LED or Halogen light source configurations

Discussion in 'Darkroom Equipment' started by Curt, Jul 3, 2008.

  1. Curt

    Curt Subscriber

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    I've seen enlarger light sources as covering a flat plane, such as cold light grids and LEDs in grid arrays. I've also seen halogen bulbs as light sources for "light boxes", a Styrofoam lined box with a hole for the light to direct in and diffuse onto a plastic, Plexiglas sheet. Older enlargers even had a bulb in a can with opal diffusing glass.

    I've read the threads here, contacted http://huws.org.uk/ and I have experience with Aristo Cold Light Heads, http://www.aristogrid.com/, and Beseler Color heads. I can't remember seeing an LED group in a light box that's Styrofoam lined.

    I'm making a diffusion light box with three halogen bulbs because I have the bulbs and holders with dichroic filters in red, green, blue from a Beseler head. What I'm thinking now is why wouldn't very bright LEDs work in place of the halogen bulbs. Also why wouldn't the right color temperature LEDs work without the filters? I'm making a light source for 8x10 but it would be the same design in other sizes.

    This is where I am at the moment, I would like others to input here with their ideas and requirements.

    Curt
     
  2. Steve Smith

    Steve Smith Subscriber

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  3. jeroldharter

    jeroldharter Member

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  4. Curt

    Curt Subscriber

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  5. Curt

    Curt Subscriber

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    LEDs in grid form requires a large heat sink and is too complex for the causal diy'er. Split grade Aristo cold light sources are too expensive and are for black and white only. A diffusion mixing box can accept an external light source such as Halogen or the LED light that Steve points out. The Halogen is proven, the LED for a mixing box is not as of yet, as far as I know. I don't know how much heat it gives off, I imagine it would require a fan as does the Halogen. Halogen bulbs can be dimmed and there is the question of voltage compensation and color temperature in LEDs.
     
  6. bdial

    bdial Subscriber

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    There are MR-16 base LED bulbs that operate on 12 volts, but they don't provide anything close to the light from a 200+ watt Halogen bulb that's typical for the big enlargers, though 3 might work for your purposes. Superbright LEDs is one source http://www.superbrightleds.com/
     
  7. srs5694

    srs5694 Member

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    FWIW, I tried green and blue MR-11 bulbs from them in my Philips PCS130/150 enlarger. (This enlarger takes three 14V 35W MR-11 halogen bulbs; I only replaced the green and blue ones.) They lit up, but they were so dim by comparison to the originals that I'm sure the exposure times would have been ridiculous. (I didn't bother trying to expose paper with them, they were so dim.)

    Since I ran this test, though, the site has come out with a newer model that produces more light; however, this is a "white light" bulb, vs. the separate green and blue bulbs I used in my first test.

    I haven't looked at the MR-16 offerings all that closely, since my enlarger takes MR-11 bulbs.

    I've heard that LED technology is progressing rapidly, with light output doubling every few months. Thus, I figure it's only a matter of time (months to a few years) before LED bulbs that might function as drop-in replacements for tungsten or halogen bulbs in at least some enlargers arrive on the scene.
     
  8. JBrunner

    JBrunner Moderator Staff Member Moderator

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    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.
     
  9. srs5694

    srs5694 Member

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    What I didn't make clear is that the PCS130/150 uses three bulbs, each with a filter (red, green, or blue), so the light is already dimmed in the out-of-filter wavelengths. Of course, it's possible that the filters let lots more light through than just the optimum, but still....
     
  10. galyons

    galyons Member

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    The newer technology LED's solve most of the concerns and lack of knowledge raised in this thread. Please do not confuse "brightness" with effective exposure illumination. Graded and VC paper emulsion are sensitive to light in very narrow wavelengths.

    A bright quartz halogen lamp produces a broadband spectrum. Very little of it's spectrum is in the desired bandwidths for B&W papers. Most of the energy from quartz halogen, (and other tungsten light sources, as well), goes to produce heat and red spectrum light. In many cases 80% or more of the energy from QH is heat. The QH light is filtered, either additive or subtractive to narrow the spectrum actually used in an enlarger head. The tubes in VC coldlight heads have filters, as well, to achieve the desired spectrum.

    As we all know you lose light with a filter and you cannot create a spectrum with a filter that is not in the originating light source.

    The newer, high-powered LED’s are available in very narrow light bandwidths. The Luxeon brands, as an example, produce approximately 80% of their light in +/- about 10 nm. LED’s are simple electronic diodes. The circuitry is dead simple. LED’s are devices that need current limiting. So, a typical grid array is serial terminated with a current limiting resister.

    LED’s, for our purpose, are exponentially more efficient in producing the desired light wavelengths. A suitably specified LED produces approximated 80% of it’s light in the target wavelength compared to QH at about 3-5%. Again most of the energy from QH goes to HEAT!

    The heat LED’s produce is small, especially so when compared to the output. The critical heat management point is at the silicon chip juncture. Environmental heat is not an issue. With the number of LED’s that we need to produce the desired paper exposure, heat management is significantly less than required for the typical QH head. Excess head reduces the LED light output and decreases the expected duty life. It does not heat your darkroom!

    I am pretty excited about currently available LED technology. It is a growing technology, so the future, (pardon the pun!) is bright!

    Cheers,
    Geary
     
    Last edited by a moderator: Jul 7, 2008
  11. Dave Miller

    Dave Miller Member

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    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?
     
  12. srs5694

    srs5694 Member

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    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.
     
  13. galyons

    galyons Member

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    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











    g
     
    Last edited by a moderator: Jul 7, 2008
  14. RH Designs

    RH Designs Advertiser Advertiser

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    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.
     
  15. hka

    hka Member

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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.
     
  16. Bob F.

    Bob F. Member

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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.