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Why does LCD relect back if CRT does not (screen with gain)

Discussion in 'Projectors, Screens & Video Processors' started by Tempest, Feb 23, 2005.

  1. Tempest

    Tempest
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    Bit puzzled about this.

    You can buy nice screens full of glass beads which seem to be what you want if you have a massive CRT projector.

    But, you are told not to get something like this, of pretty much any screen with any gain if you have a LCD projector as you will get hot spots caused by the light reflecting back into your eyes.

    So, why don't you have this problem with a CRT?

    Can you actually have ANY gain screen with a LCD projector to make the image shine a bit more than a matt surface without any problems, or is it just a no no.
     
  2. RTFM

    RTFM
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    Tempest,

    CRTs are not nearly as bright as modern single lens, fixed panel LCD and DLP
    projectors. Also with CRT you are projecting 3 separate images which are spaced apart and each is anywhere between 6 and 9 inches across so they are not point sources of light like LCD or DLP which would probably hotspot on a beaded screen.
    If you do use high gain screens with LCD they will give you a brighter image on axis but brightness will fall off quickly if your viewing position is a few feet off center. The higher the screen gain, the sharper the fall off in brightness as you go off axis.

    Jeff :rtfm:
     
  3. MikeRJ

    MikeRJ
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    I thought the objection to using high gain screens with LCD projectors was the screen would show up the poor black levels more, which is why low gain grey screens are recommended for LCD's?
     
  4. Tempest

    Tempest
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    Thanks for the reply......
    I knew the lenses were spaced apart, but did not realise they were a lot duller than a normal LCD projector.

    In your opinion, what's the maximum gain YOU would recommend for a screen for a LCD projector without running into the hotspot issues?
     
  5. RTFM

    RTFM
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    Probably no more than 1.3.

    Depends on the brightness of the projector, screen size and application.

    I normally recomend matt white screens but grey screens can be of benefit with bright LCDs that have poor black level.

    Jeff :rtfm:
     
  6. GrahamMG

    GrahamMG
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    Spot on!
     
  7. PJTX100

    PJTX100
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    Hi GrahamMG

    So are you saying that a matt white screen is OK? It's just that I remember your recent "please no painted MDF or Hardboard" plea.

    See also this recent thread.

    http://www.avforums.com/forums/showthread.php?t=191148

    Cheers...PJ :thumbsup:
     
  8. GrahamMG

    GrahamMG
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    Eh????

    Painted MDF or painted hardboard is not a good screen for film use, period..... Matt white or grey screen material from the likes of Stewart etc. is quite different as you well know. Certianly dearer than a tin of "dulux" but the results can (and almost certainly) be very different..... ;)
     
  9. PJTX100

    PJTX100
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    OK, now we know... :)

    ...PJ
     
  10. Liam @ Prog AV

    Liam @ Prog AV
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    Unfortunately the chap on that thread has no reference setup for comparison and is likely running default (read bright and nasty) settings from his (first) projector. When it comes to calibrating for colour accuracy et al I imagine the painted hardboard will have an incredibly difficult time getting it right and in all likliehood will be quite unevenly presented accross it's surface area. Hourses for courses though, not everyone who buys a projector is doing so to see the image exactly as intended. Many are simply after a big telly with bright colours and decent blacks
     
  11. PJTX100

    PJTX100
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    Thanks Liam. I think you've summed me up in 1 sentence!
    :) ...PJ
     
  12. Liam @ Prog AV

    Liam @ Prog AV
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    Lol!
     
  13. GrahamMG

    GrahamMG
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    Hi Liam.
    Yes indeed...
    As a leveller I can see why people would want to make a screen with MDF etc. and a tin of white paint with cheap LCD pj's as decent screens can easily cost as much or more than the pj..... I would urge due consideration of a decent screen though as it will last a long time and survive an upgrade or three....
     
  14. Tempest

    Tempest
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    Thanks for the replies to my original question.

    I did try (just for fun) an old reflective silver/grey screen last night, and it gave an interesting result.

    Ignoring the horrid physical surface quality of the screen) it did give more punch to the image,

    In the dvd "Vertical Limit" near the beginning there are some tiger things playing in the snow.

    On my blackout material the snow was just a dull flat white, wheras from this reflective screen, it was definatly whiter with a cleaner slighly more punchy look to it.

    I hang it so it was half way over my current screen and the white on my REAL screen did look a little dull in comparison.

    I guess it was just that it's reflecting the light back off the surface into my eyes, rather than it just dispersing.

    The blacks were a little worse, but not a massive amount, and to be honest, I could live with that, given the whole picture was lifted up in brightness by a small but still nice amount.

    I have no idea of the gain whatsoever as this is just a VERY old cine film/slide type screen from boots the chemist. But It did not appear to suffer from any hotspotting that I could really see.

    Shame there is no way to measure it.
    If anyone COULD measue it, I could always send a small offcut for testing.
     
  15. GrahamMG

    GrahamMG
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    Grey screens would normally improve the perception of black level. 1.3 gain is a good match to DLP devices, slightly les gain for LCD but still grey material is a good place to start. Various other influences will naturally have an effect as well like light level, natural light, reflections, wall colour etc......
     

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