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tschaeikaei
Joined: 08 Apr 2013
Posts: 490
Location: Germany/Saarland
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| Posted: Sun Sep 14, 2014 11:23 am Post subject: Measuring Gain |
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Hi guys,
as i am currently trying different paints on small boards (i'm building a hard torus).
Would it make sense to project a picture on a white wall / piece of paper (something known to have gain 1)...,
measure the light reflected back and then measure some of my samples and measure again?
Divide the second mesured lumens through the first number and that's the gain factor?
Is it that easy?
Regard, Julian
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Marquee 9500U edgeblend P43 | NEC 9PG
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redfox001
Joined: 16 Mar 2009
Posts: 2257
Location: The Netherlands
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| Posted: Sun Sep 14, 2014 1:05 pm Post subject: |
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Use a paint with titanium dioxide white. That has the right color temperature. The gain will be close to 1 or even above when the underground is not transparent.
If you go to a art painter supply shop you can get the painters cloth linnen not the caton that has a nice gain as I measured once and the ftl was just a little below gain 1.3 pvc cloth. I use the linnen cloth works very well can be cleaned and is oil resistant. It is very cheap too as painters need a lot of it and are not very rich  Think you can glue it to a background.
He you can even buy just the paint there it is called gesso.
http://www.howcast.com/videos/109102-How-to-Gesso-a-Canvas
http://www.youtube.com/watch?v=qkkaGbFE1rE
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garyfritz
Joined: 08 Apr 2006
Posts: 12088
Location: Fort Collins, CO
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| Posted: Sun Sep 14, 2014 2:39 pm Post subject: Re: Measuring Gain |
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| tschaeikaei wrote: | as i am currently trying different paints on small boards (i'm building a hard torus).
Would it make sense to project a picture on a white wall / piece of paper (something known to have gain 1)...,
measure the light reflected back and then measure some of my samples and measure again?
Divide the second mesured lumens through the first number and that's the gain factor?
Is it that easy? |
Yes, it's that easy. ftL off your paint / ftL off gain-1 = gain of your paint.
The gain-1 gesso redfox01 suggests works fine on a flat screen, but don't use it on a torus. You want higher gain in a torus, usually around 3 or so. That higher reflectivity reflects the light in the direction you want so the torus can concentrate the light at your viewing location.
A gain-1 surface doesn't work with a torus. A gain-1 torus won't be any brighter than a gain-1 flat screen. Not only will you lose the focused light that produces the higher gain you're looking for, the gain-1 surface will disperse the light so much that it will strike the adjacent surfaces of the curved screen. You'd lose a lot of ANSI contrast because bright areas of the screen would wash out darker areas of the screen.
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tschaeikaei
Joined: 08 Apr 2013
Posts: 490
Location: Germany/Saarland
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| Posted: Fri Sep 19, 2014 3:09 pm Post subject: |
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No, no. I won't use any white material as torus paint.
I am just trying to find a gain 1 surface to compare my silver spray to.
My HCFR probe is nearly finished and after that is done, i will try "gesso".
But i don't have an idea how it is called in German. Chalk? Gypsum? CaSO4?
Thank you, Julian
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redfox001
Joined: 16 Mar 2009
Posts: 2257
Location: The Netherlands
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| Posted: Fri Sep 19, 2014 4:23 pm Post subject: |
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I think you can just get a paint with ral9010.
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garyfritz
Joined: 08 Apr 2006
Posts: 12088
Location: Fort Collins, CO
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| Posted: Fri Sep 19, 2014 5:53 pm Post subject: |
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Silver paint is VERY VERY hard to work with. It shows up every tiny imperfection. Pure silver paint also has way too much gain. If you're mixing silver with a white paint, that can be a reasonable way to get good gain. But it's a real research project to figure out what proportion of silver to use.
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