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  1. #1

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    difference in LED or Dichro filters

    Curiosity strikes!
    When split grade printing with a color head the recommended filtration is max Y & C. When constructing a head, mention is made of Blue and Green LEDs. Why the difference?
    Heavily sedated for your protection.

  2. #2
    RH Designs's Avatar
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    VC papers are sensitive only to blue and green light. A yellow filter removes blue light, leaving green (and the rest) so is equivalent to a green LED, and magenta removes green leaving blue. One is an additive system (LED), the other subtractive, but the result is similar.
    Regards,
    Richard.

    RH Designs - My Photography

  3. #3

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    It had to be that simple didn't it.
    Thank You
    Heavily sedated for your protection.

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    Bob F.'s Avatar
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    Added point: green light activates the lower contrast component and blue light the harder contrast component of the emulsion. Hence, a mixture of the two provides the variability of the contrast from hard to soft.
    Last edited by Bob F.; 03-20-2009 at 11:47 AM. Click to view previous post history. Reason: Double-checked...

  5. #5
    MattKing's Avatar
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    If you turn your cyan filter to max, you can pretend you are using an additive system .

    I sometimes do this, when I need longer exposure times.

    Matt

  6. #6
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    Quote Originally Posted by MattKing View Post
    If you turn your cyan filter to max, you can pretend you are using an additive system .

    I sometimes do this, when I need longer exposure times.

    Matt
    ?

    I believe, all this does is add density. Cyan filtration removes equal amounts of green and blue light.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  7. #7

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    Quote Originally Posted by RalphLambrecht View Post
    ?

    I believe, all this does is add density. Cyan filtration removes equal amounts of green and blue light.
    An ideal cyan filter (there is no such thing) will have no effect. It transmits all the blue and green but blocks the red. In general, the magenta (for blue) and yellow (for green) controls on the dichro head are all you need to control VC paper contrast.

    For LED heads it's a different story. They use red, green, and blue lights in an additive fashion, but the controls may be set up to imitate the familiar magenta, yellow, and cyan. In that case, the cyan control sets the brightness of both the blue and green lamps, the yellow control sets the brightness of the green and red lamps, and magenta sets the red and blue lamps. You should be able to use the yellow and magenta controls pretty much like the dichro controls to change contrast, but the cyan control will add exposure to both the high and low contrast emulsions. In any case, the light would be different than what you get from a dichro head, so some calibration will be needed.

  8. #8
    MattKing's Avatar
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    Quote Originally Posted by RalphLambrecht View Post
    ?

    I believe, all this does is add density. Cyan filtration removes equal amounts of green and blue light.
    Actually Ralph, the cyan filter removes the complements to green and blue - it removes magenta and yellow.

    But you are right, in combination with the magenta and yellow filters it does add neutral density.

    Oh, and "Way Beyond Monochrome" is one of my favorite references (along with the tables from your website) .

    Matt

  9. #9

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    Some basic color theory:

    Visible light is often described as consisting of three primary colors: red, green, and blue. In reality, of course, it's a continuum of wavelengths; red, green, and blue are simply labels we apply to particular ranges of wavelengths. The RGB distinction is good enough for many purposes, though.

    The primary colors can be combined (added) in various ways:

    • red + green = yellow
    • red + blue = magenta
    • green + blue = cyan
    • red + green + blue = white


    Note that these are additive colors. You'd get these colors if you had LEDs of these colors (or other light sources with appropriate filters in front of them) and mixed the light from these LEDs. This is how TVs and computer monitors work, incidentally. What gets confusing, and has resulted in some incorrect information posted above, is that filters work on a subtractive system. In a subtractive system, you start with white light and can subtract colors, which produces the color name we apply to the filter:

    • white - red = cyan
    • white - green = magenta
    • white - blue = yellow
    • white - red - green = blue
    • white - red - blue = green
    • white - green - blue = red


    When using an additive enlarger (something with RGB LEDs or other light sources with RGB filters), the mixture of red, green, and blue lights is handled more-or-less directly by controlling the relative brightness of these three colors. When using most enlargers with cyan, magenta, and yellow filters, a white light source's red, green, and blue wavelengths are blocked by the cyan, magenta, and blue filters, respectively. The two systems are theoretically identical, although of course imperfections in the filters and the spectral sensitivity curves of the papers may result in some deviations from a theoretical ideal. For instance, theoretically adding cyan filtration will have absolutely no effect on most B&W enlarging papers, since cyan filtration removes red light (not green and blue light, as incorrectly stated in one earlier post), and B&W papers are insensitive to red light. In practice there may be some small effect of cyan filtration because the filter might block a little green and/or blue light or because the paper may have a small sensitivity to the red light that's blocked by the filter.

    FWIW, I own a Philips PCS130 enlarger with PCS150 color controller. This combination uses three halogen bulbs with red, green, and blue filters in front of them. The color controller lets me adjust the brightness of the three primary colors independently of one another. This works differently from the system that nworth describes ("the cyan control sets the brightness of both the blue and green lamps...", etc.); the "red/cyan" control adjusts the brightness of the red lamp, the "green/magenta" control adjusts the brightness of the green lamp, and the "blue/yellow" control adjusts the brightness of the blue lamp. I found that using this system makes it easier for me to understand how the emulsions respond -- but I'm not sure if that's really the system or if how it all works just finally "clicked" with me at the time I bought this enlarger.

  10. #10
    RalphLambrecht's Avatar
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    Quote Originally Posted by MattKing View Post
    Actually Ralph, the cyan filter removes the complements to green and blue - it removes magenta and yellow.

    But you are right, in combination with the magenta and yellow filters it does add neutral density.

    Oh, and "Way Beyond Monochrome" is one of my favorite references (along with the tables from your website) .

    Matt
    You, nworth and srs5694 are right, of course. The cyan filter removes red light and transmits blue and green. However, my logic was as follows:

    If B&W paper is only sensitive to blue and green light, and dialing-in cyan requires longer exposure times without changing the contrast, then cyan must remove blue and green light at equal amounts. A strong cyan filter transmits less blue and green than a weak cyan filter, correct?
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

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