Brightness and Contrast: what is the minimum?

Discussion in 'Archived Threads 2001-2004' started by Stasulos, Sep 13, 2002.

  1. Stasulos

    Stasulos Stunt Coordinator

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    From your experience what do you think is the minimum for projector's brightness and contrast to have a decent picture? Say for the average room with some ambient light.
     
  2. Michael TLV

    Michael TLV THX Video Instructor/Calibrator

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    Greetings

    For an RPTV ... we shoot for between 15 to 20 ft-l of light output measured off a light meter using the 100 ire windowbox pattern.

    This is for contrast setting.

    Brightness is for whatever the AVIA or VE patterns tell you given your viewing environment.

    Regards
     
  3. Stasulos

    Stasulos Stunt Coordinator

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    Very clever... I meant plain and stupid projector specifications in case you didn't understand... But thanks for advice anyway, I'll look to get those windowboxes IRA or whatever...
     
  4. JohnnyG

    JohnnyG Screenwriter

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    Michael was being quite serious. Without a light meter, what's your reference? A number on a menu is meaningless as the correct setting not only varies from model-to-model but even from set-to-set of the same model.

    As a sweeping generality, try 50% of the contrast setting. On some sets this is still too high and on others, too low.
     
  5. Gabriel_Lam

    Gabriel_Lam Screenwriter

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    With the 1100 ANSI lumen PJ350, you're probably going to have about 950 or so lumens ignoring the manufacturer's optimistic guestimates.
    After you calibrate it for the correct colors and contrast, you'll probably lose about 15% of the brightness, MINIMUM (though with a business type projector, you're probably losing more). This gives you a max of 807.5 lumens.
    With your 130 watt P-VIP bulb, you'll lose about 20% of the illumination rather early in the bulb life, but it will stay close to that range for the life of the bulb (unlike Xenon bulbs which steadily decrease from this until they reach about 50% of illumination near the end of life). You're at 646 lumens now.
    If you're viewing 16:9 movies on your 4:3 native projector, you're giving up 25% of the available illumination unless you're using an anamorphic lens. You're now at 484.5 lumens.
    With a front projector, with a I'd aim for about 25-30 ft-lambert of illumination with a dimly lit room, and upwards of 50-80 ft-lamberts with a rather bright room. For reference, a direct view CRT is often about 80-100 ft-lamberts and modern plasma displays now get over 200 ft-lamberts. THX specifications (& SMPTE) call for 16 plus or minus 4 ft-lamberts of illumnination in the most ideal of situations (that's why movie theaters are pitch black):
    http://www.thx.com/theatres/sound_criteria.html
    Now, putting aside screen gain for a second, if you want to aim for 30 ft-lamberts, with 485 usable lumens on a 16:9 image from your PJ350, you can have a 16:9 screen with an area of just over 16 square feet in size. If you want to aim for 50 ft-lamberts, you screen has to be under 10 square feet, again ignoring screen gain. If you want to factor in screen gain, simply modify this area with the gain of the screen, assuming you're viewing in ideal angles.
    Of course, if you have a light meter, this whole process is much easier and more precise. It takes the guesswork out.
    Much of it depends on what you're watching too. Sports tend to be much more forgiving of ambient light because the scenes are brighter. Dark scenes tend to be much more important in most movies, so ambient light really ruins the picture.
     
  6. Stasulos

    Stasulos Stunt Coordinator

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    Gabriel, thanks very much. I'm getting 50*67 screen, which is what? around 20 sq.feet? so I guess I need to get one of those glass beaded or high gain screens.

    Surely the distance between the screen and the projector kicks in at some point in your calculations. I'm trying to put a projector at 12 ft from the screen. Is that acceptable? Where can I read about it on the internet to be able to calculate this stuff myself?
     
  7. Gabriel_Lam

    Gabriel_Lam Screenwriter

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    50*67/144 = total square feet.

    Your screen is 23.26 square feet, in a 4:3 format. With a unity gain (1.0), you'll get roughly 28 ft-lamberts of illumination over most of the life of the bulb:

    464/23.26 = 28.1 ft-lamberts

    As you can see, we used the 4:3 output of 646 lumens in this calculation because of the screen format.

    With 28.1 ft-lamberts, you should be ok with a little ambient light, with the understanding that this will still negatively affect your viewing pleasure, especially with programs that have a lot of dark scenes.

    Please do take my recommendations with a grain of salt though. Numbers aside, personal preference does play a big part in what will look the best TO YOU. I, for instance, love a bright, very punchy picture, with vibrant colors. Shadow detail is important to me in movies, but I'm not as much of a stickler for absolute black level as long as we get the detail and ambience. When I have a superbowl party, I'd hate to have to do so in a pitch black room. I'm on a 3000 ANSI lumen projector (manufacturer spec), with 800:1 on/off contrast, on a .95 gain screen (Stewart Grayhawk, with a wide 160 degree viewing angle) just about the size of yours.

    The distance between the screen and the projector actually plays very little into the brightness calculation. Because stray light does not affect our calculations on desired illumination (and there's not that much of it anyway, compared with the white point), and because the only thing cutting down the light when increasing the distance is the difference in dust particles in the air, we're not talking a big ammount.

    What you really want to do is mount the projector as far back as possible and yet maintain a small enough picture (fully zoomed out, of course) to fit on your desired screen size, in this case 50*67. Though I'm sure Viewsonic and the other brands do offer sizing calculators, they're still just guides. There's no substitute to actually having the projector and measuring the output size by hand if you want to get the mounting down to the inch.
     
  8. Stasulos

    Stasulos Stunt Coordinator

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  9. Gabriel_Lam

    Gabriel_Lam Screenwriter

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    When you zoom all the way out, you project the image through the cleanest, most neutral part of the lens, the center. You minimize distortion, and the corner focus tends to be slightly better. It tends not to be a huge difference for most projectors, but it's definitely worth it.
     

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