Lighting Q.O.T.W. (10/30/09)

[icewolf08]

So, I am going to try to start something new that the CB staff has been discussing. You all know that we have a "Question of the Day" forum, but we thought that we could stimulate some more discussions in the specific forums with a QOTD or in this case a QOTW "Question of the Week" (because everyone needs time to think about it and answer). So we will do our best to get at least one question up every week, and if you have suggestions for QOTW, please PM them to one of the Mods ([user]icewolf08[/user] or [user]grog12[/user]) and we just might ask them!


So I am going to kick this off with a question that I don't actually have an answer to, but have been wondering about for a while:

Our followspots are located in one of our FOH lighting positions (we call it Beam 1). The beam has a safety grating so that you can't fall out. When we run our followspots at a dim intensity you can see shadows of the grate on the stage. Why?

Additional details: The spots are Robert Juliat Topaze 1200W arc discharge fixtures. The front end of the fixture is probably around 1' from the grate at the closest. The throw distance to the front edge of the stage is about 45'. Also, I have never noticed this effect with any of our conventional fixtures (including 10˚-26˚ Source fours and 6x16 360Qs).

 

[fredthe]

My guess would be that when you "dim" the followspot, you are actually reducing the area of light from the constant intensity light source... resulting in less scattering, and thys better able to cast shadows from the close-in safety grid. (like a donut giving better definition to a gobo.)

I'm sure there's a more technical explanation, but it's late...

-Fred

 

[JChenault]

I'm not familiar with the fixture. When you run it at dim intensity, what is really happening inside of the fixture.

Is there a mechanical shutter that is closing in from all sides?
Is there a round douser that you are moving in from the edge to the center of the beam?
Something else?

I assume that at 'bright' you do not see a shadow.

 

[icewolf08]

I'm not familiar with the fixture. When you run it at dim intensity, what is really happening inside of the fixture.

Is there a mechanical shutter that is closing in from all sides?
Is there a round douser that you are moving in from the edge to the center of the beam?
Something else?

I assume that at 'bright' you do not see a shadow.

Here is an exploded diagram of the dowser. Much better than me trying to put it in words.

 

[Les]

That is very similar to the dimmer found in the Altman Comet.

My guess is that the more you 'dim' the fixture, the more the stray light is actually being reflected off of the dimmer mechanism and being redirected somewhere else - perhaps through some ventilation slots in the bottom of the fixture. This doesn't happen with the conventionals because in their case, the light source is actually becoming dimmer whereas in the followspot, the light source is maintaining a constant intensity though it is being blocked and reflected by the shutters. It has to go somewhere and in the case of your followspots, the light goes through the bottom and down through the gridiron thus causing shadows on stage. Again, only a guess

 

[icewolf08]

That is very similar to the dimmer found in the Altman Comet.

My guess is that the more you 'dim' the fixture, the more the stray light is actually being reflected off of the dimmer mechanism and being redirected somewhere else - perhaps through some ventilation slots in the bottom of the fixture. This doesn't happen with the conventionals because in their case, the light source is actually becoming dimmer whereas in the followspot, the light source is maintaining a constant intensity though it is being blocked and reflected by the shutters. It has to go somewhere and in the case of your followspots, the light goes through the bottom and down through the gridiron thus causing shadows on stage. Again, only a guess

To get the effect you describe the light would have to be reflected towards the stage, which would defeat the purpose of the dowser. Also, all of the vents on the fixture are baffled so that there is only a minimum of light leak. There are also no vents on the fixture around where the dowser is.

 

[JChenault]

I haven't seen a reply so I will take a shot.

My belief is that the dowser that dims the light is actually narrowing the beam as it closes down. This means that the light is coming from a smaller source and acting more like a point source. If the point source is small enough you would get the effect you describe.

You could test this by putting some smoke in the air, or holding a sheet of frost in front of the unit - taking it down to 'shadow intensity' and seeing what happens to the beam size. If you test it let us know what you see.

Units that dim without a dowser would not change the size of the beam so they would not cast a shadow.

 

[rmarston]

Here's a guess - The RJ followspot has a more parallel light beam due to its longer lens train, so its focal point is much longer than that of an S4 so the safety grate located in front of the spot becomes an out of focus template and becomes more definable on stage when the amount of light leaving the spot is reduced. - What do ya think?

 

[Kelite]

Here's a guess - The RJ followspot has a more parallel light beam due to its longer lens train, so its focal point is much longer than that of an S4 so the safety grate located in front of the spot becomes an out of focus template and becomes more definable on stage when the amount of light leaving the spot is reduced. - What do ya think?

After seeing fog in front of a 26* S4 and watching the beam, I would say you're on to something...

 

[xander]

A possible experiment would be to use some neutral density in front of it to dim to the point you are talking about. You could then see if it has to do with the mechanism of dowsing or the optics or maybe something to do with how our eyes observe it? Intriguing...

Tim

 

[photoatdv]

Experimentty time I think:

Set it to the intensity in question and have an assistant hold their hand in front of the spot and find the focal point in front of the fixture (if it's not out of reach-- if so point it to the side or something), where their hand shows up like a gobo or bunny ears in front of a flashlight . Now open it all the way. Is the focal spot in the same place?

I think you're moving the focal spot by changing the size of the source. Somehow... scratching head. Might be time for my flashlight and some props...

 

[dramatech]

The proposal by fredthe in Post #2 and JChenault in post # 7, are basically saying the same thing, although in a slightly different way, and I think is the answer.

 

[PeteEngel]

The shutter (douser) in the Topaze is a 4 blade, in plane shutter. it removes light equally from the pool of light without reducing the size of the beam. one effect of this is that it kind of acts like an F-Stop on a camera, so at the distance described (close to the lens) and at a low intensity, it is shifting the focal point a bit, thus drawing the screen into focus on the deck.

 

[PeteEngel]

also, you wouldn't see this with a S4 or 360Q since the tungsten lamp dimmed by a dimmer acts differently from an arc source with a mechanical shutter.

 

[J0rdster]

My guess would be that when you "dim" the followspot, you are actually reducing the area of light from the constant intensity light source... resulting in less scattering, and thys better able to cast shadows from the close-in safety grid. (like a donut giving better definition to a gobo.)

I'm sure there's a more technical explanation, but it's late...

-Fred

Honestly, I think it's a simpler matter, as Fred described. Because the grate in those beams is thicker than say, chicken wire, it blocks/absorbs more light. The optics and reflector on a RJ aren't as advanced as those in a Source Four, with a reflector and optics that tend to capture the "scatter."

OR, I think it might have something to do with the type of lamp and how it burns at lower intensities.