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PHOTOMETRICS

# Stage light beam angle, throw distance and lux

A narrow aerial beam and a stage wash can have similar power ratings and very different coverage. Start with the width you need at the target, calculate a useful angle range, then compare the fixtures' photometric data at that throw.

Published 6 November 2026 by BEAM TECHNICA

## Calculate the approximate beam diameter

For a circular beam hitting a surface square-on, estimate diameter with 2 × distance × tan(angle ÷ 2). Use the full beam angle in degrees and measure distance along the beam from fixture to target. The diameter uses the same units as the distance.

At a 10 m throw, a 5° beam covers approximately 0.87 m, a 15° beam 2.63 m and a 30° beam 5.36 m. To cover about 3 m from 12 m, the same calculation in reverse gives approximately 14.3°. That is a starting point for choosing a zoom setting. Oblique aiming stretches the footprint, and beams with asymmetric optics need separate horizontal and vertical coverage checks.

## Compare angles measured at the same light level

Photometric charts can show both a beam angle measured at half the peak intensity and a field angle measured at one-tenth of peak. The second measurement normally describes a wider area. Check how the manufacturer defines each angle before comparing fixtures or putting the figure into a coverage calculation.

A calculated diameter describes one contour, so it does not tell you that the entire circle is evenly lit. If you are overlapping fixtures for a wash, read the intensity distribution across the beam and check where adjacent fixtures meet.

## Compare centre lux at the intended throw

Lux measures illuminance at a surface. Compare readings for the same distance, colour, output mode and zoom where possible. A narrow white setting can have much higher centre lux than a wide saturated-colour setting on the same fixture.

If you only have one measurement, the inverse-square relationship can give a rough estimate in the far field with the same optics and output. For example, 2,000 lux at 5 m is approximately 500 lux at 10 m under those assumptions. Use the manufacturer's distance chart or a measurement at the intended throw when available.

## Put the venue measurements in the equipment brief

Record target width, throw distance, surface orientation, desired overlap and the colours the show will use. Compare a narrow moving beam for aerial effects with a zoom wash for area coverage on the basis of that brief. During a demo, check the edges and the light level at the subject as well as the centre of the beam.

## Related guides and equipment

[How to choose a moving head beam light](https://beamtechnica.com/guides/how-to-choose-moving-head-beam-light)[AXIS 300 moving beam](https://beamtechnica.com/products/moving-head/axis-300)[MEGAWASH moving wash](https://beamtechnica.com/products/moving-head/b-eye-1960)

## Technical references

- [Robe Lighting — LEDBeam 150 photometric report](https://www.robe.cz/res/downloads/photometrics/LEDBeam_150_Photometric_report.pdf)

- [Robe Lighting — 800X LEDWash photometric diagrams](https://www.robe.cz/res/downloads/photometrics/Robin_800X_LEDWash_photometric_diagrams.pdf)