how to measure screen gain?

Discussion in 'Displays' started by trey-m, May 19, 2005.

  1. trey-m

    trey-m Stunt Coordinator

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    How could one go about measuring the gain of their own screen (or painted wall)?
     
  2. John Whittle

    John Whittle Stunt Coordinator

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    If you want to do it yourself, you're going to need a foot-lambert meter. Then you take a know gain screen (usually a flat/matt white) and put a light on it and measure the foot lamberts (btw the SMPTE standard for screen brightness in a theatre is 16 foot-lamberts measured open gate from the projector with the shutter running and measured at the center and all four corners with a drop off of 1-2 ft-lamberts).

    Then you insert your test material and measure the amount of light reflected from it and by comparing the two you can wind up the gain.

    Foot-lambert meters are necessary for motion picture screens since they integrate the dark and light (from the shutter) to get the necessary reading. In your case for comparison you can probably use a good reflective light meter. An incident meter reads the light falling on it, the reflective meter reads the light bouncing off the surface.

    It takes a lot of light to make a big difference. Getting from a gain of one to a gain of two requires 100 percent more light and that's just one f/stop (going from f/5.6 to f/8 for example).

    You can do more to increase screen brightness by killing all ambient illumination in a room (as long as the fire department doesn't require exit signs) since it's the ambient light that really kills alot of contrast and brightness on a screen by "washing" light across it.

    You best bet (if you're also considering painting the screen) is to get a small sample of a couple of screen material (maybe a square foot each) and tack them up on the wall and just do a visual inspection in your own environment as to the difference between them and go with the manfacturer's specs. You can compare that to you paint by painting a equal square foot of masonite and using it in the test.

    John
     
  3. trey-m

    trey-m Stunt Coordinator

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    Thanks John for the detailed explanation. Very informative. In light of the price of the tools required (pun intended), I think I'll leave measuring the screen gain to the professionals.

    I like your suggestion for comparing screens/paints. I'll try to implement that before I make my selection (even if just comparing two different paints).

    And I'm going to take my exit signs down. [​IMG]
     

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