The beam angle and the distance to the surface set how wide the lit circle is. The beam diameter grows with the tangent of half the beam angle.
What the terms mean
| Symbol | Meaning |
|---|---|
| D | Beam diameter at the surface |
| d | Distance to the surface |
| θ | Full beam angle, in degrees |
Worked example
A 30° beam at 3 m gives D = 2 × 3 × tan(15°) = about 1.6 m across.
See the Beam Angle Comparison and CBCP Comparison.
Beam angle and spot size
The beam angle and the throw distance together set how wide the lit circle is. The geometry is straightforward: the beam diameter grows with the tangent of half the beam angle — diameter ≈ 2 × distance × tan(beam angle ÷ 2). So a 30° beam at 3 metres lights a circle about 1.6 metres across, while a 60° beam at the same distance covers roughly 3.5 metres.
Narrow beams (under ~25°) concentrate light for accent and long throws; wide beams (40°+) cover area for general and wall lighting. Remember the trade-off: for a fixed lumen output, a narrower beam gives a brighter, smaller pool (higher candela) and a wider beam a dimmer, larger one. Use the beam-angle and distance relationship to plan fixture spacing so the pools overlap for even coverage.