Lighting
Beam Angle
01 — Definition
The angle within which a fixture holds at least 50% of its peak intensity. The narrower the angle, the tighter and more intense the spot; the wider it is, the further the wash spreads, with less intensity per unit area. Coverage diameter = 2 × throw distance × tan(angle/2).
02 — In the field
At 10 m throw with a 20° beam angle, the diameter covered = 2 × 10 × tan(10°) = 3.5 m. At 5° you drop to 0.87 m — barely a face at that distance. Always work out the coverage before you settle on a beam angle.
04 — Sources
- A Practical Guide to Stage Lighting, Shelley, 2e éd. 2009, p. 28 — angle de faisceau = cône interne à 50 % de l'intensité centrale