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How do I know what gain I need?

Discussion in 'Projectors, Screens & Video Processors' started by l33750rz, Oct 16, 2005.

  1. l33750rz

    l33750rz
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    Hi, I am about to buy a PJ and I need to find a good screen. I understand that gain has to do with the ambient light present in the room. However, what I do not understand is knowing which gain you need for certain rooms. If someone can please explain to me to figure out which gain I need for my room, that would be great!

    BTW: I have Greyish / White walls and the room is 16' x 14' wide

    Thanks in advance! :)
     
  2. binbag

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    What type of PJ?
     
  3. l33750rz

    l33750rz
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    Sanyo Z3 / Sanyo Z4
     
  4. DEANO-B

    DEANO-B
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    I use 'Foamex' which cost me £20 for a 6'6" wide screen and is excellent! :thumbsup:
     
  5. Timmy C

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    Do you watch in a fully blacked out room?
     
  6. l33750rz

    l33750rz
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    I have 2 windows, but I will put black-out material over them when I would like to use the PJ.
     
  7. Timmy C

    Timmy C
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    I don't know if this applies to LCD's as I've always had DLP projectors but from what I've gathered you should be looking for a gain of around one if you're watching in a fully blacked out room. My screen is 1.3 and I tried my brothers Z3 on it and I was very surprised how good the picture was considering the cost of the pj.

    If you are possibly going to be watching in less than ideal conditions then you might want to consider a grey screen that should improve the black levels.

    I would say a good place to start would be here: http://www.beamax.com/

    They are very helpful and although I've not owned one of their screens, they often get good reports on this forum. I'm sure they can tell you everything you need to know.
     
  8. sbowler

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    If your pj has around the 1000lumens brighness then a 1.1gain should be enough, I found i have the brightness turned down to -15 on my panny 700.
     
  9. kurtz

    kurtz
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    The gain of the screen is a measure of the light reflected back at the viewer. It isn't really to do with ambient light, but is sometimes used to compensate for ambient light once the prime objective of gain - the brightness of the picture - has been taken care of.

    The gain of the screen you use depends on many factors - the projector's black level capability, the relectivity of the walls and ceiling, the required width of viewing cone (only really applicable for high gain screens), any ambient/stray light and so on... But the prime factors are the size of the screen and the brightness of the projector.

    Here's a useful article on screen gains.

    You haven't mentioned screen size, but I would guess you'd go for a 96" diagonal 16:9 screen (83" wide). This will give you just over three feet each side to mount your front speakers and a seating distance of about 12 feet. The throw for a Z3 would then be about 10-12 feet.

    Although a Z3 is rated at 800 lumens, probably after calibration and a few hundred hours bulb use you'd probably be looking at 600 - 700 lumens depending on how optimistic you are.

    Now, a 96" diagonal screen is 84x47" = 3948 square inches, which (div by 144) is just over 27 square feet.

    And foot-Lamberts = (projector lumens x screen gain) / screen area


    So with a unity gain screen, (650 lumens * 1.0) / 27 square feet = 24 foot-Lamberts.

    I think the American Society of Motion Picture and Television Engineers (SMPTE) recommend 16 ftL as a brightness in a blacked out movie theatre. Some enthusiasts adhere to this to create a true 'cinema experience' - others like their picture a bit brighter and punchier and aim at 20 ftL or even a bit more if they intend to watch sports as well as movies.

    So with 24 ftL you should have a pretty bright picture. If you wish, you could choose to reduce the brightness by going for a 0.9 gain or 0.8 gain screen (which would give you 21.6 ftL or 19.2 ftL respectively) and still be around the 20ftL mark.
    Going for a 0.9 or 0.8 gain screen would help improve blacks (because you're using LCD technology which isn't known for giving as good absolute black levels as DLP) and help darken any backwash from your light walls and ceiling.

    A different approach would be to go with a bigger screen, which will dim the picture. For instance, a 108" diagonal would be 34.5 square feet and give you 18.8 ftL on a unity gain screen - probably as dim as you would want to go.

    Now, I've made an assumption about your screen size and a guess ;) about the available projector brightness after a period of use. You can eliminate one of these variables by deciding your actual value of screen size depending on your seating distance, speaker placement and projector placement.

    Hopefully I've shown you some of the factors involved in choosing screen gain:
    • choose your screen size
    • calculate how much brightness you've got to play with (depending on how bright a picture you'd like)
    • decide if you want to lower the projector black level or compensate for ambient light or backwash in the room

    Remember that backwash from the room can always be reduced by painting the room (and ceiling) in a darker colour - even going from white to a wheat colour can make a difference - so if you do decide you want more ANSI contrast this is something you can do later.

    A practical thing you can do is when you have a projector demo, ask what size the screen is and what gain it is. Check if the projector is in an economy or high brightness mode. Then decide if you like the brightness. When you get back home you can plug the figures into the equation to work out the foot-Lamberts and then you'll have a reference of what 20 ftL, or 25ftL (or whatever it turned out to be) actually looks like.

