How do I calculate the size a light will be when it hits the stage?

Question

I'm trying to design lights for a show. How do I know how big a light will be when shining from our catwalk?

Answer 1

Good question. This is called photometrics, and understanding how this works can help you choose the best locations to put your lights, or the right type of light for the job.

Every lighting manufacturer publishes what the multiplying factor is for their particular lighting instrument. If you know the degree of the fixture, here are some standard MF.

Multiplying Factors by Field Angle (not to be confused with Beam Angle)

50° = .93 MF

36° = .65 MF

26° = .46 MF

19° = .31 MF

10° = .17 MF

5° = .09 MF

You'll also need the throw distance. You can measure this from the stage, using a laser (disto) measure, or on a scaled section view drawing. Remember, you want to measure based on the head height of your actors. Typically this is done at 6' off the stage deck, to ensure everyone is covered, but if you have a youth cast you might want a different height.

Formula to calculate beam size:

Multiplying Factor x Throw distance = Diameter of area covered (MF x T = D)

Now you have the pieces to calculate the size of your beam, by inputting the Multiplying Factor and Throw Distance into the formula above.

Alternatively, this formula can also be used in reverse, and that's how most lighting designers use it. Maybe you know that you want your pools of light to be 8' in diameter. And you know that you want your front light at a 45 degree angle, which is the most natural angle for front light face sculpting (anywhere from 40deg to 50deg, trust me). Your 1st electric is approximately 45 degrees from the current position, so you can measure from your head height to the 1st electric, and that is your throw distance. For the sake of this, let's say that is 28'.

We'll reverse our formula:

Diameter of area covered ÷ Throw Distance = Multiplying Factor (D ÷ T = MF)

We'll divide 8' by 28', and that gives us a Multiplying Factor of .285

19 degree fixtures have a MF of .31 so that is just slightly larger.

That's how you choose the right lighting instrument for the job.