Beam diameter is the calculated footprint of a beam at the assessed plane.
Beam diameter is the approximate beam footprint from beam angle and effective height. It is a geometry result, not a measured usable-light value.
Technical meaning
Beam diameter is the approximate width of the beam footprint where the nominal beam reaches the assessed plane.
The calculation assumes a simple symmetrical cone from beam angle and effective height.
Calculation use
Beam diameter is calculated with 2 x effective height x tan(beam angle / 2).
Spacing estimates compare beam diameter with room size, fitting count and nominal centre spacing to identify overlap or gaps.
Not the same as
Beam diameter is not coverage quality. It does not show lux level, uniformity, edge fall-off or glare.
Beam diameter is not the lit area guaranteed by the manufacturer. Actual distribution depends on photometry, aiming and surfaces.
Australian context
Australian downlight and floodlight notes should treat beam diameter as a layout cue that sits beside target lux, mounting height and the applicable project limits.
Examples
Example
Value
Planning note
60 degree at 2.4 m
about 2.8 m diameter
Geometry estimate before lumen and spacing checks.
90 degree at 2.4 m
about 4.8 m diameter
Wider footprint but not automatically better task lighting.
24 degree at 5.0 m
about 2.1 m diameter
Narrow high mounting can still form a concentrated pool.