"Lumens" vs. "Ansi Lumens"

Discussion in 'Archived Threads 2001-2004' started by RonJ, Nov 29, 2002.

  1. RonJ

    RonJ Auditioning

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    I take it one ansi lumen is = to atleast 3 just "lumens"? Am I right?
    I saw a nice sony projector (VPL-HS10) that looks to be a high
    quality projector but is only rated at "1200 ansi lumens" however many
    projectors that appear to be lower quality are rated at "3000 lumens."
    Thanks.
     
  2. Gabriel_Lam

    Gabriel_Lam Screenwriter

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    ANSI lumens and "lumens" are used pretty interchangeably these days. Most 3000 lumen projectors are for business use and tend to be biased towards a very cool (high temp) picture. The reason the VPL-HS10 has just a 1200 lumen rating is their 180w UHP bulb and non-MLA panels (MLA increases the pixel fill ratio and makes the image brighter).
     
  3. RonJ

    RonJ Auditioning

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    I didn't quite understadn that explanation. Is there a FAQ or something that might help me out? I know that there is an ~$8500 JVC projector that has slightly more than the 1200 (but less after calibration whatever that means) so I'm assuming that 1200 is pleanty for a large screen home theater application (10-12 feet across).
     
  4. Gabriel_Lam

    Gabriel_Lam Screenwriter

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    It depends.

    A screen that is 12' across is about 125.6" wide and 70.6" tall. This means the screen is 61.6 square feet.

    Let's say you want the THX & SMPTE recommended 16 ft-lamberts of illumination on the screen with an ideal setting (no ambient light). We'll hold screen gain constant at 1.0 for a minute.

    We would need:

    16 * 61.6 = 985.6 lumens of illumination.

    Let's say we start with the Sony HS10 with 1200 ANSI lumens (manufacturer rating, with new bulb).

    UHP bulbs lose roughly 20-25% of their max illumination rather early in their life, and stay roughly there until their demise. Let's say in this example we only lose 20%. We're now at:

    1200 - 20% = 960 lumens

    UHP bulbs tend to run rather hot, upwards of around 8000 degrees K. For video, you want white to be 6500 degrees K (also known as D65). After calibrating the projector, you'll lose upwards of 40% off the max illumination, but let's assume the HS10 loses a very very low 10% of this number. We're now at:

    960 - 10% = 864 lumens

    As you can see, with a 1.0 gain 12' diagonal screen, assuming you'd like the screen to be 16 ft-lamberts, there's no way you can reach it with 1200 manufacturer rated ANSI lumens. We're only at about 14 ft-lamberts, which is still reasonably acceptable. If we choose a higher gain screen (ie. the Stewart Firehawk with 1.35 gain), we can bump this number:

    864 * 1.35 = 1166.4

    1166.4 / 61.6 = 18.9 ft-lamberts of illumination.

    A few things to keep in mind:

    1. Our eyes perceive brightness on a logarithmic scale. This means, a doubling of available light will seem like less than a doubling of brightness.

    2. If you have ambient light, you're going to need a LOT more brightness to compete. Remember, a direct view TV set runs upwards of 80 ft-lamberts or so (though less when calibrated), and some of the newer plasma units can hit upwards of 200 ft-lamberts.

    3. In general, the higher your screen gain, the better it can reject ambient light. However, it also limits your viewing angles.
     
  5. RonJ

    RonJ Auditioning

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    Thanks. Don't quite understand all the technical terms but I get the idea. Maybe I need a projector with higher illumination. I wasn't aware of all these losses from calibration and bulbs. The good news is the theater room will be in the basement. There is a sliding door and a small window but I plan to hang heavy thick black curtains over both so there hsould be 0 light in that room other than the projector during movie viewing.
     
  6. Kelley_B

    Kelley_B Cinematographer

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    Get the HS10, it is a sight to behold! I don't think you could go wrong plunking $2500 down on the HS10.
     
  7. Gabriel_Lam

    Gabriel_Lam Screenwriter

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    RonJ:
    Calculations aside, you really should see it for yourself. Everyone has his own preferences. Not everyone thinks a bright, punchy picture is necessary. Even if they do, not everyone preceives 16 ft-lamberts as the minimum cut off point. I'm at about 65 ft-lamberts on my setup. [​IMG]
    You actually have a lot of flexibility though, because you have tight ambient lighting control. If you can, go see some projectors and let your eyes decide.
     

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