The fixture produces luminous flux; the optical system determines how it is distributed. With the same output and mounting height, a narrow distribution may produce ≈ 400 lx on-axis while a wider one may produce ≈ 90 lx. Here is how to read a beam-angle spec, and what mounting height does to it.
Four ideas to carry into the next specification conversation you have.
A 15° spot pushes lumens far down with minimum spill. A 60° flood lights a wider area at lower intensity. Choose for the task, not the fixture catalogue.
At 6 m a 60° beam covers ≈ 7 m diameter on the floor. Double the height, quadruple the area, quarter the lux.
With the same LED source, the reflector finish changes the distribution, the peak intensity and potentially the luminaire's delivered output. Evaluate visual comfort in the project model.
Racked aisles want a dedicated asymmetric aisle optic. Display wants a focused 10-24° beam. Offices want a wide or batwing distribution, verified by modelling. The reflector is a design decision, not a cosmetic one.
Approximate diameter of the lit circle, assuming a circular beam. Real fixtures use IES files for accuracy.
| Beam angle | Coverage @ 3 m / 6 m / 9 m | Typical use |
|---|---|---|
| 10° (narrow spot) | 0.5 / 1.0 / 1.6 m | Façade accent · merchandise |
| 24° (spot) | 1.3 / 2.6 / 3.8 m | Retail downlight · gallery |
| 36° (medium) | 2.0 / 3.9 / 5.8 m | Office downlight |
| 60° (flood) | 3.5 / 6.9 / 10.4 m | General lighting · corridors |
| 90° (wide distribution) | 6.0 / 12.0 / 18.0 m | Open warehouse · low-bay areas |
| Aisle optic (asymmetric) | per photometry | Racked warehouse aisles |
| Wide / batwing (distribution shape) | 10+ m (lateral spread) | Office desks · classrooms (verify by modelling) |
Six ideas that change how you read a photometric distribution.
When a spec sheet lists "36° beam," that is the full angle across which luminous intensity stays above half of peak (FWHM). Outside that, spill continues out to the field angle, the full included angle at 10% of peak. In this example a 36° beam corresponds to a field angle of about 49°; the relationship varies by optical system. Plan for both.
An anodised aluminium reflector with high specular reflectance keeps the beam directional, useful at high mounting heights. A matte white powder-coated reflector scatters light into a softer, wider distribution; verify its effect on visual comfort in the project model. Reflectance values are material- and product-specific, read the datasheet.
A real IES/LDT file tells you intensity in candela at every angle. A glossy brochure showing a clean cone is marketing. Always ask for the photometric file before specifying.
On the beam axis, using the point-source (far-field) approximation, illuminance drops with the square of the direct distance from the luminaire to the calculation point. A 30,000 cd fixture gives ≈ 3,300 lux at 3 m and ≈ 830 lux at 6 m. Halving the mounting height quadruples the on-axis lux.
A luminaire may combine an LED module, a lens or refractor seated over it, and a reflector cup; their functions and order vary by product. Specify and evaluate the complete optical system, not just the beam angle.
Light-coloured reflectors bounce stray light around the housing and lift total lumen output, but the cup itself reads bright on the sightline. A matte-black reflector absorbs that internal scatter instead of re-emitting it, which can lower the luminaire's apparent brightness. Its effect on output, distribution and UGR depends on the luminaire, the room, the layout and the observer position; verify with the correct IES/LDT file. Reach for black in meeting rooms, classrooms, control rooms, museums, and high-end retail where contrast matters. A large portion of NLC's office downlights and recessed luminaires (LUXO, ICON, AGILE, HANDY, MINI, CYLINDER, and more) ships with white, mirror-silver, and matte-black reflector options as a build-to-order choice, specify the colour against the lumen/glare balance the room needs.
Every NLC fixture ships with its photometric file. Our DIALux service models lux at the work plane for the reflector and mounting height you actually have.
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