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Brightness theory & practice

Discussion in 'Projectors, Screens & Video Processors' started by Chris Bellamy, Feb 21, 2003.

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  1. Chris Bellamy

    Chris Bellamy
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    Perhaps one of you (Gordon, Roland, etc.) can sort out where I’m going wrong here. It’s not terribly important, but has been bothering me like grit in a shoe:

    I believe that due to beam current limiting a crt has a higher peak output for small bright areas than large areas (ie: the whole screen), hence the large discrepancy between ANSI lumens and peak lumens. Since film/video generally contains only small proportions of white, the peak lumens figure has been invoked frequently (particularly recently on AVS) to explain that crts can be much brighter in real life use than the simple ANSI figure would suggest.

    However, I had always sort of assumed that crts are setup and calibrated using a full screen window for 100IRE, rather than say a 20% window. In that case, the crt would never be called on to use its extra “peak lumens” capacity, since outside of DVDs with illegal gamut, no film source should exceed 100IRE at any point on the screen. Maybe I’m wrong – are crts calibrated to 100IRE with only a small or a full screen white window? If a small screen window is used, then how small?

    Can someone sort me out here… I put this point to someone on AVS, but received only deafening silence

    regards,
    chris
     
  2. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    Chris,

    I use a small window pattern off AVIA or VE to do greyscale of CRT. It's an interesting point though. I have a friend who has been experimenting with greyscale on his projector and finds he prefers the calibration with full field patterns. There is a measureable difference between the two.

    When my new measuring device is with me I will investigate this further.

    Gordon
     
  3. Chris Bellamy

    Chris Bellamy
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    thanks, Gordon,

    it would be very interesting if you can check whether doing setup using a small 100IRE window has implications for poor linearity of light output/greyscale at the top end for large very bright scenes (eg: in Fargo) where the projector might be struggling for juice (technical term!) compared with the small window used for calibration? My next question I suppose would be what is the best compromise window size to use for 100IRE setup.

    Your new "device" sounds interesting: what is it, a CIE-calibrated light meter/photometer (which I believe are visciously expensive, but the only way to measure FtL/nits accurately)?

    chris
     
  4. Gordon @ Convergent AV

    Gordon @ Convergent AV
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