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CRT screen calcs. ANSI or Peak?

Discussion in 'Projectors, Screens & Video Processors' started by Nic Rhodes, Sep 30, 2001.

  1. Nic Rhodes

    Nic Rhodes
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    Guys and Girls

    Can people have a final look over my calculations before I buy the wrong screen. I think I know what I want but felt it prudent to double check the thinking. Stewart's are expensive and I therefore don't want to make a mistake.

    The projector coming is a CRT based unit. It is quoted as 1000 lumens with new tubes. Mine is a good second hand unit.

    Gordon's excellent screen article on the site describes what to look for in screens and the calculations necessary for the right brightness.

    If I was going for a panel projector then the ANSI lumens figure and the Lumens figure would be similar. This is because these projectors use a high intensity lamp that burns at a constant brightness.

    A CRT projector works differently. It has two brightness specs. ANSI and Peak. Light in my projector comes from a beam of electrons hitting a phosphor coating. The primary limitation of the brightness is the current that this ‘beam' can pull. A full white screen will have a much lower brightness that a 10% white block on a black background. Hence the difference between the two figures. The ANSI (lower figure) is based on the whole screen and the Peak is based on a 10% of the face of the CRT having the bright area. There seems to be 5:1 or 6:1 ratio between the two figures for most projectors. This would make a 1000 lumens CRT projector have a 167 to 200 ANSI Lumens figure. I have used a 185 average.

    I therefore do the calculations based on:

    (screen gain) x (Projector output in ANSI Lumens)/ Sqft of screen

    This gives a ftL figure. Foot Lamberts. TVs are normally 32-35, a cinema 11-12 and a room where you can control all light it needs to be at least 8ftL. (Like the projectors proposed location).

    Well I have done the calculations using 185 ANSI, 1.3 screen on the two screen sizes I am interested in and get:

    72 inch wide 8.9 ftL<br />80 inch wide 7.2 ftL

    this looks boarder line re the recommended 8 ftL min. I have however seen this projector on a 100 inch screen (from memory) and that would give 4.6 ftL. It looked great and far from dull. In fact I thought it looked great.

    I think my calculations are fine re &#8216;the letter of the law' but I am questioning the wisdom of using the ANSI figure for CRT calculations. With films, surely the Lumens figure is far more accurate re brightness and the ANSI figure would only be for &#8216;Power Point presentation' stuff. If so and I put a 1000 Lumens figure into the calculations instead of 185 I get

    72 inch wide 48.1 ftL<br />80 inch wide 39.0 ftL

    and at 100 inch

    100 inch wide 25.0 ftL

    Now everyone at the Event saw the Cine7 (a 1000 lumens projector (185 ANSI)) on the same 100 inch wide screen and I can't ever remember anyone commenting on it was too dull. In fact everyone loved it.

    I think I will be going for a 80 inch wide 16:9 screen and I am not too worried that it will be too dull, I just can't make the calculations tally with what I have seen! What figure should I use for calculations?
     
  2. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    Well Dominic,

    I think your comment about the Event was telling. The 8ft Lamberts thing is a guide only. What happens when light output gets low though is that you seem to loose the POP of the picture. Hard to descibe but the CineMAX iamge really loooked 3D when you were at the back of the big room at the Event while the Cine7 only pulled that off on a few scenes.

    I've just run the Stewart screens programme for a NECPGextra (not sure the difference in light output to extra spec). At 72" wide 16:9 aspect screen, using Studiotek130 they say you'll achieve 9.52ftLamberts. 80" wide they say 7.71ft Lamberts.

    As you say, the 9 in the shop doesn't exactly look dim! See you tomorrow.

    Gordon
     
  3. Roland @ B4

    Roland @ B4
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    Oh if only we could control those little iris things in our eyes it would be so simple.

    For those who haven't read it yet, Gordon's screen article is on his webb site.<a href="http://www.convergent-av.co.uk/pr01.htm" target="_blank">Convergent-av.co.uk</a>

    You do need to use the ANSI measurement for the calcs to work properly but I recon 6 ft/lamberts is still a stonking good picture.
     
  4. Nic Rhodes

    Nic Rhodes
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  5. Nic Rhodes

    Nic Rhodes
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    One other thing that comes to mind with a CRT and light output.

    Many HCPC owners recommend 72 Hz at the best frequency to drive the projector at as it is exactly 3 x the film speed. It minimises judder as was demonstrated very well at the event. If the CRT is run at these frequency, then the electron is scanned across the phoshpor quicker, thus refreshing the picture more often. Surely this aslo produces more light from The CRT. If this is so, then are all light measurements done at standard 50 / 60 hz frequencies?

    Isn't this a good way to get more light output (at the expense of tube life?)?
     
  6. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    If it were me I'd go for a 7ft wide image and look to sit 12-14ft from the screen. (16:9 aspect ratio). Further away may even be better.

    Re higher refersh rates. I think you're right about increased light output but there's also the problem that as the phosphor can't decay fast enough before being hit again you may loose detail in some scenes where there's motion. Depends on the phosphor in the tubes I guess. When I get a real nutter unit in I'll try this experiment and see what happens.

    Gordon
     
  7. Nic Rhodes

    Nic Rhodes
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    Just for Reference the BarcoMax is a 9 inch unit, 1200 Lumens, 300 ANSI Lumens. This gives a 'POPPING' 7.5 ftL on the 100 inch StereoStereo screen. Now everyone loved that.

    Assuming my projector is a little less bright than a new unit is capable of....

    I feel a sleepless night coming on!
     
  8. Jeff

    Jeff
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    It looks that Don Stewart is doing well out of the event. Maybe he will make a personal appearence at the next one. <img src="smile.gif" border="0">

    Jeff
     
  9. Roland @ B4

    Roland @ B4
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  10. Nic Rhodes

    Nic Rhodes
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