Beam Angles and Coverage: How Many Fixtures for the Stage You Have

In this guide:Beam angle · The relationship · Overlap · Working it out · Choosing · Picks · FAQ

Fixture counts are usually guessed, then corrected on the day by moving everything. A small amount of arithmetic beforehand gets it right first time and stops you buying four fixtures you did not need.

The whole subject rests on one relationship: beam angle plus throw distance gives you pool size.

What beam angle means

Beam angle is the cone of light a fixture produces, quoted in degrees. A narrow angle throws a small, intense pool a long way. A wide angle covers a large area at lower intensity from close up.

Two figures are sometimes quoted. Beam angle is measured where intensity has fallen to half, and field angle where it has fallen to a tenth, so the field angle is always the larger number. When comparing fixtures, make sure you are comparing the same one.

The relationship, without the trigonometry

The useful approximation is that the diameter of the pool is roughly the throw distance multiplied by a factor for the beam angle.

Throw distance is the actual distance from the fixture to the surface, not the height of the bar. A fixture on a 5 metre bar lighting the front of a stage at 45 degrees is throwing about 7 metres, not 5.

Even coverage needs overlap

Pools do not butt together neatly. Light falls off towards the edge of the beam, so two pools placed edge to edge leave a dim band between them.

The working convention is to space fixtures so that the edge of one pool reaches the centre of the next. That looks like a lot of overlap on paper and produces an even wash in the room.

For front light on an acting area, that usually means fixtures at roughly 2 to 3 metre spacing on the bar for a typical school or function room. Wider spacing and you get scallops, which are visible the moment anyone walks across the stage.

A worked example

A school hall stage 8 metres wide and 4 metres deep, with a lighting bar 5 metres up and 3 metres in front of the stage edge.

• Throw distance to the middle of the acting area is roughly 7 metres.
• A 36 degree fixture gives a pool about 4.5 metres across at that distance.
• With overlap, effective spacing is about half that, so 2.2 metres.
• 8 metres of width divided by 2.2 gives four fixtures across the front.
• Add the same again for back light, and the front-light layer is eight fixtures.

That is a real answer, arrived at in two minutes, and it is far more useful than buying six because six was on offer.

Choosing angles for the job

• Front light on a small stage: 25 to 36 degrees is usually right.
• Big washes from a short throw: 50 degrees and above.
• Specials and long throws: 15 to 19 degrees.
• Zoom fixtures cover a range and cost more, which is worth it for hire stock that goes into different rooms every week.
• Fixed angle fixtures are cheaper and fine for an installed rig in a room that never changes.

The fixture families that carry these angles are covered in fixture types explained, and where to point them in lighting a stage.

Frequently asked questions

What beam angle do I need?

It depends on throw distance and the area you need to cover. As a rough guide, 25 to 36 degrees for front light on a small stage, wider for close washes, narrower for long throws and specials.

How far apart should lighting fixtures be?

Close enough that the edge of one pool reaches the centre of the next. That produces an even wash. Edge to edge spacing leaves visible dim bands.

Is a zoom fixture worth the extra?

For hire stock and touring, yes, because the same fixture works in every room. For a fixed installation in a room that does not change, a fixed angle fixture is cheaper and just as good.

Does beam angle affect brightness?

Yes. The same output spread over a wider cone is less intense at any point. That is why a wide fixture looks dimmer than a narrow one of the same wattage.