Throw Distance for a Target Beam Pool
The throw distance a fixture needs to cast a target beam (pool) diameter.
Example
You enter
- Target beam (pool) diameter 10.5796
- Beam angle (full cone, deg) 20
- Distance unit ft
You get
- Throw distance needed 30
Checked against published worked examples from: ETC.
Details, formula, and sources
Scopethrow = D / (2 x tan(beam angle / 2)). A 20 degree fixture makes a 10.6 ft pool from a 30 ft throw; a wider 40 degree beam reaches the same pool from just 14.5 ft. Answers 'how far to hang it for an X-foot pool' instead of the pool at one throw. Geometry only - the illuminance still falls with the square of the throw (check the level with lighting-beam). The fixture cut sheet governs.
Formula or tablethrow = target_pool_diameter / (2 x tan(beam angle / 2)); the inverse of beam_diameter = 2 x throw x tan(beam angle / 2).
Source and editionFirst-principles theatrical photometry - the beam-spread geometry the form fixture photometric charts (manufacturer cut sheets) publish, solved for the throw, by name; public domain.
Free accessPublic first-principles photometry. The beam / field angle comes from the fixture's photometric data.
What governsEstimate. AHJ and licensed professional govern.
API field namestarget_pool_diameter, beam_angle_deg, distance_unit, throw_distance
- Geometry only throw from beam spread; the illuminance still falls with the square of the throw (use lighting-beam for the level)method
- Beam vs field angle enter the angle being designed to; beam angle is to 50% intensity, field angle to 10%fixture photometrics