Jump to content

Beam Angles


Recommended Posts

Hi guys,


Very basic question to you guys but something which has been getting me thinking over the past few days. After googling and Blue Room wiki'ing, I'm not much wiser.


Beam angles. How do you guys work out what beam angle you are going to need? For example how do you go about working out that you are "going to need a source four 36 degree". Also, where is this 36 degree measured (if that makes sense).


So if you are working out a plot for a show, you know the distance of bar from the stage, the height how do you go about working out your angles?


I guess the other question I am also asking, is what exactly is the angle (I know it is how wide/narrow the beem is) but where is it measured from.



Hope that makes sense and thanks for your help! I haven't been in a situation where this has mattered you see (at school it's just a case of pointing a flood or focussing a fresnel, no high-tec source fours or anything!









Link to comment
Share on other sites

This is where all the trigonometry you learnt comes in handy! This is one of the reasons real plans are still popular. You simply stick a protractor on the symbol and then read off the angle. 36 degrees in your example should be taken as the maximum, so if you want to do tha maths then you need to know the throw to the subject and the width the beam needs to be. Half of this beam diameter is the opposite angle in the calculation, with the adjacent being the throw - bung in the tangent and multiply by two gives your beam angle. You just need to make sure the fixture you select is capable of being a bit wider. If maths is a bit grim, then a scale drawing and protractor is probably easier and you can use the drawing for all sorts of things. If you use CAD in the drawing or visualisation styles it does this for you. The thing to remember with beam angles is that the spec is what you expect it to be. For profiles the angles are usually pretty obvious because of the natural hard edges, but with Fresnels, the soft edge confuses things. At what point do you consider the beam to be gone? When it drops 25%, 50% or is gone? Most people hover around half-gone, but check the specs. Some manufacturers just state the angle.


So the beam is measured at the lantern. One point when working out angles. Unless the beam hits the surface or subject at a right angle, you will have an ellipse, not a circle - so you need to consider width and height when you do the maths or drawings.



Link to comment
Share on other sites

This is why Tom, plans are drawn in "section" view as well as "plan view".


For example, if in a venue you have your stage at one end - and there might be a thrust on it, or a cat walk etc etc going on the "traditional theory" of hanging lights at a 45 deg. angle -using a section plan you are able to see how many FOH trusses you need to hang to ensure coverage from furthest DS up to the point of No 1 bar postion.


In most pros arch theatres, you will normally only need one or two (aside from side perches , dress circle bars etc) but in an arena or hotel venue where usually there is no fixed stage, then a section plan is vital to see how far the beam is going to cover.


The same for a "plan" drwaing as well. You can draw the beam angles from the symbol to where they have to light - and this can help you determin, depending what size beam angles you use , how many lamps you need on a bar / truss to get an even cover

Link to comment
Share on other sites

Beam Dia = Distance X (2 X Tan (Beam Angle / 2))


That, of course, is quite correct, but it's the mathematician's answer ;)


For most practical purposes (beam angle in degrees * distance / 60) is within a few percent.

Link to comment
Share on other sites


This topic is now archived and is closed to further replies.

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.