Michael Keller (keller@wvinter.net)
Sun, 07 Mar 1999 12:03:42 -0500
Actually, when you use lith film to make halftones, you make the basic exposure
in camera, which gives dots in highlights and midtones, and the shadow dots are
put in using a yellow light exposed directly to the screened film (basically a
flash exposure through the screen). So while I can't comment on your specific
idea, it seems using different colors of light might have some effect. Obviously
the lith film is more sensitive to blue light than yellow, which is why the
yellow filter is used for a short flash exposure.
Liam Lawless wrote:
> The important thing is that I would expect a blue light to affect lith more
> than a magenta light (blue+red) of the same intensity. Now, suppose we
> project an original camera neg onto lith from a colour enlarger head with
> maximum magenta filtration dialled in, and a pale cyan filter (say about
> 25C) held under the enlarger lens. Lots of magenta light will pass through
> the shadows of the negative and its colour will be relatively unaffected by
> the pale cyan filter that it must also pass through, but in the highlights
> where only a little magenta light can pass, the effect of the filter under
> the lens is much greater, and magenta+cyan, according to the rules of colour
> printing, equals blue, which has more actinic power on lith. The same
> filter factor applies to both shadows and highlights, so what I believe we
> have done with our filters is to hold back the shadows and augment the
> highlight exposure at the same time.
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