A fixture's beam angle tells you how a cone of light spreads. Combined with the throw distance (how far the light travels to the surface), it tells you the diameter of the pool it makes — essential for spacing a wash so there are no gaps or hot overlaps.
Beam vs. field angle
- Beam angle — measured where intensity falls to 50% of centre. The "bright core".
- Field angle — measured at 10% of centre. The usable edge of the pool, always wider than the beam.
The geometry
The pool diameter is simple trigonometry:
Diameter = 2 × throw × tan(angle ÷ 2)
A 20° fixture at 8m throw makes a pool about 2 × 8 × tan(10°) ≈ 2.8m wide.
A 20° fixture at 8m throw makes a pool about 2 × 8 × tan(10°) ≈ 2.8m wide.
Narrow beams (5–15°) punch long distances with tight pools — good for aerial looks and big rooms. Wide beams (40°+) wash close surfaces. Match the angle to the throw, not the other way around.