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Screen Gain

Discussion in 'Projectors, Screens & Video Processors' started by Supernosh, Aug 4, 2003.

  1. Supernosh

    Supernosh
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    Just a quickie!

    What does screen gain mean and do?

    I have seen screens with gains from 1 - 1.8? What real difference does it make?

    If you went from a 1 gain to a 1.1 would the difference be considerable?

    What is the best gain to go for for a ae100 PJ?

    Thanks!
     
  2. Gary Lightfoot

    Gary Lightfoot
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    Screen gain is the amount of light reflected, and is relative to the reflectivity of magnesium IIRC (somebody correct me please - my mind has gone blank).

    So a screen that reflects back the light it receives is unity gain. The more light reflected back, the brighter the image. Screens with gain greater than unity mean they are more directional, and have a viewing cone.

    Unity screens spread the light in all directions, so you can view from anywhere and see an equaly bright image. The higher gain screens will be less reflective the further outside of the screen are you look at it from - say 45degrees from the ceneter for example.

    Higher reflectivity can also make black look lighter. Most lamp projectors can't produce true blacks anyway, and this bothers some people.

    For your projector it will depend on how big you want the image (bigger means a dimmer image). If you're going for a 6ft or smaller screen, unity gain should be fine (blackout cloth is a cheap alternative to screen material if you want to experiment first), but if you want a bigger screen, then you'll need more gain or the image will be too dark.

    Cinemas aim for between 15 and 30 ft lamberts of reflected light (originaly to reduce flicker from the 24fps that film runs at), and this works in the home too.

    Divide the lumen output of the projector by the surface area of the screen, then multiply by the gain, and you will have how much light is refelcting back, and give you an idea of what kind of screen you would need.

    I.E 7ft wide screen:

    84" x 47.25" = 27.56 sqr feet. Divide that into 800 (IIRC) lumens of the AE100 and you get 29ft lamberts. Factor in bulb wear (they dim after a few hundred hours) and misleading lumen figures by manufacturers by reducing by 30 to 40%. You then end up with around 20ft lamberts at unity. Multiply by 1.2 for example if you want a brighter image with a 1.2 gain scren: 24ft lamberts.

    Grey screens are another option that produce better black level and reject ambient light if you don't have full light control.

    There is a screen forum over at www.avsforums.com that may help some more.

    Gary.

    HTH
     
  3. ReTrO

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    Exactly right Gary. It's a ratio to the light reflected off a block of cut magnesium carbide.
     
  4. Supernosh

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    thanks for that guys.

    So what gain should I be looking at for my ae100? I use it mainly in the dark and have about a 7ft picture.
     
  5. ReTrO

    ReTrO
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    A matte white 1:1 screen will do you fine. Don't go with a grey screen unless you really can see an adavntage yourself, personally I con't see much point in them.

    Nothing higher than 1.3:1 would be recommended with a fixed panel projector. (Generally, not always true)
     
  6. Supernosh

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    but does that mean that the higher the gain the better the picture? i.e. if I went for a 1.3:1 would that give a better picture than a 1.1?
     
  7. Gary Lightfoot

    Gary Lightfoot
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    Not necessarily.

    It can make the blacks look more grey than they already are, and you may possibly get hotspotting if the projector is already bright enough, which yours would appear to be. Too reflective may enhance artefacts inherent in the source too, so that would make the image appear worse than it is.

    On the other hand, you may like the image it gives over a less reflective screen. Some have even said that it makes screen door more apparent though. Using the ft lamberts calculation it looks like a unity gain is adequate for your screen size.

    If you're not sure what would be best for you, you can make a screen from wood and blackout cloth - make a wooden frame and stretch/staple the blackout cloth to it. The cloth is obtainable from most curtain shops and is around £5 a meter or less. You can use 2 x 1" wood to make the frame. I used 69mm x 18mm wood to make mine. There are pics on my website.

    It should cost less than £28 to make for all materials, and it will give you the opertunity to see what a unity gain screen will look like with your projector. You can then decide if you need a higher gain screen or not.

    If brighter was better, there wouldn't be the need for low gain or no gain screens. :)

    Gary.
     
  8. Supernosh

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    would it be true to say that a screen with a gain of 1 would be like projecting straight onto a wall?

    If that is the case then would it be worth getting one?
     
  9. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    SN: No it wouldn't. The paint finish on the wall will determine its reflective characteristics. Also the wall may not be flat.....

    Gordon
     

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