# Equation for Beam Angle

Discussion in 'Lighting and Electrics' started by Tyler, Dec 8, 2008.

1. ### TylerMember

Messages:
26
7
Location:
000000000000
0000000000000000000000

Last edited: Nov 9, 2009
2. ### icewolf08CBModCB Mods

Messages:
4,082
695
Occupation:
Controls Technician - TAIT Towers
Location:
Lititz, PA
In is simple trig:

In this image, A is the throw, B is 1/2 the field diameter, and C is the beam edge or hypotenuse of the right triangle created by A and B. Lets call the beam angle X. Most of these calculations require a good scientific calculator.

So you know A and B. To get beam angle: X=2*atan(B/A)
NOTE: atan = Arc Tangent or inverse tangent

So you know the beam angle (X) and B. To get throw: A=B/tan(X/2)

So you know beam angle (X) and A. To get Beam width: A*tan(x/2)

Cheever and (deleted member) like this.
3. ### derekleffewResident CurmudgeonSenior TeamPremium Member

Messages:
4,643
3,130
Location:
Las Vegas, NV, USA
Nicely done, Alex. [user]Tyler[/user], for more, see the CB Collaborative Article: http://www.controlbooth.com/forums/collaborative-articles/7664-mathematical-formulas-lighting.html.

One caution: Don't get "beam angle," "field angle," "beam," and "field" confused. When referring to a 26° ERS, the number is referring to the fixture's field angle. Usually, when planning a wash from multiple fixtures, one wants to overlap fields, but not beams. Softening by "running the barrel" or adding a light frost (X114, X119, X132) covers a multitude of sins/errors in judgment.

Do they still print trig. tables in the back of textbooks?
Edit: Backstage Handbook, Third Edition (Blue Cover). Paul Carter. Broadway Press, 1994, page 226.

Last edited by a moderator: May 30, 2014
4. ### gafftaperSenior TeamSenior TeamFight Leukemia

Messages:
12,930
3,055
Occupation:
Theater Manager & T.D.
Location:
Seattle, Washington
FYI 1994 is the most "current" edition of the Backstage Handbook. Maybe if we are lucky 2009 will bring us a 15 year update.

Last edited by a moderator: May 30, 2014
5. ### CavezziMagnumMember

Messages:
62
1
Location:
Atlanta, GA USA
From your keyboard to whatever you believe in's ears.

6. ### waynehoskinsActive Member

Messages:
782
49
Location:
Arlington, TX
Wow, how time flies. I got mine when I started college in '99 and the book was a mere 5 years old .. that can't have been nearly 10 years ago now, can it?

Messages:
4,643