White LED vs Tungsten bulb for VC filters

Discussion in 'Contact Printing' started by buze, Oct 12, 2006.

  1. buze

    buze Member

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    I'm not fantastically happy with the contrast I get out of my cheapo VC paper and the light bulb I use for contact printing. I added some white paper over the (15W) bulb to get longer exposure and thats great, but the contrast is all over the place..

    I'm thinking of getting a LED "bulb", and out some Ilford Multigrade filters in front to have a better control on the exposure time and the resulting contrast.

    Question is, these LED lights are low power (great : longer exposure) and give white light; is the white light what I need ? Are the VC filter calibrated to white, or to Tungsten ?

    Is the whole plan valid ? I'm trying to have longish exposure (30 seconds or so) and some reasonable contrast. Right now it's a bit "mushy".

    Side question that I can't find any definitive answer on : is a RED safelight the one to use for VC paper ? I've seen brown/greens and various other shades...
     
  2. Lee L

    Lee L Member

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    The white LEDs I know of use a blue LED that in turn excites a phosphor to emit full spectrum light, but very often with a greater amount of blue light. The cheaper ones tend to emit more blue with color temps around 8000K, but you can also find them with temps around 5000K down to 2700K, the lower end of that being similar to a 15W tungsten lamp. See a typical range of the color temps here: http://superbrightleds.com/edison.html

    Since VC papers respond to blue and green light, you might find a higher color temp lamp a bit more efficient. I'd also diffuse most bare LEDs, as they tend to have a strong pattern to the light they project.

    Most VC papers will work fine with either an amber or red safelight filter. If your safelight emits at 580nm or longer wavelengths, you're usually OK for VC filters.

    Lee
     
  3. srs5694

    srs5694 Member

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    Here's an alternative you might want to consider: Get two LEDs for your contact prints, one green and one blue. You can then do a contact print with different exposure times from each source (5s green and 10s blue or 7s green and 6s blue or whatever) to control contrast. Depending on how fancy you wanted to get, you could rig up some sort of timer to do them both automatically, sequentially or simultaneously.

    (This assumes you're using bare bulbs hung over the negatives and paper to make contact prints. If you're using some sort of apparatus that takes a bulb, you'll have to determine whether or not you could squeeze two bulbs into the space occupied by the one you're using now.)

    One other concern: LED bulbs' power ratings (in watts) are not directly comparable to those of tungsten bulbs. Watt for watt, LED bulbs put out a lot more light than tungsten bulbs. You might want to look for light output specifications in lumens for both types of bulbs.
     
  4. Loose Gravel

    Loose Gravel Member

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    I wouldn't use LEDs. They are not blue enough. You need a violet LED. Since manufacturers are making LED light to be seen by humans, the blue is on the green end of things unless you are getting special deep blue, violet, or UV LEDs. Even the whites, although high color temperature, as discontinous in the spectrum and mostly wimpy blue and down. Get a halogen bulb. They have a very high color temperature with a continuous spectrum. They even have a little UV. Your filters will work better.

    Some fluorescent tubes would work, but you'd need an integrating timer for they are terribly variable with light output.
     
  5. Donald Miller

    Donald Miller Member

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    This brings up a question about something that I have been considering developing for some time.

    Would there be any interest for a variable light source to use in contact printing with VC papers either now or in the future.

    I may be wrong but it seems that VC may be the "last indian standing" when it comes to silver papers.

    What I have in mind is a light source with variable stepless contrast control for VC materials.

    Please share your thoughts on this. Is there any interest?
     
  6. Lee L

    Lee L Member

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    I have the parts on hand, and plans to build a combination VC flasher (similar to RH Designs) and contact printing source with potentiometers on blue and green high brightness LEDs running off 12VDC like my LED safelights. Just haven't had the time to build it yet.

    Lee
     
  7. Loose Gravel

    Loose Gravel Member

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    Don

    Seems as though you can get continuousl VC with halogen light and filters, especially if you add some Rosco blues to your filter pack. The standard blue LEDs probably won't go above about 4.5 or 4 grade, if I had to guess. All the filtering in the world won't put more blue in, not even white LEDs. There are some near-UV LEDs that you could use, but I don't know much about this and I'm guessing they are expensive. Coldlight would be a good choice. It isn't so hot for enlarging because it isn't bright enough, but that problem goes away with contact printing. Stabalization problem comes back to get you, so you'd need a controller and you could only do one color at a time or get two controllers.

    How much are those old Minolta (?) pulsed xenon light sources these days. Would that work? I thought about 2 Halogen lams, one with green and one blue filtered, but you can't turn down the intensity without a color shift. Bad idea.
     
  8. Donald Miller

    Donald Miller Member

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    The information that I have has indicated that LEDs are not effectively controlled through the use of variable resistance like potentiometers.

    The problem that exists, as I look at it, is to get even light coverage over a 16X24 inch area (in the case of a 12X20 inch negative. I don't know that LEDs are the way to go for this. The number of LEDs needed to get even coverage is going to be tough to design even for an 8X10 coverage. Looking at HUWs design of the 4X5 enlarger head, the area and design considerations are at a factor of at least four. The costs would soon become prohibitive.

    Cold light would require a massive two tube grid (blue and green) to cover the area. Some type of control is needed to switch the tubes in or out.

    I have worked with 395 nm UV LEDs. The light output is relatively low. Certainly not on the order of a BLB lamp. There again this addresses only the high contrast emulsion and does not deal with the low contrast emulsion.

    I have some designs in mind but would only move forward if the demand appears to exist.
     
  9. Loose Gravel

    Loose Gravel Member

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    at a time when one can barely give darkroom equipment away, I don't think there is much demand. You would never re-coup costs and your time spent would be better used shooting or printing. Just my two-bits.
     
  10. buze

    buze Member

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    Well, a blueprint (excuse the pun) for an affordable, versatile contact print light source that does VC would be nice...
     
  11. Donald Miller

    Donald Miller Member

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    Buy a medium format dichroic enlarger. Should be dirt cheap and readily available.