Lighting
Beam Angle
Definition
The beam angle of a fixture is the angle of the cone within which intensity is at least 50% of its peak value. A narrow angle gives a tight, intense spot, and a wide angle spreads a wash over a larger area with less intensity per unit area. The diameter covered at a given distance is 2 × throw distance × tan(angle/2).
In the field
At a throw of 10 m with a 20° beam angle, the diameter covered is 2 × 10 × tan(10°) = 3.5 m. At 5° it falls to 0.87 m, barely the width of a face at that distance. Coverage is therefore calculated before a beam angle is chosen.
Related lessons
Sources
- Shelley, A Practical Guide to Stage Lighting, 2e éd., Focal Press, 2009 : angle de faisceau = cône à 50 % 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