Lighting
Beam Angle vs Field Angle
01 — Definition
Two ways of putting a number on a fixture's spread. The beam angle marks where intensity falls to 50% of peak — the bright core. The field angle marks the 10% point — the still-usable edge, with its dimmer fringe. Field angle is always wider than beam angle, and since some manufacturers quote only the more flattering figure, it pays to compare like for like.
02 — In the field
When laying out coverage, take the field angle (10%) for the usable edge of the pool and the beam angle (50%) for the bright centre. Comparing one fixture's beam figure to another's field figure gives you a wrong spacing plan.
04 — Sources
- Shelley, 2e éd. 2009, p. 28 — faisceau 50 %, champ 10 % de l'intensité centrale