How does a front projector work?

Discussion in 'Displays' started by DanielKellmii, Feb 7, 2005.

  1. DanielKellmii

    DanielKellmii Supporting Actor

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    Does anybody have a link explaining how a little front projector can make such a great image. Is it analagous to film? e.g. An image is created on a small screen, and a powerful light and some optics projects it on a surface.
     
  2. Leo Kerr

    Leo Kerr Screenwriter

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    In a very generalized nutshell, that's exactly what's happening.

    The devil is in the details.

    CRT front projectors have three 6-9" CRTs with colored filters, and project the RGB that way. THey've a lot of quirks, but once they're set up, they can give you a fabulous image.

    LCD front projectors (now) split the light into three colors, pass them through three (typically) ~2" panels, recombine the image through a prism array, and ship it out the lens.

    DLP projectors, have the mirrors doing pulse-width-modulation of the light, so the actual D-A is done by your eye. Single-chip ones do sequential color with a filter wheel spinning very quickly. This is inexpensive, but it also gives rise to the 'Rainbow effect' - because at any given time, only one color is being projected, and if your eyes move, the image breaks. Three-chippers (much more expensive,) don't do the color-wheel thing.

    D-ILA / LCOS projectors use Reflective LCDs.

    There are other, less common technologies, but those are the big 4 right now. More to come... sometime.

    Leo
     
  3. ChrisWiggles

    ChrisWiggles Producer

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    not all are filtered, in fact relatively few are. Filtering can improve the primary chromaticities for accurate color gamut, but is not essential for a CRT.
     
  4. Brian Gi

    Brian Gi Stunt Coordinator

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  5. Leo Kerr

    Leo Kerr Screenwriter

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    Chris,

    are you suggesting that some CRT projectors are using blue, red, or green phosphores?

    That seems really.... strange? I thought the general tendancy was to use a high-efficiency 'white' phosophore and color it in the lens...

    Leo
     
  6. ChrisWiggles

    ChrisWiggles Producer

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    Leo!

    They all use colored phosphors. White phosphors are incredibly rare, I don't know of really any normal display that uses white phosphors, other than perhaps specialty displays or old monochrome displays. If you are talking about CRT projection, yes each tube's phosphor emits a certain color, just like the colored phosphors on a direct view CRT. In *addition* some CRTs use colored lens elements, or colored glycol coolant to achieve accurate color primaries, but the phosphors still emit a specific color of light. If you replace a tube in a CRT display, you need to specify which one it is, because yes the phosphor itself emits a specific color. The tubes themselves are fairly interesting, and the phosphor coatings can also be designed for specific applications, like a fast-green P43 tube for 3d applications, or other fast-refresh simulation needs.

    If you have a CRT projection display, it is easy to see whether, and which tubes have color filtering added (usually just R and/or G), by looking in the lenses with the set off. If the lenses or glycol are not filtered, you will see the phosphor as white(when not emitting light), if they are filtered, you will clearly see the color of the filter.
     
  7. Leo Kerr

    Leo Kerr Screenwriter

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    Yeah, it wasn't until after I had moved on that it occurred to me, it's more efficient using phosophores, anyway...

    Leo
     

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