    Another useful tip is to play some of these 'what if' scenarios out on the projectorcentral.com calculator. Here's a link to the calculator for the Z3 calculator for the Z3

    This should give you an idea of the basic principles you can take into account when making a choice.
     
  10. oconnpad

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    excellent post kurtz.

    as a complete novice to this, i have to say you've given me some great information there.

    Thanks
    Paddy
     
  11. MikeK

    MikeK
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    Yes, but I think the estimate of 650 lumens from a best-video calibrated Z3 is way optimistic TBH - more like 300 lumens (to be fair, kurtz did say he had guessed here)!

    This puts a rather large spanner in the resulting calculations, as a 27sq ft unity gain screen will only be lit to around 11fL - too dim for many/most people!
    With such a screen you'd then be left with having to operate the projector in modes not best suited for video, in order to drag enough brightness out of it, such as full lamp mode, fully open iris etc etc.

    Bottom line is that if you are going to calculate the screen requirements, then you can't just guess the light output of the PJ - you need to know it really.

    BTW, I wouldn't realy on projector central's screen calculator - while it may be OK for deciding screen sizes based on throw ratio and distance etc, it's screen brightness guide is often simply inaccurate.
     
  12. kurtz

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    Yes, I guess I must have been too optimistic - which is a rarity for me!

    But, again it does highlight some important principles:

    • never trust the manufacturer's quoted lumens they are usually too high using an unrealistic colour balance (eg. too green)
    • manufacturers quote lumens based on "high lamp mode" with the iris open whereas the most cinematic picture is found in "low lamp mode" with the iris stopped down
    • bulb brightness deteriorates with age and manufacturers will measure with a new bulb

    Hard facts on actual projector brightness are rare because few people have the ability to measure them. Also the room's reflectivity affects their measurements.

    I did a search for "lumens" and found this interesting post by Gary Lightfoot (who is able to measure lumens) and he has a rule of thumb to reduce the manufacturer's quote by 40%. But this is only a rule of thumb as some manufacturers will be more realistic than others.

    l33750rz:

    From MikeK's estimate it seems that something like a 1.5 gain screen would be a safer bet for a 96" diagonal - or 1.3 gain and a smaller screen. You can see that variations in projector brightness can make a big difference.

    When I got my projector I looked into the subject of screens and realised (like you) there were a several unquantifiables. I solved the screen size/gain dilemma in a more empirical way by getting the projector first then figuring out the screen second. Before I bought the projector I realised that I would need some surface to test it out on and check for dead pixels, colour-uniformity etc, so I primed and sprayed a sheet of MDF matt brilliant white. When I got the projector I tested it and lived with it first to learn its black level and brightness characteristics. After a couple of months when the bulb had settled in I painted the room a darker colour and painted the wall behind the screen chocolate and went for chocolate curtains in the room to reduce backwash. Then I lived with it a bit more before finally deciding to build my new "proper" screen and spray it with digital light grey goo paint. So I kind of "homed in" on the screen size and gain and I didn't have too much invested in it and I could afford to make mistakes and learn as I went along. I'm a self-taught sprayer and had several mishaps learning by spraying cardboard cartons and MDF offcuts and the final screen took a bank holiday weekend in the garage to get right.

    This was OK for me because I have a dedicated room, so its not too disruptive to keep fiddling around. The goo greatly improved the picture - and not just from a gain point of view but I had not expected its surface texture to be noticably smoother than the sprayed white emulsion, presumably cotributing to less scattering of light and better fine detail. So I kind of treated the process as a learning experience in itself as well as a lower risk means to getting a screen.

    Perhaps you could do a similar (and less time consuming) sort of thing and paint the screen wall a plain white to begin with and try your projector to see what screen size/brightness suits you. This may be the nearest you get to 'knowing' the brightness of your projector. Then make or buy your screen.
     
  13. Gary Lightfoot

    Gary Lightfoot
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    I try to aim for 12ft lamberts though I can live with less if the CR is rerasonable (2000:1+). If the image is too bright it can highlight image noise related to the technology, such as DLP dithering or mpg artefacts, so for me brighter is not necessarily better (plus you get better blacks if that's important too you). With Hi Def sources (and less artefacts) that could change though.

    Always set your white (contrast) and black (brightness) levels using a test disk like DVE and that will also ensure you won't have an elevated bklack level which can highlight artefacts or levels which mean you are mising image detail in the brightests or darkest images. There's a basic calibration 'sticky' in the LCD/DLP forum if you want to have a look.

    Forgot to mention - 16ft lamberts is derived from a film projector without film in the gate, and 12ft lamberts is with film in, so that's the figure to aim for if you're emulating the cinema. In reality cinemas can be around the 9ft lamberts mark due to wanting to extend lamp life. I'm currently running quite happily at 9FL myself on a scope screen.

    Gary.
     

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