Blue Tube CRT projector question

Discussion in 'Archived Threads 2001-2004' started by Leo Kerr, Mar 11, 2002.

  1. Leo Kerr

    Leo Kerr Screenwriter

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    Okay, in my grand experience of CRT projector maintenance, every projector I've ever gotten my hands on to rework, recalibrate, realign, re-etcetera, has had a couple of common issues.
    One is, they're old. Not particularly calander age, but power on / video displayed hours on the tubes. (Considering that where I'm working now, we put about 3000 hours per year on them. They tend to get serviced every couple of years. Minimum hour-count for the last three I've worked on is in the 10k to 15k hour range. Yes, I really do mean 15,000 hours.)
    That isn't my question, but it is the lead-up to my question.
    Every CRT projector I've worked on has been impossible to get sharp focus on the Blue tube. Sometimes, it's close. Other times, it's clearly yucky.
    Is there something magical about blue CRTs that are designed to be out of focus, or do they age faster, or have I just been lucky?
    (by the way, this grand experience totals seven different CRT projectors; not just one or two.)
    Leo Kerr
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  2. Vince Maskeeper

    Vince Maskeeper Producer

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    Even on brand new CRT projectors, you will find this problem. It has to do with how our eyes percieve BLUE, and while I can't quote all the specifics of the physiology- long story short is:
    How our eye receives the color BLUE (and actually all high freq light, Blue, Indigo, Violet- I believe) and where it focuses it in the back of our optics, we are unable to see it in crisp focus.
    Here's a blurb I stole from AVS:
     
  3. Guy Kuo

    Guy Kuo Supporting Actor

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    The blue CRT has to be driven the hardest to keep up with the other two and produce an acceptable white balance. This ages that tube faster and contributes to that gun seeming to always look less focused. However, that isn't the whole story for you will see that on new projectors blue is also blurred, primarily because it is intentionally defocused. By defocusing the blue gun, about 20 to 30% higher blue light output can be squeezed out of the blue gun. That means the overall picture can be brighter without running out of blue. This is a reasonably good trade off because humans are fairly poor at seeing fine details in blue. The blurring of blue does not blur the overall image as much as might be expected.
     

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