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White screen versus grey screen...

Discussion in 'Projectors, Screens & Video Processors' started by PJnewbie, Jul 12, 2004.

  1. PJnewbie

    PJnewbie
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    Hi guys,

    Following an earlier thread, I am very confused about whether grey screens are noticeably better than matte white for DLP projectors such as my IF 5700.

    Some people say it gives a better image, some say white is perfectly good if the lumens output is sufficiently high. Some say the grey kills the brightness too much and kills the colour accuracy whereas others say the contrast is much improved over a white one!

    What does everyone think? I am using a white wall currently and am pretty happy, but would like a proper screen to boost the image a fair tad.

    Incidentally, I want 16:9 fixed frame if that matters....
     
  2. Kramer

    Kramer
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    Moved to Screen forum.

    :smoke:
     
  3. ReTrO

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    With your Infocus 5700 you'll not see any benefit from a grey screen as the projector has a very good black level to start with, as it has the nice new Matterhorn DMD.

    We have the Sharp equivilent, the XV-Z200, at work and this works best on a 1-1.3 gain white screen.
     
  4. Gary Lightfoot

    Gary Lightfoot
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    I agree with Retro - in normal conditions, you probably won't benefit from a grey screen, and the majority will be more than happy with a 1 to 1.3 gain screen. A good painted clean matt white wall is about as good as a plain matt white screen anyway, so you probably won't benefit a great deal from a unity gain white screen unless the wall is bumpy and uneven.

    I'll try to comment on some of the remarks you mention though:

    In a room with ambient light, you will indeed see a better image as a grey screen reduces the affects of ambient light and cross reflections. A screen is only as dark as the room lighting conditions will allow, so a grey screen will give you an improved black level.

    If the grey screen has a gain of less than unity, such as 0.8 or 0.95, then the image may look dull compared to a unity gain matt white screen. That's a condition of the gain, not the colour. If the grey screen has some gain like the Da-Lite grey screen (HCCV? 1.1) or the Stewart Firehawk (1.35) (Vutec make a Gey Dove screen too) then it will look as bright or brighter than a white screen, depending on the reference white screens gain. There will be a slight colour shift with a grey screen, but this can depend on the screen make-up - the Da-Light may have a worse effect than the Firehawk or Greyhawk for instance (they use different materials so will behave differently), and the colour shift or overall effect can be down to personal preference as to whether it's an improvement or not. Apparent contrast is improved due to the percieved black level, and the reduction in effect of the ambient light in the room (if there is any).

    I've seen the Stewart Firehawk side by side with a white screen (a single screen made from both materials) in a very brightly lit room which had many windows (daytime) and the lights were on. The Firehawk image was noticably better than the white screen, which was washed out with very little detail. The Firehawk produced a very watchable image which was surprisingly good despite the high ambient light in the room. Very impressive I must say. IIRC, the white screen was a unity gain Stewart screen.

    The Human brain is easily fooled and has a bad memory for colour and white level. It is better at percieving black level though. So, with a grey screen, the peak white level will be light grey, but the brain will perceive it as white, and that is what you'll see. With the Firehawk/white screen demo, the difference in peak white was noticable because there was a white reference, but the difference was negligable.

    Do you need a grey screen? Probably not unless you watch with light coming in through the windows, though some benefits from a higher gain screen have a similar effect to a grey screen with gain. The viewing cone is tighter so more light is reflected back to the viewer, and cross reflections are reduced in the same way (they come in from the side, and go back the same way). You don't benefit from the reduced black level a grey screen will give, or extra contrast you will percieve. If you have a blacked out room, those benefits are negated though.

    If you need a brighter image, you can calculate if it's enough - the reflected light is measured as ft Lamberts - you need 12-14 to emulate a cinema, so you can probably count that as a ball-park range. To calculate it, you just need to divide the screen area by the amount of lumens, and multiply it by the gain (if any):

    so a 7ft wide 16:9 screen would give 27.5 sqr ft, and say your projector has an advertised lumen output of 1000, you can reduce that by up to 40% to allow for advertising 'bloat' and for lamp aging, so by dividing 600 by 27.5 you end up with about 22ft lamberts. If you calibrate the white and black levels, you may end up with less (full calibration generaly reduces lumens). A bigger screen will of course give you reduced ft lamberts, so using the above calculation, you can work out how much you'll need to achieve the kind of picture you'd like.

    Going above a gain of 1.3 can give hotspotting, but Retro can probably give you a much better idea of what his screens will look like with reguard to higher gains, hot spotting and colour shift etc if you decide you need something greater than 1.3.

    HTH

    Gary
     
  5. PJnewbie

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    Thanks very much indeed guys for the replies, much appreciated.

    Basically all my walls are white, but I tend to watch mostly at night with little ambient light. I really just wanted a fixed frame screen assuming it would improve over the wall by a fair amount.

    It seems I would be fine with a white screen then all things considered, and I cannot justify the Da-Lite costs anyway! I was looking at the Goo system (paint) as an alternative.

    ReTrO - what screen would you recommend then, and Gary do you have any recommendations on manufacturer/supplier and what gain would best suit me?

    Thanks again very much.
     
  6. Gary Lightfoot

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    What size screen will you be wanting? You'll need to know that as it directly affects how bright the image will be - bigger will be dimmer, and the bigger you go, the more chance of needing a higher gain.

    Gary.
     
  7. PJnewbie

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    Well the image I am currently projecting onto my wall is approx 9 ft across, but would settle for 8 ft.
     
  8. Gary Lightfoot

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    Assuming 600 lumens with your pj after lamp aging of a couple of hundred hours, a matt white 9ft wide screen will give you approx 13ft lamberts, which should be enough in normal blackout conditions and dark matt coloured walls. Add ambient light and reflective walls and it may not be enough.

    What size are you currently projecting at? If you're happy with that, and the white wall is similar to a matt white screen, then that should suit you. If you want something a bit brighter, then Retros advice of something up to 1.3 gain may be what you need (A 1.3 gain screen will boost 13ftL to 17ftL).

    There are quite a few screens available, such as http://www.drhscreens.co.uk www.ellie.co.uk http://direkt2u.co.uk and Retros own company. Other names such as Beamax are mentioned, so see what other screens get best reviews. A fixed screen will nearly always be better than a pull down or electric, as it remains flat and doesn't ripple - unless you get a Stewart screen of course. :)

    Gary.
     

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