From: Sandy King (sanking@clemson.edu)
Date: 11/14/01-08:36:48 AM Z
Joe,
I am not measuring the sensitivity of the dichromate solution itself,
though I think it has been fairly well established that the maximum
absorption is around 360nm, and as I recall, slightly more for
ammonium than potassium dichromate. That is in the UVA range, of
course, because the absolute maximum absorption of the dichromates is
actually in the UVB range at 200-250nm, but that energy is useless to
us.
What I have done quite simply is to print the Stouffer step scale
with number of different processes using light sources with
significantly different spectral distribution charts. By inspection
and simply analysis it becomes apparent very quickly that light
sources that emit a greater percentage of their energy at long
wavelengths (say 450nm) gives images of higher contrast than sources
that emit a greater percentage of their energy at shorter wavelengths
of 350nm and below. If you plot a characteristic curve you will find
quite simply that long wavelength light gives a greater gamma (or CI
) than light of short wavelength.
Just by example, the tonal scale of cyanotype varies from about 8 to
as many as 11-12 steps, all things equal but the exposing light.
The reason for this phenomenon, apparently, is that long wavelength
light penetrates deeply into the emulsion, which causes high
contrast, while short wavelength light is diffused and scattered on
the surface. I had long suspected this to be true for carbon printing
from printing with different light sources and was not at all
surprised that testing verified the premise. However, the fact that
this concept also applies to a number of other processes (cyanotype,
vandyke and traditional kallitype), and in virtually a linear
progression similar to carbon, was and is quite a surprise to me.
However, this has nothing to do with the spectral sensitivity of
Pt/Pd, does it? However, if anyone is willing to send me a few
small sheets of platinum paper I would be willing to test with the
same light sources and post the results.
Sandy King
>In a message dated 11/13/2001 17:52:52, sanking@clemson.edu wrote:
>
><<I have strong
>supporting evidence from recent tests that several other processes
>(carbon, gum, cyanotype, vandyke and traditional kallitype) have
>significant sensitivity to light in the near ultraviolet and violet
>between 400 and 475 nanometers.>>
>
>Hi Sandy,
>
>How are you measuring sensitivity? I ran a spectrosomethingorother on a
>dichromate solution about 2 years ago (assuming absorption was indicative
>of/equivalent to sensitivity) and found a secondary peak absorption spike
>around 370nm which was pretty close to the maximum 365nm peak reported
>elsewhere (Ware, etc., hmmm, should that be "elseWare") so I went with that
>and tried to find a UV/light that output efficiently in that region. (I
>further assumed any pigment would decrease the sensitivity.) However, the
>spike was fairly narrow as I recall and the curve flatlined in the visible
>with just a minor bump out around 600nm. By analogy, the curve looked like
>far UV would be Everest, the near UV 370nm spike would be the Adirondacks and
>the visible 600 bump would be Florida.
>
>Joe
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