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LED Variable Contrast Diffusion Head - How to space the LEDs for even light

  1. L Gebhardt
    I'm going to be making a 5x7 variable contrast diffusion head for my Durst 138S. I've got some LEDs and drivers on order to experiment with. I've built a white light version for a different enlarger, and it worked pretty well. But I had trouble getting the light to be even because I didn't know exactly how to space the LEDs. To make it work I ended up making a mask for above the diffuser to dim down the center, which overall cut down on the light possible.

    I'm looking for ideas on how to arrange the LEDs to get as even of light as possible. An additional constraint on this is it must fit in the condenser slots on the head, which are a hair over 2 inches tall. I figure the top will have the lights and heat sink, and the bottom slot will hold the diffuser. So I will have about 2 inches of space between the LEDs and the diffuser. Power and the controller will be external (an arduino controlled f/stop timer is the plan). Based on the controllers and LEDs I ordered I will have 12 to 13 of each color to play with. The more I can fit in there the brighter it can be. The controllers will allow dimming so I'm not worried about it being too bright.

    Any ideas on how to model this in the computer, or even on paper, to determine the best arrangement of the two colors of LEDs for evenness of both, and to maximize the light? Trial and error with mounting the real LEDs is something I hope to avoid, since they require drilling holes for the mounts. My only simple idea is to make approximations of the light spread on transparencies and arrange them all by hand to most evenly cover the required area. But it seems like there should be a better way.

    In case anyone is interested the LEDs I'm trying first are CREE XP-E Green 3W LED On Star and Cree XT-E Royal Blue and I'm going to use the Mean Well ELN 60-48D. There are also lenses available for the LEDs, but I'm not sure if these spread of focus the light. I have some ordered for testing, but I'm not sure there will be enough room for them given my need to split the head into two parts to fit the enlarger.
  2. David Grenet
    David Grenet
    Hi there,

    Check out this LED head - the author calculated the layout for a 4x5 but I imagine it would scale okay...
  3. hgernhardt
    hgernhardt
    Sadly the author didn't give any explanation of the methodology for position calculation. Anyone got any leads on how to do that?
  4. L Gebhardt
    L Gebhardt
    Thanks David. I have seen Huw's head before and have contacted him in the past about how he calculated the layout. He wrote a program to do it using measured data from the LEDs. He said it took a few hours to calculate (he tried every combination). I guess just scaling this up would probably make the most sense for a first try. Or, I may end up taking his software approach and calculate the optimal placement based on my LEDs and sizes. Probably won't take more than a weekend to write.
  5. stormpetrel
    stormpetrel
    Hi guys!

    I have just acquired a durst 139 which is very similar to the durst 138. My plan is to make a similar multigrad LED source I did for my omega 4x5 (violet & yellow green LEDs controlled separately, full story here: http://www.apug.org/forums/groups/sm...ht-source.html). This source will replace the existing bulb (xenon bulb in my case).
    The size of the source will be 130cm. I will try to post my progress here in this group.
  6. L Gebhardt
    L Gebhardt
    Stormpetrel, I'm looking forward to seeing your progress.

    I've been prototyping my head over the last few weeks. I have based my controller logic off of http://www.brodie-tyrrell.org/fstoptimer/. My original plan to use the Meanwell 60-48D fell through since it didn't respond quickly enough to the dimming / on / off commands. I have switched over to using four A009-D-V-1000 Buck Blocks (http://www.ledsupply.com/0a009-d-v-1000.php) to drive the LEDs, two for each color. These can be controlled rapidly with a simple transistor from the Arduino. So far everything is working well, but it's all just a tangle of wires. I also need to build the actual head instead of just a few bare test LEDs. I'll build up a circuit board and the box for the head soon.

    I plan to make this run in a mode where contrast grade is input and the speed stays constant. Most systems do this around a middle gray, but I'm hoping to calibrate things to hold a barely off white tone constant. Then as you add contrast all the lower tones will darken. This isn't done yet. but I do have a nice split grade option working where the blue and the green are exposed separately (for the same time) to allow dodging in only one hard or soft light.

    I also plan on building a second cluster of LEDs to drive with the same controller so I can switch between diffusion and condenser modes. From my tests a simple white plexiglass or milk glass diffuser with even illumination on it in the condenser lamp house of the Durst 138S gives good results. So this should work well with LEDs behind the diffuser.

    By having two options for light I should be able to use the condenser for most images, but switch to diffusion when I need to use masking techniques, or have a negative with excess contrast. I'll post some pictures when it starts to look like something.
  7. L Gebhardt
    L Gebhardt
    I posted an update with my progress over on my blog, http://www.trippingthroughthedark.co...138s-progress/.
  8. gianear
    gianear
    Hi guys!this led lighting
    which has better heatsink and good quality!
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