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  1. #11
    Dave Miller's Avatar
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    Quote Originally Posted by JBrunner View Post
    It's possible that you could have reasonable exposure times as the light is the correct wavelengths for the emulsion. A huge portion of the light in filament bulbs is useless to the paper. You see it. The paper doesn't. It takes less of the "pure" color than you might think. Not very handy for focusing though.
    Presumably one would build in white LED's into the array to use for focusing in a similar way that we use the "white light lever" on a conventional head; or is this too simplistic?
    Regards Dave.

    An English Eye


  2. #12

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    Quote Originally Posted by Dave Miller View Post
    Presumably one would build in white LED's into the array to use for focusing in a similar way that we use the "white light lever" on a conventional head; or is this too simplistic?
    Patrick Gainer wrote a piece for (I believe) Photo Techniques called "Hazards of the Grain Focuser" (IIRC), in which he examined the optimum method of focusing with a grain focuser. He found that green or white light produced the best results. Issues with the way the human eye focuses caused focusing with other wavelengths/colors to be sub-optimal.

    My understanding is that "white" LEDs don't produce the same sort of broad-spectrum light that most light sources produce; it's really just a few wavelengths, which our eyes perceive as white-ish. If that's correct, I don't know how this would interact with focusing issues. If I were using an LED light source with separate green, blue, and possibly red LEDs, I'd focus using the green LEDs alone, at least pending further experiments along the lines of those Gainer conducted.

  3. #13
    galyons's Avatar
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    Quote Originally Posted by Dave Miller View Post
    Presumably one would build in white LED's into the array to use for focusing in a similar way that we use the "white light lever" on a conventional head; or is this too simplistic?
    Dave, "white LED's" emit most of their light in blue and green. Most of the light is in the very same wavelengths as the blue and green that we use to do VC printing. Cool white LED's have more blue. Warm white LED's have more red. Some brands use filters.

    Our eyes are most sensitve in the blue-green range. In my short time using my LED head, I find just running both the blue and green give a more "whiteish" appearance for focusing, although it is really the blue-green that we are most sensitive to. I do not know that there is any benefit to "whiter white" for focusing. Intuitively, one would might surmise that focusing in the same light spectrum as exposing woud be more accurate, (The old blue filter debate?). But adding red LED's for illumnated alignment of paper and running them with the blue and green for focusing would give a more white focusing light. At least you get some additional benefit from the aded array!

    Cheers,
    Geary











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    Last edited by galyons; 07-07-2008 at 03:44 PM. Click to view previous post history.
    But your flag decal won't get you into Heaven any more. They're already overcrowded from your dirty little war.
    Now Jesus don't like killin' no matter what the reason's for, and your flag decal won't get you into Heaven any more. – John Prine

  4. #14
    RH Designs's Avatar
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    Quote Originally Posted by Dave Miller View Post
    Presumably one would build in white LED's into the array to use for focusing in a similar way that we use the "white light lever" on a conventional head; or is this too simplistic?
    Heiland's LED head uses an RGB array, the red ones were added so that focussing could be done in something approximating to white light, and there's the additional advantage that by switching on the red ones only you have the equivalent of the red filter that you would swing under the lens of a white-light enlarger when determining the best position for dodgers etc.
    Regards,
    Richard.

    RH Designs - My Photography

  5. #15
    hka
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    Now for some months I use a Heiland LED coldlight source on a Durst M805 and I'am very happy with it.
    I don't discovered any problem with focussing. On the other hand the exposure times are shorter than when using the old lightsource. And as Richard mentioned already above the red light function is a masterpiece. No more stress to find the right position for dodging or so. The white light looks a bit blueish but after a while you don't see it anymore.
    It's a pleasure to use it.
    harry

    Release, the best you can do...

  6. #16
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    "White" LEDs are indeed far from pure white ('tho emitting over quite a wide spectrum). The majority actually use a blue LED to excite phosphorescent material rather than illuminate directly which is why there is a strong blue component. The phosphor is then used to generate the other light frequencies required, including more or less red to give warm (more red) or cool (less red) "white". This is also why they have shorter lifetimes than standard LEDs: the phosphor deteriorates.

    There are also white LEDs with red/blue/green individual LEDs in the same package but they are rarer and more expensive, but more efficient.

    I really must find the time to have a bash at this some day: I have a spare enlarger to use as a guinea pig...

    Bob.

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