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  1. #21
    RH Designs's Avatar
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    Ilford publishes the spectral response curve for Multigrade IV papers in its data sheet, here.
    Regards,
    Richard.

    RH Designs - My Photography

  2. #22
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    Quote Originally Posted by RH Designs View Post
    Ilford publishes the spectral response curve for Multigrade IV papers in its data sheet, here.
    Unfortunately, the curves for B and G sensitive parts have been combined to create a "total" spectral response curve.

    However, we can deduce that 500-520 nm for green should be spot-on. Do you agree?

    Typical cheap power RGB led has green at about 520 nm and blue at about 460 nm (an example: http://www.satisled.com/3w-rgb-led-e...olor_p234.html ). However as I have shown above, the bandwidths are not that sharp, thus the G and B in typical RGB leds almost overlap.

    AFAIK, blue exposure causes both blue- and green-sensitive layers to react in VC papers (in the other words, they have only-blue-sensitive and blue-and-green-sensitive layers). Thus, if our green is "green enough" not to expose the blue layer, and our blue is "blue enough" to expose the blue layer, it should work and give us all contrast grades from 00 to 5. I said earlier as an educated guess that the green in typical leds is probably spot-on, and this seems to be backed up by the evidence.
    Last edited by hrst; 04-17-2011 at 11:07 PM. Click to view previous post history.

  3. #23
    Steve Smith's Avatar
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    Quote Originally Posted by hrst View Post
    AFAIK, blue exposure causes both blue- and green-sensitive layers to react in VC papers (in the other words, they have only-blue-sensitive and blue-and-green-sensitive layers).
    It's the other way round.

    All of the emulsions are sensitive to blue and some are sensitive to blue and green.

    http://www.ilfordphoto.com/Webfiles/...8932591755.pdf


    Steve.

  4. #24
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    Quote Originally Posted by ic-racer View Post
    Or even better, a 120v halogen lamp and non-fading dichroic filters
    Or even better one to three 120volt, correct wattage, correct color temperature halogen bulbs behind either RBG or CMY Dichroic filters that project into a Styrofoam lined tube with two Plexiglas diffusers.

    Each "channel" can be use independently as in split grade or VC paper application or all three can be used for focus or white light with VC filters under the lens.

    Run it off a CVS and put a fan in a box and run the hose to the head with a light tight baffle on the air intake side.

    The possibility of industrial or commercial dimmers is a possibility also.

    I'm nearly finished with this type of head for my Durst 138 enlarger. It's a diffusion head with a cooling fan as compared to a cold light diffusion head using a tube or tubes. I'm making a 5x7 head right now and will visit an 8x10 head a little later.

    For my negative carrier; I really never liked glass; I'm using a "Zipper" glassless carrier. It mounts a 5x7 negative without scratching the hell out of the edges like the Beseler Negaflat does. I can also put a 4X5 negative in at 90 degrees and use the enlargers masking system, a great feature of the Durst enlargers.

    For 8x10 I'd make one just like the Zipper carrier.

    Best,

    Curt "the 'ive had it with out of date bulbs, heads, lights and lamps that are problematic."

    What kind of light source did that fellow in California by the sea use on his big converted studio camera enlarger? Old technology? So what? It worked pretty well for him and he had a nice Stetson hat to boot.
    Everytime I find a film or paper that I like, they discontinue it. - Paul Strand - Aperture monograph on Strand

  5. #25

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    Quote Originally Posted by hrst View Post
    Unfortunately, the curves for B and G sensitive parts have been combined to create a "total" spectral response curve.

    However, we can deduce that 500-520 nm for green should be spot-on. Do you agree?

    Typical cheap power RGB led has green at about 520 nm and blue at about 460 nm (an example: http://www.satisled.com/3w-rgb-led-e...olor_p234.html ). However as I have shown above, the bandwidths are not that sharp, thus the G and B in typical RGB leds almost overlap.

    AFAIK, blue exposure causes both blue- and green-sensitive layers to react in VC papers (in the other words, they have only-blue-sensitive and blue-and-green-sensitive layers). Thus, if our green is "green enough" not to expose the blue layer, and our blue is "blue enough" to expose the blue layer, it should work and give us all contrast grades from 00 to 5. I said earlier as an educated guess that the green in typical leds is probably spot-on, and this seems to be backed up by the evidence.
    hrst said what I was trying to say but didn't quite say it right, or to put it a different way, the spectral response curves and contrast curves for the two layers are not available separately.

    As to the choice of green LED, you should probably use an LED with a center wavelength closer to 530 nm to be sure to not be in the toe of the blue curve while at the same time being fairly close to the peak sensitivity for the green layer. As to the blue LED, it is hard to say from the combined graph. However, I think an LED with a center wavelength of roughly 425 might be a good guess, or maybe even something close to 415. This would likely keep you away from the toe of the green curve while being close to the peak intensity of the blue curve.

  6. #26

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    On second thought, taking a ruler to the paper and measuring the vertical distance between the curves, I think a blue LED of about 430 nm might be a little better than the values I gave earlier. This is about where there is a minimum vertical distance between the curves. This assumes that the curves are drawn fairly accurately, a possibly dubious assumption. However, if you had to shade the wavelength one way or another it should probably be toward 420 nm in order to stay away from the toe of the green emulsion.

    For the green curve I think 520 nm might be pretty good, but if you had to shade that value one way or the other I would shade it more toward 530 in order to stay away from the toe of the blue emulsion.

  7. #27
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    Quote Originally Posted by Steve Smith View Post
    It's the other way round.

    All of the emulsions are sensitive to blue and some are sensitive to blue and green.
    That is exactly what I said and I know it very well.

  8. #28
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    Quote Originally Posted by hrst View Post
    That is exactly what I said and I know it very well.
    Looking back, you did say that but you said the reverse a few lines earlier!


    Steve.

  9. #29
    RH Designs's Avatar
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    Quote Originally Posted by hrst View Post
    we can deduce that 500-520 nm for green should be spot-on. Do you agree?
    Yes, green is not a problem for an LED enlarger, although I do have a couple of RGB LEDs in which the blue is correct at 430nm but the green wavelength is too long at 560nm. 525nm LEDs work very well. Normal 470nm blue LEDs won't achieve much better than about grade 4 on Ilford MGIV without extra filtration. I've not been able to find any easily available 430nm power LEDs as yet.
    Regards,
    Richard.

    RH Designs - My Photography

  10. #30
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    Quote Originally Posted by Curt View Post
    Or even better one to three 120volt, correct wattage, correct color temperature halogen bulbs behind either RBG or CMY Dichroic filters that project into a Styrofoam lined tube with two Plexiglas diffusers.
    Sounds like the makings of a good thread (with pictures !)

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