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Discussion Starter #1
The question is:

How much light do i get back in the end?

Ok, let's start with the theory....according to wbassett:

''gain is the ratio of brightness of your projection screen material to a white standard such as barium sulfate or magnesium carbonate. These materials are used by the industry to set a flat white “Lambertian” light distribution where every point in the audience would see the same image brightness. A gain of 1 represents a screen as bright as magnesium carbonate. A gain of 1.1 means it is 10% brighter and a gain of 2 means a screen is twice as bright.''

http://www.hometheatershack.com/forums/general-screen-discussion/5977-gain-other-confusing-topics.html


wbassett is very very wright in the definition, but the question is another one:

How much light do i get back?
Have all different materials the same gain?
Have they all the same reflection properties?


Let's make an example:

I have a board of melamine and a board of mdf painted with the same exact primer and with the same exact paint mix. Will i measure the same amount of light off the screen in both cases?


What do you think?:)
 

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Let's make an example:

I have a board of melamine and a board of mdf painted with the same exact primer and with the same exact paint mix. Will i measure the same amount of light off the screen in both cases?


What do you think?:)
It will depend on the primer and paint mix used. Some primers have more gloss than others and some paints and paint mixes are more translucent than others. The glossier the surface painted and the more translucent the paint the greater chance you will have of the gloss under the paint showing through.

It should be noted that having any visible gloss from either the substrate being painted or the primer under the paint showing through is a BAD thing. It will soften and blur the reflected screen image which with today's high resolution projectors is not desirable.
 

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Discussion Starter #5
Don, i made just a test. I primed with the same primer, only once, a melamine board and a sanded mdf board. Then i paint them twice using a roller with the same paint mix.

The outcome was a bit of a surprise for me because i measured 18% more light from the melamine than the mdf. The mdf board and my plasterboard (my actual screen) gave me the exact same values. All three of them were primed and painted with the same primer and mix.
 

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Don, i made just a test. I primed with the same primer, only once, a melamine board and a sanded mdf board. Then i paint them twice using a roller with the same paint mix.

The outcome was a bit of a surprise for me because i measured 18% more light from the melamine than the mdf. The mdf board and my plasterboard (my actual screen) gave me the exact same values. All three of them were primed and painted with the same primer and mix.
That is surprising. What paint mix did you use? We have found that the regular interior latex house paints sold here in the U.S. are quite opaque unless thinned with a lot of some kind of clear medium. You might want to check out Mirror Experiments in the Developers forum.
 

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I believe those readings are foot Lamberts since they are measuring reflected light and not incident light.

I remember that Mech took readings from his commercial screen samples to verify that our gain measuring protocol was giving the same results as respected commercial companies. It took a lot of internet searching to find the correct procedure in detail!
 

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I believe those readings are foot Lamberts since they are measuring reflected light and not incident light.
You 're probably wright, but since we don't know the exact amount of the incident light that was hitting the board at that time but only the intensity of the source (1000,500,250 W), we can't know the real ratio of reflection (not the gain).
 

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You 're probably wright, but since we don't know the exact amount of the incident light that was hitting the board at that time but only the intensity of the source (1000,500,250 W), we can't know the real ratio of reflection (not the gain).
Actually, the magnesium carbonate reflectance should be so close the the incident light striking it that only a very precise (and expensive) light meter could tell the minute difference.
 
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