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  1. #1
    BetterSense's Avatar
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    Additive color printing brainstorming

    This is a conceptual exercise, but I want input from experienced color printers.

    Suppose that instead of building a color pack with CMY filters, or using a dichroic head, I want to print with 3 separate exposures through R G B filters. Suppose I make an automatic shutter mechanism to make the 3 exposure sequence through the different filters for me. Since I am good at electronics, I make myself an enlarger timer to drive my color-exposure-sequencer. I can make the electronics do whatever I want. Now it's a question of what would be the best and most convenient for the printer:

    The scheme I am imagining, I would conceptualize the R G B exposures in terms of their relative ratio to one another, which I would use for color control. I would have my timer store this ratio of the 3 R G B exposures to each other, and then I could set the total exposure time be whatever I wanted. When I went to print, I would enter my desired exposure time...say, 10 seconds...and if I had set a color balance of 20%/50%/30%, I would automatically get 2s of R, 5s of green, and 3s of blue. If I changed the color balance, the total time would stay the same. Do you think this would be a smart system or would it be better to have the ability to individually increase the exposure time of one color while leaving the other colors' exposure times the same? With the scheme outlined above, increasing the amount of one color would automatically reduce the other colors' exposures to keep the total exposure time the same. Is that what you want when color printing?

    If I wanted to do something like this, what filters would I use (what specific Lee filter gels or kodak/wratten numbers).
    f/22 and be there.

  2. #2
    holmburgers's Avatar
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    I like your question! This thread might have some hints... (including the Wratten #'s)

    http://www.apug.org/forums/forum40/8...eet-paper.html
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  3. #3
    Greg Davis's Avatar
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    Kodak recommends Wratten 29, 47B, and 99 filters. PE recommends the Wratten 98, 99, and 70 filters for cleaner whites. You could also get the additive color head made for Beseler enlargers that does this for you.
    www.gregorytdavis.com

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  4. #4
    BetterSense's Avatar
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    Where can you buy these filters? I can't find them at Freestyle. I have a Lee/Roscoe gel swatchbook; do you think there is equivalent RGB filters in there I could use?
    f/22 and be there.

  5. #5

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    With the scheme outlined above, increasing the amount of one color would automatically reduce the other colors' exposures to keep the total exposure time the same. Is that what you want when color printing?
    Personally, I'd rather operate with the more traditional method, where the operator chooses plus or minus adjustments to CMY plus a "density" adjustment, all going plus or minus roughly in equivalent cc filter values. If you've never learned to work this way, though, the auto expos system might be better, I don't know. But since you'll be programming this into a system, I presume, why not just keep both options? It seems it would be a relatively minor programming change, given that you WILL need some exposure adjustment already. Certainly it will be easier than building in "slope control" adjustments, which I think you may also want. At least stay open to the possibility of building these in.

    I think that the more difficult parts of the job will be the hardware, setting up the light source(s), filter mounts, etc. I don't know how much you know about this, but here's a little background: The traditional industry method has been to use subtractive systems, where a single white light source is set up. There would be a mechanical shutter in the system, so that the lamp can operate in a low power mode, whilst not exposing the paper. Preparing to start the exposure, the lamp is switched to full power, where it very quickly reaches a stable operating temperature. Next, the shutter opens to begin the printing exposure, with white light. Somewhere in the light path are what they call "filter paddles", equipped with sharp-cutting cyan, magenta, and yellow dichroic filters. The filter paddles are essentially arms, attached to rotary solenoids, which hold one of these filters at the business end. So that the filters can be swing into, or out of, the light path. So the exposing method is: start with white light, essentially red+green+blue combined together. When enough of one color, say 2 seconds of blue light, is achieved, then the yellow filter paddle swings into the light path - this immediately terminates any blue light exposure, while still allowing red and green to come through. As the next color reaches it's aim exposure, say 3 seconds of green light, the magenta paddle enters the light path. Although magenta filtration only blocks green, the yellow filter (already in the beam) continues to block blue. Thus, only red light is coming through. When enough red expousre is reached, a cyan filter swings into the light path. At this point, all light is totally blocked, at least withing the limits of what the filters can do, and the mechanical shutter finally closes.

    If you consider how much light energy is being used during a full, color ajdusted exposure, you will probably see that the subtractive system is much, much more efficient at using light, and this is one main reason why it has been so much used in industry. Also, because it has been so commonly used, I'd expect that surplus parts - filter paddles, etc - might be found on the used market. Just something to keep in mind while you do your planning.

  6. #6
    Greg Davis's Avatar
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    You would need to have the spectral data of the Roscoe filter and compare it to the data of the Wratten filter. There is a guy in Florida selling the Wratten filters on ebay and you may find the ones you need. B&H will order any of the Wratten filters for you either from Kodak or Tiffen.
    www.gregorytdavis.com

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  7. #7
    Photo Engineer's Avatar
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    Well, I actually recommend the 98, 99 and 70 for purer colors due to the cleaner separation between the filters. However, the method in the OP is what was used in the original Kodak Color Printer series. It worked very well for years.

    It is inefficient and takes much longer exposures. It is also much harder to adjust color balance to the untrained eye. A special sensing system did this job in Kodak printers.

    PE

  8. #8
    holmburgers's Avatar
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    Check on eBay. You'll probably have to get gelatin filters, and if you wait long enough you'll be able to find a large lot for cheap. Buying them individually would be easier but is generally more expensive per filter. I got lucky and found a huge lot of 3"x3" gels with a lot of useful ones in it.

    #61 Green is also an option as is #25 Red (though perhaps too broad). I have a Kodak filter handbook which has extensive information about all the filters and if you find a large collection you'll find this book invaluable.
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  9. #9
    BetterSense's Avatar
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    I think that the more difficult parts of the job will be the hardware, setting up the light source(s), filter mounts, etc.
    Na, I already have it designed and half-built. It uses magnets to attach to the enlarger so I can switch it from my 4x5 to my 6x6 enlarger.

    start with white light, essentially red+green+blue combined together. When enough of one color, say 2 seconds of blue light, is achieved, then the yellow filter paddle swings into the light path - this immediately terminates any blue light exposure, while still allowing red and green to come through. As the next color reaches it's aim exposure, say 3 seconds of green light, the magenta paddle enters the light path. Although magenta filtration only blocks green, the yellow filter (already in the beam) continues to block blue. Thus, only red light is coming through. When enough red expousre is reached, a cyan filter swings into the light path. At this point, all light is totally blocked, at least withing the limits of what the filters can do, and the mechanical shutter finally closes.
    That is pretty smart. It sounds like it would give faster exposures, and it wouldn't be that much harder to build and program. But I heard that modern color paper is very fast anyway, almost inconveniently fast for enlarging; is that true? Plus, I have also heard that using sharp cutting R G B filters is an actual advantage in image quality due to better separation.

    It is also much harder to adjust color balance to the untrained eye.
    Do you mean using RGB additive printing will be harder to learn that subtractive printing? I don't have any subtractive printing to 'unlearn' if that's an advantage...
    f/22 and be there.

  10. #10

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    I'm in the middle of building RGB LED-based enlarger to try exact same idea. I've got all electronics done, and in the
    process of writing the firmware for the timer. The cool thing about RGB additive enlarger is that theoretically color
    filtration can be found with just one test strip. Fix one color, and run perpendicular test strips for the other two.
    Find intersection with the desired color, and you are done. I don't know yet how well this works in practice, and
    I can't wait until my enlarger is done. Anyway, I'm very interested to hear about your progress.

    Eugene.

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