CRT color lens filters? Can we add these to improve the picture?

Discussion in 'Archived Threads 2001-2004' started by RyanDinan, Mar 20, 2002.

  1. RyanDinan

    RyanDinan Stunt Coordinator

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    Hello everyone,
    I know Toshiba (and maybe Hitachi) allready have red/green lens filters with their sets, to improve the "purity" of the light the phosphor emits. However, I believe Sony and the majority of other manufacturers do not have such filters. Would it be possible to find such filters, and retrofit them to sets that do not allready have them? Are the filters on the Toshiba's separate, or are these filters "built-into" the actual mechanical lens assembly?
    I've looked online for filters, but the only hits I seemed to get were Toshiba's pages [​IMG]
    I know Guy Kuo mentioned doing this to a projector of his using certain gels, but I was wondering if there was some really good glass filters available?
    Thanks for any responses/comments!
    -Ryan Dinan
     
  2. JohnnyG

    JohnnyG Screenwriter

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    Ryan, the filters on Toshiba sets are built into the top part of the CRT assembly (the 'cap' to the coolant reservoir).

    I'd also be very interested in any sort of add-on. The Panasonic sets do not have filters, but according to March's Home Theater magazine, could certainly benefit.
     
  3. RyanDinan

    RyanDinan Stunt Coordinator

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    Hi John,
    I've been truing to get ahold of my local camera/photo shop here to see if they might have any good red/green gel filters that may do the job. The problem is, I have no idea what kind of filter to get. I do know that I want my red blue and green phosphors to match as closely as possible to the SMPTE-C phosphor standards per the 1931 CIE Chromaticity diagram. www.imagingscience.com has the actual xy coordinates that the phosphors are supposed to be.
    After measuring mine last night with my colorimeter, mine were off by a fair margin - enough to investigate this further (I have a Sony HS10 by the way). I just don't know if a "red" filter would help bring the red closer, or bring it farther away from the SMPTE-C standard coordinate.
    Apparently, typical consumer red and green phosphors have too much yellow in them - the Reds are actually red-orange, and the greens are green-yellow. And now that I *look* at the reds and greens, I can easily see that they are definatly not as pure as they could be (damn this forum! [​IMG]) I would think that any good red/green filter would help filter out some of that unwanted yellow, which is really what we want to do.
    However, I've read that in doing this color-correction with filters/gels, you lose contrast, due to lower light output. We have to find the right intensity filters...
    Doing this should also help grayscale accuracy and tracking.
    -Ryan Dinan
     
  4. Guy Kuo

    Guy Kuo Supporting Actor

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    It won't and shouldn't do anything to help grayscale and tracking. Those are grayscale issues which you should try to keep separate as you consider the addition of filters to shift the primary chromaticities.

    You probably won't find any ready made color filters or gels which are exactly the color needed. But there is a way out as I'll mention later. Before we get any further, this is something which is likely to confuse a lot of people. We are barely able to get most to grasp the difference between grayscale and color decoding. With the addition of correction of shifting the primary chromaticities, a lot of people are simply going to be left in the rubble of confusion.

    Here's the deal. You have three color guns, each of which produce a primary color of a cerain chromaticity (xy). Those three are mixed together to form the final image. The particular balance of those primaries is made to achieve D65 via the grayscale adjustment of the display (cuts & gains). The colorization info from the color difference signals are used to alter how far the guns deviate from that underyling grayscale image to create colors. The variable we are about to add is that the color (xy coordinate) of those CRT phosphors aren't exactly right so colors end up a bit off even with perfect color decoding and grayscale.

    So, you color correct the guns with filters which shift the chromaticities closer to the actual desired values. Then you readjust gray scale to once more achieve D65 with the corrected primaries. Then with correct color decoding, correct grayscale, and correct primaries your set delivers the color actually encoded in the video signal.

    The red gun is very often too green and the result is something which is more orange for the red primary. Green can also be off, but I'd shy away from correcting green because optical clarity of the green gun is so vital. Adding a filter to the red will drop the clarity of the red subimage, but won't be as noticable as fogging the green image with a filter.

    The filters you will be able to find are not going to be really great optical quality. They won't even have exactly the density and coloration you need. So I'd concentrate only on the red gun. Go to your theatrical lighting store and obtain several lighting gel sheets which are in the light magenta to antique rose colors. You'll need a variety to find one which is usable. The darkness or intensity of the filter will actually be surprisingly low. In fact, you'll they are probably too much filtration and be stuck. That is unless you take advantage of the fact that a filter placed near the front surface of the lens won't cast a sharp image of itself on screen. You can punch multiple small holes about 1/4 in diameter into the filter gel to reduce its effective intensity. Evenutally, after much experimentation you'll get the red primary to be less orange. With your colorimeter it will be a LOT easier.

    Then you have to redo grayscale, by REDUCING green and blue drives. Don't try to compensate for the red filter reducing red gun light output by jacking up the red gun a lot. Just a little maybe, but it is better to drop the greend and blue to match the new lower red intensity. Adjust for D65 and go check out how red in images look now.

    NOT by any means QED. It takes an enormous amount of time and effort. I've done it for myself, but I would consider this a tweak only for someone who already knows how to adjust grayscale correctly and has their color decoder already free of red push. The end result is a richness to red that is otherwise impossible to achieve, but light output is lower. This is not something for the weekend DIY'er to casually attempt. You have to know what you are doing to the color space or the end picture will be pretty awful.
     

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