Discussion in 'Darkroom Developing and Printing' started by Bujor, Aug 22, 2003.

  1. Bujor

    Bujor Guest

    As far as I can observe, all consumer enlargers work in CYM. I built one which
    works with three sources of light in RGB. My analyzer reads CYM. DOes anybody
    have any idea on a simple way of equivalency?

    Thank you,
    the rookie
    Bujor, Aug 22, 2003
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  2. Bujor

    Dan Dunphy Guest

    Using primary (RGB) filters, you must make 3 exposures, one of each
    color. If you put all three filters in the light source, you turn
    white into gray. I believe this is the way the Minolta works.

    Colorado Springs, CO
    My advice may be worth what you paid for it.
    Dan Dunphy, Aug 28, 2003
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  3. Bujor

    Bujor Guest

    Thanks for your input.

    I know what happens when you mix filters on the same light source. What I
    didn't tell you is that I have three different sources, each having it's own
    filter (25, 58 and 47B. If you are interested, I have the exact frequencies and
    %Ts, as I scanned the filters on my UV-VIS spectrometer). I think this detail
    changes basis for discussion a bit... However, my mean color analyzer provides
    readings only in CYM.

    If you have color related experience (as I don't have ANY, and obviously I
    didn't read the right articles yet), help me interpret the readings in terms of
    time of exposure given to the three sources.

    Simplier put: could you please explain what happens in two different instances,
    when you have two readings, say, 90Y and 10Y on your Y pontentiometer after you
    zero the analyzer. You dial those values on your enlarger head. Which setting
    appears more yellow when reflected on your white enlarger table: the one for
    dialing 90Y or the one for 10Y? Maybe I can then figure out how long must I
    expose (or correctly cut exposure, because Yellow is negative Blue) with the
    blue source. And so on...

    I can't think what the final outcome would look like. Maybe I'll get a negative
    from a negative... I don't see it now... In film you have three layers sensible
    to CYM. Nevertheless, your camera exposes it to RGB light, and you get a
    negative... I have to read more on this.

    I know, it seems complicated, but I'm sure I can get it in the end. Afterall,
    this is the way the process should have been physically done, but people
    engineered it more economically.

    the rookie
    Bujor, Aug 28, 2003
  4. Bujor

    fnovau Guest

    I understand you are mixing RGB additive and CMY subtractive printing
    systems. If you have RGB lights sources forget about measuring CMY
    with your color analyser. I believe you could try to measure RGB on
    the easel and to compute directly your RGB exposure times, the way
    modern automatic color printing systems do. For this you must measure
    LATD (large area transmission density) of your negative and apply a
    simple equation to compute your RGB exposure times. Equation is:
    log(t/tref)=S*(LATD-LATDref) where t is exposure time, tref is
    exposure time of reference negative, S is slope constant, LATD is
    density of negative to be printed and LATDref is density of reference
    negative. If interested in this I can describe process.
    I got your e-mail with some details of your system. You control power
    trough triac but it's not clear for me if you pretend to control power
    applied to light source and exposure time or both. Take care because
    if you apply a variable power to your light source you will get
    different color temperatures and so different ratios of blue to red
    -Francesc Novau
    fnovau, Aug 29, 2003
  5. Bujor

    Bujor Guest

    First to answer your last question: I can control the voltage of each source
    separately (as you said, to a certain extend, because of bulbs color shift),
    and I may control the time of exposure for each source too, through simple 555
    circuitry timers (I don't want to introduce this though, unless I have to). The
    filters that I use now have transmission patterns pretty much juxtaposed on
    film sensitivities ( if Blue for film is 80%, then filter will give a 20%T).

    Sure I would like to knor the process with LATD, please send it if you can

    the rookie
    Bujor, Aug 29, 2003
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