- Desired area size (typically 8'ish)
- Throw distance from hanging position to center of your area
- Beam angle (and corresponding multiplication factor) of the instrument
If you multiply the throw distance by the beam angle's multiplication factor, you will get the beam diameter at that distance.
Throw Distance x Multiplication Factor = Area Size
Note: You'll often start with a desired area size, then look for a light position that gives you the best angle for your lighting idea, then determine which size of instrument gets you closest to the desired area size. So I sometimes use the formula this way:
Area Size/Throw Distance = Optimal Multiplication Factor
To Determine Throw Distance
To figure the throw distance from a hanging position to the center of a light area, you will need the horizontal distance (run) and the height (rise) from your light position to the center of your light area. You'll find the hypotenuse of those two measurements by using the Pythagorean Theorem.
a2 + b2 = c2
a = run (center of light area to light position on ground plan)
b = rise (5' off stage deck to vertical height of light position)
c = throw distance
Remember that your light area is always 5' above the stage deck - so that you are lighting the actor's face and not the floor.
To Find A Beam Angle
Each instrument has a beam angle and a field angle. For lighting the stage you are most concerned with the beam angle. You can get the beam angle from the manufacturer specification sheet for that particular instrument. Once you have that beam angle, you can use this chart to find the equivalent multiplication factor for that beam angle. For your convenience, I have included beam angles and multiplication factors for all our instruments at George Fox on our lighting inventory sheet.
EXAMPLE
Suppose you want to light an 8' light area with a source four ERS. The ideal light position is 18' horizontal feet away (plan view) and 24' above the stage deck.
- Determine the throw distance. Remember to subtract 5' from the height!
182 + 192 = throw distance2
324 + 361 = 685
Square root of 685 = 26.2
Throw distance = 26.2' - Find optimal multiplication factor.
8' (desired light area diameter) / 26' (throw distance) = optimal multiplication factor
8/26=.31 - Select instrument size that gets closest to optimal multiplication factor.
The beam angle of a 26degree source four is 18degrees, with a multiplication factor of .32. - If we double check:
From a throw distance of 26', a 26degree source four (MF = .32) would give us a beam diameter of 8.3'. That's close enough for us, and it's usually better to be a little over than a little under.
