Film-Tech Forum ARCHIVE: Relative screen "Gain"

This is topic Relative screen "Gain" in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Lindsay Morris (Member # 1430) on 07-31-2013, 03:56 AM:

Along with other issues of digital conversion the so called screen gain has been queried.

I sort of wonder just how one can compare a flat solid white (VERY white) screen compared with the sound screens in most cinemas in use now.
The sound screen being perforated lets a fair bit of light pass through and the ones I have been behind I would guess that about 20% passes through.
So how would a flat white solid screen stack up against a flat white perforated screen???.
I can appreciate that the silver types appear brighter square on but fall off considerably to the sides and having worked years ago at a cinema that had a silver screen one could certainly see the difference as you moved about the auditorium.
I am tending to go for a figure of 1 for the solid screen. Would that be somewhat in the ballpark.

Posted by Carsten Kurz (Member # 5396) on 08-04-2013, 04:51 PM:

What about the issue of L/C/R speakers behind a perforated screen if you have the choice?

TechSpecs of perforated screens quote around 6%-8% surface 'loss' for standard perforation, so light loss should be in the same ballpark. That's not much, and you can easily compensate for it by using a light 'gain' screen. I would prefer a perforated medium gain screen (1.4-1.6) , depending on your auditorium seating/viewing angles - we are still talking about your outdoor location, I guess.

- Carsten

Posted by Dave Macaulay (Member # 813) on 08-04-2013, 07:13 PM:

Generally a block of solid magnesium carbonate is used as a 100% reflective reference matte surface. Put a light on screen, measure with a spot meter ponted to avoid perfs if possible, place the block where the spot meter is aimed and read again: divide the brightness reading of the screen from the reading from the MgCO3 block and multiply by 100 = basic screen gain.
Measuring a gain screen is not as easy, you need to measure at various angles from the source axis.
The loss from perforations is simple, you just subtract the percentage of screen area in the perforations. A 100% reflective screen with 10% perforation area will be 90% reflective.

Posted by Lindsay Morris (Member # 1430) on 08-04-2013, 10:45 PM:

Carsten,

Most definitely the outdoor situation with existing screen as there is NO WAY a conventional screen can be held taught enough in the winds experienced at the location to maintain acceptable focus.
So speakers behind are not even a consideration.
Currently a BIG multi-cellular horn (12 throat) on top pointing down into the audience with bass and sub woofer bin underneath the screen & column arrays on the L & R edges (10M apart). The columns are above audience heads by around 1-2m and spray out a broad horizontal beam into them with the columns slightly angled inwards.Sound overall is quite good despite the layout. But outdoors it is a whole different ball game than inside a box.

What prompted the question is trying to sort out what we might need for light from a digital source and also sort out lens sets suitable for throw and in all that bobs up the question of screen gain.
So going by what Dave has just explained I reckon that by guesstimate I will run with a gain of 1 and see how that all pans out.
Currently run a Century CC with a Lumex yellow lamp house behind it running 65A on the xenon and current illumination is very bright...so bright at times on very light prints I tend to drop the amps back to 60A to reduce flicker.
Never ever of course have actually attempted to measure the FL on the screen but currently on 35mm have plenty of light.
Thanks for info.

Lindsay

Posted by Carsten Kurz (Member # 5396) on 08-05-2013, 04:03 AM:

There are a number of threads here regarding screen paints for drive-in screens. I guess some of them reach slightly higher gains than 1.0. Picture-quality wise, around 1.0 is always preferred.

What screen size was it again, and what projector are you aiming at?

- Carsten

Posted by Stephen Furley (Member # 1277) on 08-05-2013, 04:18 AM:

quote: Lindsay Morris
The sound screen being perforated lets a fair bit of light pass through and the ones I have been behind I would guess that about 20% passes through.

Are you talking about just the light passing through the perforations, or the total, including that passing through the screen fabric as well? If the former then 20% sounds too high; the perforations cover about 5% of the screen area. If the latter then it will depend on the screen fabric used.

Posted by Lindsay Morris (Member # 1430) on 08-05-2013, 10:17 PM:

Stephen,

It has been many years since I stood behind a sound screen with just white light playing upon it to be able to observe what passed through and illuminated the speakers I was going to work on.
So it was just a big guess but non-the-less it was quite well lit back there on the stage.
More than enough to get to the speakers and check that all plugs were seated home OK.

Initially I used to paint the 10M wide screen with simple flat white or if I could get it low sheen ceiling paint that was washable.
But after a couple of seasons standing there it had a decided yellow looking cast and recently skinned it back to almost bare metal and gave it 2 coats of membrane paint that is used in big quantities here in West Australia to paint the wheat bins scattered all over the inland west coast. It is highly flexible covers very well, sticks like you know what and is brilliant white..really brilliant white. [thumbsup]

Now after 3 winters it still looks VG, has NO mouldy bits like the past and only in a few spots can I see any cracking of the sheet joins. Has lasted far better than anything else I have ever used.

So when I ventured into looking at either a Christie Solaria One + OR A NEC 900C for possible conversion and copped questions re the screen "gain" and because I had no idea on what it would be as soon as I informed the quoter of its structure it was decreed to be 0.8 or less... [Eek!]

That did NOT sit well for me as the bod had never ever set eyes upon it and no matter what I said after that... 0.8 it was.
Hence the question here and I would not the least surprised if it was ever measured it came up at better than 1.
Then when you start plugging in that sort of gain into the spreadsheets to determine what machine, lamp type or lens available the differences between 0.8 and 1 are quite remarkable.

Need to get some sense out of these installers over this coming summer or that will be the end...as film winds up in OZ so will we.
Lindsay

Posted by Lindsay Morris (Member # 1430) on 08-12-2013, 05:09 AM:

Antti,

Thank you for that link re woven screens etc in Topic
"Screens for 2D".

In all that was some very enlightening info re solid screens..exactly what I was after.

Lindsay