Lighting
Beam Angle vs Field Angle
Definition
Beam angle and field angle are two measures of a fixture's spread. The beam angle is where intensity falls to 50% of its peak and defines the bright core. The field angle is where it falls to 10% and marks the usable edge with its dimmer fringe. The field angle is always wider than the beam angle. Datasheets do not always state which of the two figures they give, so fixtures are compared on the same measure.
In the field
For coverage plans, the field angle (10%) gives the usable edge of the pool and the beam angle (50%) its bright centre. Comparing one fixture's beam angle with another's field angle produces a wrong spacing plan.
Related lessons
Sources
- Shelley, A Practical Guide to Stage Lighting, 2e éd., Focal Press, 2009 : faisceau à 50 %, champ à 10 % de l'intensité centrale
- Wikipédia (pointeur secondaire), Beam angle, définition de l'IES : angle de faisceau à 50 %, angle de champ à 10 % de l'intensité maximale · en.wikipedia.org