50 Lumens

[Charc]

I've read, from multiple sources, multiple times, that 50 lumens is considered to be the lowest level of usable light for stage lighting. I've never obtained a definition for "usable light". Is that quite dim light, but provides a clear view of the subject? Or does that mean something that might be useful for accent or something, but not enough illumination to provide a clear view of the subject? I'm not quite clear on that. And let's say I have 100 lumens of light, but I gel the light with a filter that has a 50% transmission. That means I have 50 lumens on stage, or the lowest amount of "usable" light?

What is the lowest amount of IR light? Somewhere on here was a discussion of using WFL PAR64s, with deeply saturated blue color filter run at like 5-10% as a source for an IR camera. Is this truly enough light, or was this a hypothesis?

[gafftaper]

I don't have the exact answer to your question. You can somewhat accurately think of I.R. as the heat escaping the lamp... yeah there's more to it than that, but a large part of the heat you feel is really IR. Something that only produces 50 lumens is going to be very dim... but it's still generating a lot of heat... and a lot of IR.

Also I believe you should be using some extra dark Red Gel, not Blue. I believe at the high end of the spectrum there is some cross over between Red and IR. Blue Gel would potentially block those "near IR" wave from getting through.

How am I doing? Anybody?

[Charc]

Originally Posted by gafftaper

I don't have the exact answer to your question. You can somewhat accurately think of I.R. as the heat escaping the lamp... yeah there's more to it than that, but a large part of the heat you feel is really IR. Something that only produces 50 lumens is going to be very dim... but it's still generating a lot of heat... and a lot of IR.
Also I believe you should be using some extra dark Red Gel, not Blue. I believe at the high end of the spectrum there is some cross over between Red and IR. Blue Gel would potentially block those "near IR" wave from getting through.
How am I doing? Anybody?

Good call on the red, yes it is closer to IR. I think the idea though was that color filters are designed to let as much heat through as possible, so as not to burn out. This means that the difference between deeply saturated blue and deeply saturated red would be negligible, so since blue (I would think) would be harder for the audience to pick out, you can run it at a higher intensity, getting more IR light, without having the light noticed by the audience. Though all of the preceding was a guess.

[gafftaper]

Somewhere in the thread I started a couple months back about a night vision camera, I think I remember someone saying they ran a PAR with double red gel at about 5% or 10% and that was all the light they needed for their IR camera.

[gafftaper]

Another more expensive option is to get a "cold mirror". A dichroic filter which reflects visible light and allows IR to pass through. I looked into those and if I remember right they run in the $50-$100 range.

[Footer]

Originally Posted by gafftaper

Another more expensive option is to get a "cold mirror". A dichroic filter which reflects visible light and allows IR to pass through. I looked into those and if I remember right they run in the $50-$100 range.

Also called "woods glass". If you get enough of them, they give a pretty good black light affect, if you have just one full stage, you might get spikes to glow a bit, but nothing insane.

[Charc]

Originally Posted by Footer4321

Also called "woods glass". If you get enough of them, they give a pretty good black light affect, if you have just one full stage, you might get spikes to glow a bit, but nothing insane.

I thought blacklight effects came from UV light?

[gafftaper]

Originally Posted by charcoaldabs

I thought blacklight effects came from UV light?

Yeah Footer wrong end of the spectrum buddy... get some rest. "Wood's glass" is a filter that allows UV to pass while blocking visible light. While a "cold mirror" allows IR to pass while blocking visible light (it's what they use in Selecons to cool the light down). But as he said there's a big difference. It takes a TON of incandescent light source to generate a small amount of UV. Where it only takes a small incandescent source to create a lot of IR. As we've all heard lately in the news, Incandescence is really inefficient and ton's of excess heat is wasted in the production of light. Gaseous discharge lamps on the other hand are much more efficient and are sort of tuned to a specific wave length. So it's easy to produce a ton of UV with a discharge lamp while it's harder to produce IR.

[Charc]

Originally Posted by gafftaper

Yeah Footer wrong end of the spectrum buddy... get some rest. "Wood's glass" is a filter that allows UV to pass while blocking visible light. While a "cold mirror" allows IR to pass while blocking visible light (it's what they use in Selecons to cool the light down). But as he said there's a big difference. It takes a TON of incandescent light source to generate a small amount of UV. Where it only takes a small incandescent source to create a lot of IR. As we've all heard lately in the news, Incandescence is really inefficient and ton's of excess heat is wasted in the production of light. Gaseous discharge lamps on the other hand are much more efficient and are sort of tuned to a specific wave length. So it's easy to produce a ton of UV with a discharge lamp while it's harder to produce IR.

My understanding is that the "cold mirror" technology is in source fours as well.

[gafftaper]

Originally Posted by charcoaldabs

My understanding is that the "cold mirror" technology is in source fours as well.

Yes the reflector in a Source4 is a cold mirror. It reflects visible light forward into the gate while allowing IR to pass through the reflector and into the body of the S4. However the 30% or so of the light moving directly into the gate without hitting the reflector does not get filtered so quite a bit of heat is retainted. That's one of the things that makes a Selecon so COOL (pun intended). Selecon's are banana shaped with the lamp and reflector pointing up. At about the point where the gate would be on a S4, it hits the cold mirror which is sitting on a 45 degree angle. The IR passes straight out the top into a heat sink while visible light is reflected forward into the lens tube. It makes them run much cooler up front.