Measured edge to edge across the beam at the points where intensity has fallen to 50 percent of the peak, which usually sits on axis. Narrow spots run under about 15 degrees, floods 25 to 40 degrees and wide floods beyond. Beam angle sets the size of the bright core of a pool of light; the softer edge extends out to the field angle. Together with distance it lets designers size accent pools quickly.
At 4 meters a 24 degree beam angle gives you a pool about 1.7 meters across, perfect for that painting.
Also heard as
- beam spread
- half-intensity angle
- FWHM angle
Do not mix it up with
- Field angle: Field angle is measured at 10 percent of peak intensity, so it is always wider than the beam angle.
Related terms
Field angle
The angle across a light's beam at which intensity falls to 10 percent of its peak, marking the usable edge of the light.
Candela
The unit for how much light a source throws in one direction, and the SI base unit behind lumens and lux.
Throw
The distance from a lighting fixture to the surface or subject it lights, which sets beam size and intensity.
Luminous intensity
How much light a source sends out in one specific direction, measured in candelas.
Spill
Light that falls outside the area it is aimed at, onto walls, ceilings, audiences or neighbors.