How to work out a latern angle size

[charl.ie]

Perhaps some diagrams will be useful......

 

I'll make some as soon as I get home.

[stuartsl]

ok thanks JSB understand it now, does it do all it measurments in feet?? so is the answer x in feet yeh?

[paulears]

If maths isn't for you, then LD calculator isn't anywhere near as difficult, and can do exactly what you want. All you need to do is go to the photometrics module and decide what you want to know from the little plan. so you need to know the height of the bar, the angle up from vertical to where it points a the position you want, and then the beam width (in degrees). Assuming you enter details in feet, or metric, and don't mix them, it will work out the size of the oval where it lands on the stage.

 

Basic geometry just needs some basic maths - but I too would agree that the ld calculator is pretty useful,

[DaveP]

Or maybe a bit simpler ...

 

http://design.cablepick.com/ and use the 'Photometric Visualizer'

[Suzette]

Or try this one Angle Size Calculator

Enter the distance and the beam angle and it will give you the size on stage - so a 50 deg S4 at 5m gives a size of 4.66m on stage.

I'm not sure how absolutely accurate it is but presumably you don't need it the final millimetre?

[Wuddy]

so if I had a lighting bar say 5 metres from the stage and the point that I wanted to light, and I was using a source 4 with a beam angle of 50 degree what would the light spread be on the stage? and what the easy (understandable way) of workin this out,

 

There seems to be an anomoly in the calculations for the above problem.

 

My calculation gives a beam dia of approx 6 Mtr.

 

The beam calc software gives a beam dia of 4.66 Mtr

 

I have always used the formulea {tan(beam angle)} x throw distance = beam dia, for a ball park guide

 

So I've just done some calcs against published data for Strand Cantata 11/26 and the new Selecon PC 4^o

 

Results.

 

Cantata 11

tan11 x 10Mtr

0.19 x 10 = 1.9Mtr published data says 1.9Mtr

 

tan11 x 30Mtr

0.19 x 30 = 5.8Mtr published data says 5.8Mtr

 

Cantata 26

tan26 x 10Mtr

0.45 x 10 = 4.9Mtr published data says 4.6Mtr

 

tan26 x 30Mtr

0.45 x 30 = 14.7Mtr published data says 13.8Mtr, there's an error creeping in here but its at 30Mtr, who's using a Cantata at 30Mtrs

 

Selecon

tan4 x 12Mtr

0.07 x 12 = .84Mtr published data says .84Mtr

 

tan4 x 16Mtr

0.07 x 16 = 1.12Mtr published data says 1.12Mtr

 

Does anyone have published data for a 50^o lantern to do a comparison

[smalljoshua]

According to the Stage Electrics Catalogue

 

50^o

 

05m = 4.7m

10m = 9.3m

20m = 18.7m

30m = 28m

 

Josh

[Tomo]

Given that you're talking about the Source Four, have a look at the Source Four Minisite Photometrix

 

This has photometry diagrams and built-in calculators for the most commonly-used fixed angles.

 

The diagrams might help you visualise how it works.

[Wuddy]

So looking at this, using my rule of thumb method an increasing error appears as the angle increases.

 

I'd never realised, having only used the method for short throw narrow beams for gobo projection and short range specials

 

Ah well, you learn something every day

[paulears]

The simple calculation measures beam diameter at 90 degrees to the direction of the beam - i.e. a circle. The LD software will give you a beam width, or the oval height as it falls onto the stage - does this account for the discrepancies?

[Wuddy]

The simple calculation measures beam diameter at 90 degrees to the direction of the beam - i.e. a circle. The LD software will give you a beam width, or the oval height as it falls onto the stage - does this account for the discrepancies?

 

No, I think it is just one of those wierd mathmatical things, at narrow angles my calc. works as you get above 30^o an increasing error occurs

[LeeStoddart]

Wuddy, sorry but your maths is faulty.

 

In an earlier posting you simplified

 

Diameter = distance * Tan(beam_angle/2) * 2

to

Diameter = distance * Tan(beam_angle)

 

That is mathematically incorrect - the TAN function is not linear so you can't take the /2 outside the brackets and cancel it against the *2

 

The result of the second calculation is the length of the "opposite" side of a right angle triangle, there the "adjacent" side is distance (Tan = Opposite over Adjacent)

 

The beam diameter is calculated based on an isosceles triangle of height distance by splitting it into two right angle triangles.

 

 

The fact is that at small angles the results are close but as the beam angle increases the discrepancy becomes more marked.

[Wuddy]

Wuddy, sorry but your maths is faulty.

 

Probably, when you get to my age a lot of things are faulty.

 

Yes, I saw Joshs formula and just interprited it to be what I was using. I must have got it wrong origonaly or realised a long time ago that for the beam angles I was using and the throw distances that tan(angle) x throw gave the correct result.

 

I did know that I wasn't using the isosceles split into two right angle triangles,

 

If you check my calcs in my previous post you will see that for the narrow angles the results are correct.

 

It wasn't until I joined this thread and extrapulated my formula to larger angles that I realised I was incorrect.

 

Lets put it this way for what I have required I have been near enough, might just have had to tweak the iris a little.

 

And I don't know about your experiences but you can have a pool of light 3Mtr dia and still the actor stands in the dark corner

[Don Allen]

What is also relevant when working our where to place your lights is peaked or flat beams.

 

The beam angle is the 50% power point and the field angle is the 10% power point. Beam angle is used in lighting design so if two adjacent lights have their beams overlapping at the 50% points, then you have 100% of light, (50% from each light) so you end up with a dipless focus point. As you move closer to the centre of one profile, its intensity increases and the other profiles intensity decreases so you still get 100% total. This is if the profile have a peaked beam. If the profile has a flat beam focus for gobo use, then you will get hot spots across the stage.

 

So if you grab a profile without checking its lamp focus and put it in a rig, you could end up with uneven light distribution.

 

The 10% field angle lets you know how wide the light output will be, but is too low an intensity to be usefull, but bright enough to be an untidy focus if it lights up the proscenium arch etc as an undesirable effect.

 

Try the Selecon Lighting Calculator as it gives peak and flat for profiles and flood and spot for fresnels, to see the different light levels.

 

Also have a look through the Selecon Lighting Support section of their web site.