Health and safety issues

[Pete McCrea]

In perfect conditions* it makes no difference the throw, and assuming the same screen size the brightness will remain constant as the available light is being spread over the same area.

 

In the real world, there may be a loss through diffusion due to particles in the air, and the quality of the lens will affect the output, but this can be taken into account with good lens design and manufacturer. We are doing a show next weekend - 40m throw to the screen, and there should be no discernible difference in the output of the units, compared to when we did the job last with a 14m throw. Lenses are £3000 each though which is middle of the road for a projector lens....

 

*Perfect conditions are IIRC from A Level Physics, a Vacuum.

[kitlane]

It may be a small effect but the f value of the lens will make a difference in theory. Assuming you have a different lens for a different throw, the smaller the f number the more light is transmitted.

[ramdram]

Ref the distance from light source to board, perhaps this may be of interest:

 

http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html

 

(applies to other sorts of radiation and forces too, and the sort of stuff you might learn at school anyway...)

 

As for the H&S guidelines and/or legislation, its probably better to take them seriously...I would not want to be up before the Beak attempting to explain why I decided it best to ignore such.

 

The insurance company may even decide that failure to comply with (any) regs was sufficient grounds to dismiss any claims.

 

I take the view that it is by far the most prudent course of action to follow the legislation to the letter, however daft it might appear to others.

 

As in, (from another thread on the forum) I would not want to regain consciousness in hospital, only to hear the Chairman of the theatre where I volunteer say,

 

"...and by the way, Ross, we have decided to earth the lighting bars after all..."

[Seano]

(applies to other sorts of radiation and forces too, and the sort of stuff you might learn at school anyway...)

If you learned about the 'inverse square' law at school, you might also realise it doesn't apply to this situation.

[ramdram]

I'm intrigued, how have "they" managed to overcome the laws of physics? A link would be useful if the explanation is too longwinded for the forum.

[Seano]

I'm intrigued, how have "they" managed to overcome the laws of physics? A link would be useful if the explanation is too longwinded for the forum.

Noone has "overcome the laws of physics", you've just misunderstood them.

Your own link will do fine. The first sentence reads:

"Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law." (My italics)

[kerry davies]

Ram, what Pete and Seano are saying is that for a set image size, inverse square does not apply whatever the throw length.

Same size screen, varying throw, varying lens angles = same value for 'I'.

 

The guidance for "trivial" sources such as classroom projectors is easy to follow...."Don't look into the light!"..."Safe!" No one except a jobsworth with no understanding of H&S needs be concerned about applying the guidance. The issue here is that some firms have seen a marketing tool for selling their products, and good luck to them, it's called capitalism.

 

crosspost with Seano

[Shez]

In perfect conditions* it makes no difference the throw, and assuming the same screen size the brightness will remain constant as the available light is being spread over the same area.

 

In the real world, there may be a loss through diffusion due to particles in the air, and the quality of the lens will affect the output, but this can be taken into account with good lens design and manufacturer.

Perhaps worth bearing in mind that most ultra short throw projectors bounce the light off one or more mirrors in order to achieve that throw which will probably have a similar effect to the loss via a longer throw through more air. So the difference will end up being pretty negligible.

[ramdram]

We may be talking at cross purposes. I believe what "you" are saying is that owing to the light energy, if you will, being focussed into a narrower beam, the lamp will still provide the same amount of light intensity per unit area on a screen of x area further away as on a screen, again of x area, closer to the projector fitted with a wider angle lens.

 

Cue a session on google to get more gen,

 

This link (and subsequent links):

 

http://htrgroup.com/main.php?section=minmax-throwdistance

 

certainly agrees with this to up to a point, but, then goes on to say that the throw distance is limited by the brightness of the light source/lamp, and, the inverse square law.

 

What I was saying was that (regardless of the light source, beam angle) the inverse square law applies to any light source full stop.

 

There is another link, depicting worked examples for calculating light intensities, that may help determine if a given projector/lens combination can project a bright enough image prior to installation.

[madorangepanda]

The inverse square law doesn't matter as we are focusing the beam.

 

Think of it this way, if we move 3metres away we should have a 9th of the brightness. BUT, we would have an area 9 times the size, so the energy at this distance is the same just spread across a greater area.

 

But as we use a lens it allows us to focus the beam onto the desired area.

 

Note:There may be a slight decrease in brightness due to absorption/scattering but this would be negligible in this instance.

[ramdram]

I stand corrected; thanks for the edification/micro lecturette. It seems my education did not cover collimation, ** laughs out loud **.

 

As always a quick (?) google found this (for those who might want to know more):

 

http://www.portraitlighting.net/inversesquare_law.htm

 

However according to the blurb the ISL still applies, but it seems long after the throw distance.

 

So, as you rightly said, sort of, who cares what happens to the light intensity after the target has been illuminated.