From: Ed Stander (glassact@compuserve.com)
Date: 06/25/01-07:35:04 PM Z
All:
Mike Ware contacted me some time ago re. nanometers and PT processes.
Here is his commentary:
*******************************
The choice of the best UV light sources for the various processes is a
recurrent topic that attracts much opinion but very little science. None of
the sources are monochromatic, so one has to consider the whole waveband of
radiation emitted, in relation to (1) the absorbance and quantum efficiency
of the sensitizer, (2) the internal filter effect of the coating, (3) the
glass in the printing frame, and even (4) the plastic base of the negative,
all of which can affect the outcome. My theoretical recommendation in JPS
only took into account the first two of these.
If you know of any report in the Pt/Pd archives of a proper experimental
comparison of tubes peaking at ca. 365 nm with similar-power tubes peaking
at ca. 410 nm, under identical conditions, I would be genuinely grateful to
have the reference, so I can give the question more thought if experiment
does not agree with theory.
Your message has just sent me to search my e-mail archives, where I find
that I posted the following to the Alt Proc List on 9 January 1996
>This thread has generated some very interesting observations and I would
>like to thank everyone who has generously shared experimental results,
>which are most illuminating (8~). Can I try to summarise the factors in
>this debate which seem to me important?
>
>1) The spectral power distribution curve of the light source is certainly
>the place to begin the comparison. But remember that it is the *area*
under
>that curve, between appropriate wavelength limits, that counts, not just
>the *height* of the peaks. (Technically: the spectral power per unit
>bandwidth must be integrated over the wavelength range).
>
>2) But, of course, the intrinsic response of the sensitizer to light also
>varies with wavelength, and has to be scaled into the calculation. And it
>should be remembered that the sensitivity of iron-based coatings (e.g.
>cyanotypes or Pt/Pd) is different intrinsically from that for
>photohardening of dichromated colloids (e.g. gumbi and Carbon). So it is
>essential to compare like with like, and we should not be surprised at the
>apparent 'differences' in experience which have been recently decribed
(You
>can leave the rest of your crow for the cat, Judy).
>
>3) And then there is the question of how much of your original light (at
>each wavelength) is actually arriving at the surface of the print: you
need
>to take into account the transmittance of the glass, as Klaus has pointed
>out, and any other apparently clear material in the path; e.g. plastic
>masks, and even the negative film base itself, which do have a cutoff in
>the UV, and are absorbing an important part of the light you can't see.
>
>If this were all there was to it, we might still have a chance of doing
>some useful scientific calculations and predictions. But if you are making
>a real print (a strongly recommended activity, as opposed to just amusing
>oneself with laboratory photochemistry) there is also (horribile dictu):-
>
>4) *The Internal Filter Effect* (A gasp of horror is appropriate here).
>Most 'real-life' alt-proc coatings also contain coloured substances that
do
>not contribute significantly to the light-sensitivity (i.e., the pigment
in
>Gumbi and Carbon; the potassium ferricyanide in cyanotype; the noble metal
>salts in Pt/Pd). On the contrary, these substances are absorbing your
>printing light as it passes through the coating, and giving nothing in
>return. E.g. the presence of these coloured (and UV-absorbing) substances
>is like putting a coloured filter over your light source - except that the
>filter is 'built-in', and inescapable. Once you add this into the
equation,
>it is obvious that it is not strictly valid to compare the efficiency of
>your light sources unless the coatings are *identical in all respects*.
>Two differently pigmented carbon tissues, for instance, will not be of the
>same 'speed' even if the dichromate concentration is the same.
>
>At this point, the scientist bows gracefully, admits that the real world
is
>far too complicated for him to quantify, and retires to make some prints
>instead.
>
>Mike
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