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  1. #31
    Donald Qualls's Avatar
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    Hmmm.

    I think there may still be too many engineers involved, here.

    What, if I might be so bold as to ask, is wrong with independent timers for the blue and green LED circuits, as opposed to megahertz frequency pulse width modulation to emulate a dimmer? You want Grade 4, you look up Grade 4 on the chart (calibrated according to paper curves and LED output, or by printing actual tests), dial up 30 seconds green, 5 seconds blue (to pick numbers out of the air) and hit "start" (and this could be done with two of the small enlarging timers wired to a single start switch, just to keep things really simple and completely analog -- you're only switching a couple hundred milliamps, after all). When all the lights go out, put the paper in the developer.

    As for Huw not getting a full Grade 5 -- I'd guess this is because the color of the blue LED isn't quite right. If the blue overlaps the sensitivity range of the low-contrast, green sensitive component of the paper emulsion, you'll get some response from the lower contrast even at 100% blue. The solution here is probably to get LEDs with shorter wavelength light, such as the "royal blue" mentioned above, or else to filter the blue LEDs through a full cutoff magenta printing filter to eliminate the "yellow" portion of their spectrum. No LED is a true "line" emission like a gas discharge; they have a continuum spectrum, which can be very narrow at low levels of emission, but tends to broaden at higher power outputs (I'm told). If the blue LED emits too much longer wavelength, "greener" light, it would benefit from such filtration for the hardest grade...
    Photography has always fascinated me -- as a child, simply for the magic of capturing an image onto glossy paper with a little box, but as an adult because of the unique juxtaposition of science and art -- the physics of optics, the mechanics of the camera, the chemistry of film and developer, alongside the art in seeing, composing, exposing, processing and printing.

  2. #32
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    That would make it work like the Minolta Beseler 45A head in it's additive timer functions. I would admit that your setup would be easier to use than the 45A, it's a royal pain trying to figure where it's going. But when you do get the hang of it, it works very well. I suspect using timers set on seconds and minutes would be a lot more intuitive than the 0-255 setup the 45A used, as well as less limited for longer print times.
    Gary Beasley

  3. #33
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    The effective brightness of a pulse modulated led should vary pretty much linearly with the duty cycle, the ratio of on time to total time between on's. The flashing can cause visual problems as it reaches some critical rate that varies with age. I'll have to look that up. I think the flicker frequency increases with age. If you start to get migraines when you see flicker, make the pulse frequency higher.

    I think some printing materials do not integrate perfectly with short on periods. These are not unsurmountable problems, as they have apparently been solved for some systems. I think the repetition rate should be rapid enough not to cause visual problems and slow enough not to cause reciprocity (excuse the dirty word) problems. You can vary the effective output enough at some constant pules rate to satisfy most criteria, I think.

    Of course, visual problems can be dodged by having continious light for focusing.
    Gadget Gainer

  4. #34
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    I forgot the possibility of having blue on when green is off. You may not see flicker then, I don't know, but a single knob would set blue-green ratio. Did someone already mention this? My flicker frequency is quite high, so maybe I'm too old.
    Gadget Gainer

  5. #35
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    Quote Originally Posted by JMoore
    Just wondering how those of you who are building an LED light head are coming along with your projects...
    At this moment, my 'darkroom', aka garage, is at a toasty 21F, a good 20-25 degrees higher than it has been for a few weeks. Needless to say, I'm not done. A darkroom with heat and a sink is in the works

    I've built a mixing box that works well for 6x7 , and a full size one for the Durst 138 (5x7). According to an Ilford EM10, illumination is within .2EV for a 4x5 with more falloff on the edges for 5x7. I need to improve the mixing box a bit. With temperatures where they are, I'm not testing with negatives and paper yet. I will definately need more light than the LEDs I have now for the 5x7 head, but I have not figured out the necessary increase in lumens yet.
    Jonathan
    -----------------------------------------------

  6. #36

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    The suggestion to use varying times for blue and green separately is a worthy consideration particularly for people familiar with split grade printing. However, it would also be nice to dogde and burn at specific grade values where both lamps are on in proportion needed to generate the grade.

  7. #37

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    Patrick wrote, "Of course, visual problems can be dodged by having continious light for focusing."

    So how about adding in a few red LED and leaving that on during the entire expsoure time to cut down on the flickering. It should not affect the paper and should help with focussing.

  8. #38
    Huw
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    Hi all, I am most flattered. Just to clear up a couple of points that might not be clear on my website.

    1. The 5x4 head uses royal blue LEDs, as does the 6x6 one now.

    2. The PWM is converted into linear current, no flicker.

    3. 'Give an engineer a project he throws a processor at it', true but it also communicates with the RH Analyser, saves me forgetting.

    4. I have done a unit for someone else (it's on pnet) with PWM, seems to make no difference.

    Feel free to mail me if you have any questions.

  9. #39
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    Hello Huw, not being an EE I got stalled on the schematics and part numbers etc.. I did some research with the LED manufacturers, it's as though they didn't want any LED's put into use with enlarger heads. If I could only figure it out I would still like to make a 5x7 inch size light source. If it could be dummied down, never thought I would use that term, I would enjoy building the unit. Any other people interested in a split grade filtering head using LED's that could be built and adapted to, say a Beseler or other 4x5 enlarger?
    Everytime I find a film or paper that I like, they discontinue it. - Paul Strand - Aperture monograph on Strand

  10. #40

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    Yes, I'd love something that could be attached to a Beseler 45. Trouble is that I'm not very good with electronics. I could solder robust components in place but very fine soldering would be trouble for me. Once the electronic side is done I would be fine with making it fit the Beseler.
    It would be nice to have something that was modular. I want to build an 8x10 enlarger and it would be nice if I could just connect 4 4x5 boards together to make an 8x10 board. Of course that will have to wait until the price of LEDs drops a lot more.

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