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Clearly it's physically impossible for a projector screen to have gain...

Discussion in 'Projectors, Screens & Video Processors' started by NicolasB, Jun 8, 2004.

  1. NicolasB

    NicolasB
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    ...because a projector screen is a passive device. It's not, for example, an actively pumped laser medium which amplifies a light signal that encounters it.

    So, when people inaccurately refer to a screen as having "gain", what are they actually talking about?
    :smoke:
     
  2. Zone

    Zone
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    Dictionary definition of "Gain" is "to increase".

    Screen gain as I understand it is a ratio of reflected light off a given screen material as compared to a standard white board.

    Therefore a screen gain of 1.0 reflects 100% more light than a white board, 1.2 120%, 2.0 200% etc.
     
  3. Jules

    Jules
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    I think it would be more sensible for screen manufactures to quote the percentage 'loss' of their material.
    i.e a perfect mirror would have 0% loss.
     
  4. Paul D

    Paul D
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    You are right when you say that a screen cannot "gain" on the light output from a projector.

    I understand it to mean:-
    A light "gain" in relation to a standard "plain white" matt screen.
    1.0 gain is supposed to be about the same as a quality white sheet of paper. where light is reflected in a diffuse manner in all directions.

    To get more light reflected back to the viewers, a screen reflects lights back on a narrower plain.
    The tighter this plain, the brighter the image appears as long as you are viewing in this plain.
    Viewing out of this plain would result in a darker image.
    Microbeads can be added to the screen, which work similar to cats eyes. ie you get a lot of light reflected back in a certain direction. But the down side can be "hot spotting" and "colour shifts" due to refraction.

    But then again, i could be wrong! :laugh:
     
  5. LV426

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    I'd agree with you, fulabeer.

    A screen with gain of 1.0 is equivalent to plain white.

    A screen with a quoted gain of (say) 1.2, will reflect, in one given direction, 120% as much light as plain white. There is a curved graph you can draw (something like a bell curve) that depicts how much light is reflected, starting at 90deg off centre, through centre, to 90deg off centre the other side.

    So, a screen with > 1.0 gain will only exhibit this when on, or close to, on-centre. As you move off centre, the gain will gradually drop down to less than 1.0.

    So, even though it is a passive device, it can still have gain - it's simply a measure of the extent to which reflected light is concentrated in a given direction. The total light reflected (i.e. the area under the curve) is nominally the same as a plain screen.
     
  6. Gary Lightfoot

    Gary Lightfoot
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    Gain is measured against a block of magnesium carbonate IIRC, which has a gain of one. There's no such thing as negative gain - 0.8 is still positive. :)

    Another way of looking at it (adding to the above comments) is that the screen is taking light which would normaly reflect outside of the higher gain screens viewing cone, and redirecting it into the narrower cone.

    A matt white screen has 180 degree reflectivity, when you cut that down to say 100 (Firehawk) due to the screens gain reducing the viewing cone, the light that would have been reflecting in the outer 80 degrees (40 each side) is now being reflected elsewhere - into the 100, so you're taking from Peter to pay Paul. :)

    Fresnel lenses work in a similar way by having a specificaly designed reflective surface. How the hell a screen material actualy does this I've no idea, but I think I'll have to find out. Stewart screens spray the optical coatings onto the screen, so how the coating knows where to send the light is intriguing...

    HTH

    Gary.
     
  7. NicolasB

    NicolasB
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    Corner-cube reflectors, maybe?
     
  8. Gary Lightfoot

    Gary Lightfoot
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    Sounds interesting - how do they work? Got a link?

    Gary.
     
  9. NicolasB

    NicolasB
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    Erm... not off-hand, but try a Google search. :)

    Corner-cube reflectors are something I recall from my university course (Laser Physics and Optoelectronics). Probably the best way to understand it is to try drawing a 2D equivalent. Draw 2 sides of a square:

    |
    |
    |
    |
    |
    |____________


    Then draw a ray of light coming in from somewhere in the top right quadrant so it hits one of the lines. If the line is a mirror, then it will reflect at the same angle (relative to a line drawn at right-angles to the surface) as it came in at.

    (See lower diagram).

    If you trace the path of the ray as it reflects off both surfaces, you find it gets reflected back along its original path, but with a slight sideways displacement.

    (See upper diagram).

    If you try a few different angles you find the same thing is true - whatever direction the light comes from, it gets reflected back towards the source.

    The same applies if you take this to 3 dimensions: if you have three mirrors arranged to form the corner of a cube, then any light coming in is reflected back towards its source.

    They use a corner-cube reflector to measure the distance to the moon by bouncing a laser beam off it. (The Apollo astronauts took it up there!) The same thing is used in the surface of road-signs and those reflective strips on clothing (of the sort designed to show up in car headlights).

    Edit: my second diagram originally went horribly wrong. I've replaced it with a marginally less horribly wrong one. :)
     

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  10. SimonO

    SimonO
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    This is commonly know as retro-reflective... Useful if you have to mount a projector below your normal viewing height...
     
  11. ReTrO

    ReTrO
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    Then you look at a Vutec SilverStar screen which has a gain of 6.0 and retains a 180 dgree viewing angle. It's done through using multiple layers of coating I believe. :)

    Thus ends my plug. ;)
     
  12. Gary Lightfoot

    Gary Lightfoot
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    Great info guys - but how does the physical material work? The box reflector shows how that reflects, but when spraying a coating onto a screen material, how does the sprayed optical coating physicaly reflect in the right direction? All the boxes would have to line up correctly in that instance. Is the blue line in the second image correct? It looks like the first reflection should be greater than 90 degrees.

    Gary.
     
  13. NicolasB

    NicolasB
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    Well, no, that's the whole point: it doesn't matter what direction the box is pointing in, any beam of light coming in will still be reflected in the same direction (i.e. back towards its source).
     
  14. Gary Lightfoot

    Gary Lightfoot
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    OK thanks.

    Another question - how do they make sure the boxes all end up the right way when sprayed (they could end up upside down)? Does the opctical coating contain mini boxes, or something else?

    I'm just trying to find out the true content of the optical coating so we can understand how it really works.

    Gary.
     
  15. NicolasB

    NicolasB
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    It wasn't, but I've substituted a slightly less messed up version now. :)
     
  16. avanzato

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    AFAIK the 'glass bead' screens work like cats eyes. The back of the bead has a reflective coating (the glue used to stick them to the screen). Light entering it is focused onto the reflective back and can then reflect out along the path it came in on. Round beads have no 'right way up' so however they fall on the glue when spraying they still work.

    Other coatings, Screen Goo for instance, use tiny flakes of reflective material in the paint. Effectively coating the screen with millions of micro mirrors.
     

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