Need help from a color guru.

Discussion in 'Darkroom Equipment' started by bmac, May 11, 2003.

  1. bmac

    bmac Member

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    Hey guys, I just got another Zone VI cold light for my Omega D2 on Ebay, and found out it has an aristo W31 bulb in it which give off a color temp of 3050K. Anyone know what gel I would have to usee in order to convert it to the proper temp for VC paper with Ilford Filters?

    Thanks in advance.
     
  2. bmac

    bmac Member

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    So basically, I think I need a CC filter that will convert 3050k to about 4500k right?
     
  3. Ed Sukach

    Ed Sukach Member

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    </span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (bmac @ May 11 2003, 05:08 PM)</td></tr><tr><td id='QUOTE'>So basically, I think I need a CC filter that will convert 3050k to about 4500k right?</td></tr></table><span class='postcolor'>
    No, I think the filters were based on an enlarger color temperature of something like ~ 3200K. I can not find *one* iota of information about enlager bulb temperature/ VC filtration from ANY of the paper manufacturers - anywhere.
    Certainly they were not derived from a light source with a color temperature THAT high - I thik that a source radiating light at 4500K would have to be something like a carbon arc lamp.

    I once analyzed the color "shift" from Ilford MG filters and found them to be substantially different from the suggested correction values in the data sheets. In matching the new color correction values with the dichroic head on my Omega, I had *much* improved results.
     
  4. bmac

    bmac Member

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    Ok, I found out the color temperature of my PH212 bulb in the condensor housing is actually 3400k, so I am looking at a 350k difference now.
     
  5. Ole

    Ole Moderator Staff Member

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    I think the "standard" illumination for VC filtration is an unspecified tungsten bulb - I doubt the temp is higher than 3200K.
     
  6. Thilo Schmid

    Thilo Schmid Member

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    Brian,
    you don't need to care about this as long as your light source emits a continuous spectrum. The MG-filtration is based on light blocking not on light passing. VC paper has two emulsion layers: a soft one and a hard one. The soft one is sensitive to green light and the hard one to blue light. A #5 filter blocks green light, regardless of the color temperature. But exposure correction factors do vary from light source to light source. The higher the color temperature, the less exposure correction is required for grades 4 and 5.
     
  7. bmac

    bmac Member

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    Actually, I do need to care about it [​IMG] Printing at grade 2 is producing a grade 4 or 5. Aristo makes a VC paper compatable tube, but it is $150 for the bulb alone. I can't justify spending that much right now... poor bmac... lol
     
  8. Thilo Schmid

    Thilo Schmid Member

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    Brian,
    what filters do you use? Do you use the corresponding filter for printing a grade 2 or do you print it without filtration ?
     
  9. Donald Miller

    Donald Miller Member

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    Brian,
    Color temperature may not be the only determiner in this instance. The Aristo lamp, depending on the phosphors which were used in the manufacture of the lamp, may have emissions in several distinct nm bands. These would then have the effect of arriving at an "average" which would give a certain measurable color temp. This "averaged" color temp could conceivably have a sizeable portion of it's emission band in a region that is causing you high contrast problems. This is the nature of grid light and "flourescent" types of lamps. Aristo came up with a lamp designed for VC papers for some very good reason.

    If I were going to try to work with this lamp, the first filter pack that I would add as a trial would be what others seem to use and that is 40 units of yellow. If that were not enough by itself, then I would "kick it up a notch" by another 20 units of yellow. If that were ineffective in bringing contrast down then I would begin working in the blue and green filtration.

    In my VC enlarger the filtration that is varied is magenta and cyan. The two colors that are varied in exposure of VC emulsions are blue and green. The reason for the two colors used in the Saunders system is the difference between "additive" and "subtractive" filtration that is used in printing. Saunders happens to use a "subtractive" system and others use an "additive" system of filtration.

    In the case of my Saunders VC enlarger, the light source is a low voltage halogen...and not tungsten. Tungsten is a very yellow light source and halogen much more blue (higher color temp.)

    I hope that this proves helpful. Good luck.
     
  10. Tom Duffy

    Tom Duffy Member

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    Many people have had this problem with cold light heads originally intended for graded paper printing. I think you need to add 40cc yellow to your filter draw for all to be OK. I'm sure if you call aristo, they can suggest a fix. They've dealt with the issue hundreds of times before.
    Take care,
    Tom
     
  11. lee

    lee Member

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    the new blub for cold lights (single tube) is called the V54 from Aristo. http://www.aristogrid.com It has a better response for the VC papers. It is more green in its coloration. You will still need to use multicontrast filters either in the filter drawer or under the lens. The cost for this upgrade is about $100 US. Should be a snap to change out. A 40 yellow should help but this will dramatically slow down the light response while printing as it will filter the blue light.

    lee\c
     
  12. bmac

    bmac Member

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    </span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (tschmid @ May 12 2003, 12:17 AM)</td></tr><tr><td id='QUOTE'> Brian,
    what filters do you use? Do you use the corresponding filter for printing a grade 2 or do you print it without filtration ? </td></tr></table><span class='postcolor'>
    I use Ilford Filters.
     
  13. Thilo Schmid

    Thilo Schmid Member

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    Brian,
    I have overlooked that you are talking about a cold light head. Ad Don already said, they do not have a continuous spectrum and are thus not suited for VC printing. Your head does not have a color temperature of 3400°K. This is just what your color meter might say based on two or three spectral readings. Adding another filter might not solve the problem. At least not for all paper grades.