Does anyone know of a formula that takes the beam angle and throw distance and gives the resulting light diameter at both visible width and effective width?

A handy quick referrence is the photometric data sheets for each fixture available at www.altmanltg.com Otherwise you can do it the old fashion way by drawing a vector (at the appropiate angle) and using a 1" = 10' scale measure out both legs of the vector to the distance you need and measure between the two legs to determine beam diameter.

Well you want the actual trip formula here it is... but only when a light is perpendicular to the surface it is projecting on- When the angle of the beam spread (degrees) = A, The distance from the light to the surface (ft) = d, And half of the beam spread in feet = x The total beam spread in feet = 2x = s tan(1/2*A)=(x/d) x=(d)tan(1/2*A) since x is only half... the formula you are looking for is... s=(2)(d)tan(1/2*A) I suppose if you want I could explain why you have to do all that.... just not right now, I gotta get running!

The day my math teacher told me about ... when trig would actually play a role in my life. Its funny though cause you end up using it so much like when we were trying to build a ramp and needed to do all the angles for it and stuff like that. We ended up not using it but it is some really good stuff to look up and know about if your going to be building stuff or doing anything. ~Nick

basic trig that i learned in 8th grade is the stuff i use every week but the stuff in trig class i used on the acts and thats been about it

Here is a link to an awesome lighting program that I found. It is simple and easy to use. http://www.paul-pelletier.com/