (for example ITL in Longmont, Colorado).
LM-79 measures total luminous flux, efficacy, CCT, CRI, chromaticity. LM-80 measures lumen maintenance over time (3,000-10,000 hours of aging data, used to project L70 life). A spec citing only LM-79 is incomplete.
Old fluorescent files used relative photometry, lumens from a separate lamp test, candela distribution from the fixture. LEDs use absolute photometry, both measured together, since the LED and fixture are inseparable. LM-79 mandates absolute.
DIALux models direct light + 1st-bounce inter-reflection well. It struggles with specular surfaces (polished marble, mirrors) and complex daylight. For atriums, malls, museums, switch to AGi32 or Relux for proper ray-tracing.
No standard publishes a single pass/fail percentage for calculation versus measurement, and the calculation targets maintained illuminance, not day-one lux. The gap is driven by the maintenance factor you assumed (CIE 97), the real surface reflectances versus the modelled ones, product tolerance and drive current, and the meter class, grid, stabilisation time and daylight exclusion on the day. EN 12464-1 requires measurement uncertainty to be taken into account in conformity assessment, so agree the verification method in the contract. The engine itself is a measured quantity: DIALux has been validated against the CIE 171:2006 analytical test cases.
We are building a complete photometric library for the range. Until it is published, IES and LDT files for specific products are available on request for your DIALux or Relux model.
A 14-slide explainer, ready to post straight to LinkedIn as an image carousel. Same content, faster reading.
14 slides · LinkedIn-ready. Download all 14 as high-res PNGs and post them straight to LinkedIn as a carousel. PDF version still available.
Standards are cited in their current active revisions, checked 24 August 2026. The worked figures on this page and in the carousel (a 0.6 m opening needing 3 m of photometric distance, a 24 × 181 scan giving 4,344 points, 1,200 cd at 30° from 3 m giving 86.6 lx, and 5 × 0.85 m = 4.25 m) are worked examples computed from the rules above, not measurements of an NLC product.
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