On Mon, 18 Jun 2001, Matt Mengel wrote:
> I think I have enough info to get started BUT,
> there have been some concerns here as to spectral
> problems with different light sources. Can anyone shed
> some light (sorry) on what the problem(s) would be. I
> am not at all familiar with this. If I used a source
> with the wrong color temp. what would happen? I'm
> speaking in the B&W sense of course.
The issue is that the sensitivity of photographic emulsions depend on the
frequency of the light used to expose it. This is a common variable
between different enlargers. No two light bulbs are quite the same, and
different enlargers use different technologies (cold light or flourescent,
incandescent, halogen, etc.). The upshot of this is that if you cut up a
sheet of enlarging paper and exposed it under different enlarging systems
you would get varying exposure times.
Another twist on this is that modern variable contrast papers use two
different formulations in the emulsion. One is a low contrast emulsion
that responds to one color and the other is a high contrast emulsion that
respones to a different color. I think the colors are green and blue, but
don't remember for certain. This allows you to adjust the print contrast
by changing the relative amounts of each color of light through
filtration. The limits of the possible changes depend on how much of each
of the two colors of light are emitted by your light source.
So, as far as what would happen:
- First, I'm not certain that there is a particular standard color
temperature for B&W enlarger lights. Maybe, but I don't know it. If
I had to guess, it is probably daylight at around 5500 Kelvin.
- Your enlarger will print with different exposure times compared to
your so-called "standard" light source.
- You will likely have a different contrast (filtration) range than
someone with the "standard" light source.
The good news is that you will calibrate any enlarger for your printing
times, and that you can check your maximum and minimum printing contrast
ranges. That is really all you'd need for B&W.
(For color the filtration range becomes a much bigger issue.)
- Wayde
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