Followspots through glass

[sionpanto@hotmail.com]

In my venue we use an upstairs room for all aspects of lighting control and are in the process of looking into widening the opening that we can see through. This may sound daft but I wondered if any knew of any types of glass for a window that would not alter the effect of a follow spot through it by refracting the light etc, I know it may be a long shot but ideally I want the area closed and not have a 6ft by 2ft hole open into the auditorium as I use comms quite a lot through the show to direct stage crew.

[gareth]

Bog-standard glass is fine. We regularly position two followspots in the sound control room (with an open window) and an observation room (double-glazed window) - there's no appreciable difference to the output.

[Big Jay]

Alot of West End have there followspots in Spot Rooms at the back of the Gods, and as Gareth said, normal glass wouldnt cause any problems, after all the light travels through a lens which is glass, is it not?

[Ynot]

Oddly enough we went the opposite way when we created our side rear spot ports - making sure they both had a clear shot at stage with no glass (prior to this they were behind thick single-glazed windows. My reasoning behind this was that we had a VERY bright reflected image on the back wall of the booth coming back off the window, and it always struck me that the laws of physics dictate that this reflected (not refracted) light must be deducted from the overall output, thus being a bad thing...

 

That said, I do know and have worked in venues which do house their main spots behind glass...

[back_ache]

we had a VERY bright reflected image on the back wall of the booth coming back off the window, and it always struck me that the laws of physics dictate that this reflected (not refracted) light must be deducted from the overall output, thus being a bad thing

 

I am reading a book on quantum physics at the moment that quotes loss through reflection at five percent, whilst I am sure that must change with different types of glass at least that may give you a rule of thumb.

 

Cinemas always project through glass, maybe their websites maybe a source of info on the subject, if you do find out more it could be an interesting thing to add to the wiki

[Jivemaster]

Conventionaly the projection room is glazed perpendicular to the projector beam not upright. A projection room is glazed as a hang on from the legislation relating to nitrate film stock, and to keep motor noise down.

 

Tilt the glass to an angle more perpendicular to the typical follow spot beam depression. Maybe have the window openable so that both sides can be kept clean.

[JimWebber]

I was invloved in a production a few years back, where the followspots were used through windows, and although the light level was not appreciably attenuated, (we were using powerful (2k) in a very small venue) The heat did cause the glass in the window to crack.

 

I was not particularly experienced back then, (Am I now?????) But one or more of the following may have been an issue that I was not aware about.

 

1. The glass was that wired strengthened type - Would this have had an effect?

2. Was the alignment of the lamp perfect? - I do not know

3. Was the glass already chipped, scratched or otherwise flawed?

 

So although the consensus of opinions is that yes, it does work, I did have the window crack (Not break or shatter) but that may have been something else.

 

HTH

 

Jim

[adam2]

All common types of clear glass reflect a few percent of light, and absorb a few percent.

Therefore a followspot or other light directed through glass will lose from about 3 to about 6 percent of the light output, this is not normally noticable.

With an incandescent source, a change of just 2 percent in the mains voltage would produce a greater variation in light output, and no one worries about that.

 

That said, I would still try and avoid directing a followspot through glass for the following reasons.

 

1) the loss will be greater at an acute angle

2) the light reflected into the room, although only a few percent, is still bright enough to be very distracting or unpleasant.

3) unless very well ventilated, the room is liable to get very hot

4) the glass might shatter. Glass is partialy opaque to infra red light,which is absorbed by the glass, which may get hot enough to break.

 

If directing a high powered light source through glass is unavoidable, then I suggest the use of wired glass. This is just as liable to crack as ordinary window glass if overheated but is much safer since it should only crack and not shatter.

The wire mesh should hold the pieces together and much reduce the risk of shards of glass falling and injuring persons.

[Jivemaster]

Cinema projection portholes are never wired or textured glass in fact they spend LOTS of money on specially polished flat glass plates for minnimum beam attenuation (lost light) and minimum diffraction (lost focus). The panels are in mounts with expansion joints to allow the glass to expand without cracking. To prevent ghost imaging the glass is held perpendicular to the projection axix.

[Andrew C]

Conventionaly the projection room is glazed perpendicular to the projector beam not upright.

This indicates one of the reasons I dislike architects. Knew better than me... Big bright relfection of the beam in the control room. :)

(we were using powerful (2k) in a very small venue) The heat did cause the glass in the window to crack.

Got this T shirt too. We think it is caused by the window being at the focal point of IR radiation, and the wire expanding as it warms. A piece of plain 6mm doesn't crack with 2kW incandescent lamps, and the new 400W discharge is fine with the wired glass.

 

The wired glass is a requirement at the c room is in a separate fire compartment to the auditorium. Opening window for sound? See my first point!

[sleah]

Conventionaly the projection room is glazed perpendicular to the projector beam not upright. A projection room is glazed as a hang on from the legislation relating to nitrate film stock, and to keep motor noise down.

 

Tilt the glass to an angle more perpendicular to the typical follow spot beam depression. Maybe have the window openable so that both sides can be kept clean.

 

I was going to mention this. It's also how glass is set in a radio studio, albeit for different reasons. It means the reflection isn't coming straight back at you :)

 

I would assume that you would have to use armoured/toughened glass, so that may be an alternative to the wired type, may well be better at coping with heat too.

I'm no glazing expert at all, but thinging of how the laws of physics apply, certainly regarding reflections but also heat. If the glass is tight in the frame then it will have no room for expansion, hence crack. So it would make sense that it is 'suspended' in whatever sealent it is that's used for glass (Silicone???) to allow room for expansion.

 

I may be talking rubbish, but the science makes sense :)

[Andrew C]

We did try a loose piece of wired glass in it's frame. It still cracked.

 

As I understand it, (last time we asked) there was no alternative to wired glass. The fire regulations required it. Technology may have moved on, and the regulations may have caught up.

[son of lx dad]

There are quite a few large scale touring theatre venues around the country with their follows behind glass, Milton Keynes, Manchester Opera House and Edinburgh Playhouse house spring to mind, if memory serves.

[sionpanto@hotmail.com]

Thanks for the help guys, I will look into the heat thing with the builder and glazer tomorrow. I think we will probably go down this route then as it is the option I most prefer.

[gareth]

Don't worry about the heat thing, it's a non-issue. The two pieces of glass in the window of our observation room have been getting follow-spots pointed through them for the last 33 years, and there's not so much as a crack in them.