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Damn Lies & Reality

Discussion in 'Projectors, Screens & Video Processors' started by Rob Sinden, Aug 12, 2005.

  1. Rob Sinden

    Rob Sinden
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    Looking at Rolands thread has gotten me thinking..

    Surely there is some 'proper' way of getting meaningful measurements provided the right equipment is used and all the parameters are fixed.

    Isn't this covered by the ISF, THX or someone similar?
     
  2. Gary Lightfoot

    Gary Lightfoot
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    Yes it is - D65 (not 6500k) which is usually done by someone like Gordon who is ISF trained.

    First you usually set the white and black levels using a test disk like Avia or DVE (or even THX Optimode but there are caveats), and that makes sure you aren't clipping any detail at the black or white ends of the scale. Having a black level too low means you will be missing shadow detail as it will all be crushed into black. Do the same at the white end and you can lose cloud detail for instance. Set this way will give you huge contrast ratio numbers but you will be seeing less image detail as you've crushed the detail into black or white, so it's very important to set these two things up correctly if you want to see things as they are meant to be seen.

    To get real world figures that mean anything and can be compared with others (like a benchmark), displays are usually set to D65. D65 is a white point where the colours are correctly balanced and determine how white should look (daylight 6504 is the colour of the sky in a cloudy overcast day apparently). This filters down to how white should look at all levels of grey (which is really white at less luminance) to black as well, so you get no colour tints at all. If you watch a black and white movie everything should look grey and with no colour tints, or you can use a test disk with grey ramps to do the same thing.

    You use colur bars with a blue filter to set the colour saturation and that comes with your test disk.

    6500k is not the same as D65, as a colour temp of 6500k can have a varied combination of colours to attain that colour temperature, so 6500k is a line in colour space, and not a point. D65 has specific x,y coordinates and 6500k doesn't. 6500k can look too green or blue for example.

    You need a measuring device such as a hardware colorimiter which can see light in the same way as the human eye, and a means translate it. Colorfacts is one such hardware/software combination that can do this, and it will allow you to balance the colours to the white point (D65) and colour standard (PAL, NTSC) you choose, and to plot the colour gamut, gamma and colour temperature (amongst other things). You mostly make adjustments within the projector to attain the standards you're aiming for, though sometimes you have to make adjustments elsewhere in the chain of components - such as within the DVD player (if possible) if it's clipping black and you can't set the black level properly for example.

    When this is done, you can measure your contrast and lumens for instance, and know that it is accurate for a calibrated machine, and not for something which has poor colour accuracy and incorrectly set black and white levels. This way all machines are on a level playing field and can be compared, and the measurements now mean something comparatively.

    An example that I have mentioned here before is the full on/off contrast measurement of a Sony G90 that was correctly calibrated by William Phelps to D65 which was 8300:1 IIRC. This machine was set so that shadow detail was visible. If he had set the black level (brightness) lower, he would have increased the contrast ratio, but at the expense of shadow detail. Setting the brightness too low means the contrast could be 80,000:1, but you will not be seeing the whole picture. If you then measured the visible detail you would find that the contrast ratio was closer to the smaller number.

    HTH

    Gary.
     
  3. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    There's even more to it than that. The co-ordinates for what colour red, green and blue should be in a PAL, NTSC and HDTV signal can be measured and so can the secondary Cyan, Magenta, Yellow colours to check that the colour decoding is actually correct. It's possble to have your blue filtered adjustment looking great but have way too much red in the mix........there are many parameters that are written down for how a display should be made to reproduce the many TV systems out there.......but it's along hard tedious job measuring them all and even longer and harder to explain the measurements to laymen afterwards...I'm still trying to get my head around it all.

    Gordon
     

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