From: Bob Kiss (bobkiss@caribsurf.com)
Date: 04/02/02-07:47:00 PM Z
DEAR KATHERINE,
Firstly, there are types of clear glass that greatly attenuate or even
block UV radiation so it isn't a matter of which color...it is more a matter
of what material the filter is made. Certain plastics and doped glasses
block UV very well but are, for all intents and purposes, "clear" which
means that they pass all colors which make up white light, the visible part
of the electromagnetic spectrum
Here-in lies the problem. The accepted definition of light is "that
part of the electromagnetic spectrum which is visible". UV and Infra-red
are not "light" because they are not visible...they are "energy" or
"radiation".
If you look at the spectral transmission curves of a red and a cyan
(compliments) filter you would see that the red passes...red but no shorter
wavelength colors (blue, green, etc). Cyan passes blue and green but not
longer wavelength colors (red). You can see the same with Blue-yellow and
Green-magenta; green passes the middle of the visual spectrum and magenta
only the ends, i.e. blue and red.
So, if you try to adhere to this mode of thought you will see that a clear,
UV blocking piece of glass which passes all wavelengths of light longer than
UV but no wavelengths shorter than the color violet, could actually be
considered the "compliment" of UV...if we were to insist upon one.
As April 1st was yesterday I am trying very hard to resist asking if that
makes the matter clearer.
CHEERS!
BOB
----- Original Message -----
From: Katharine Thayer <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Tuesday, April 02, 2002 5:51 AM
Subject: Spectral density [was: Re: Inkjet transparencies ]
> FDanB@aol.com wrote: [on March 8, 2002]
> >
>
> >
> > This takes us back to the Spectral Density approach. There may be other
> > ways to make the 1280/1290 produce great platinum or cyanotype negs (and
> > I hope someone suggests one) but the colorization method seems like a
> > good bet for now.
> >
>
>
> I've been puzzling about spectral density and hope there's someone here
> with enough knowledge about the physics and psychology of the
> electromagnetic spectrum to help me understand the things I'm puzzling
> over.
>
> Both pyro-stained negatives (the ones I've seen are light yellow-green)
> and Dan's orange colorized negatives are said to block the UV
> wavelengths that the platinum process responds to. My understanding of
> reasoning behind this is that the orange or the yellow-green is the
> complement of the UV wavelengths in question and therefore absorbs them
> rather than passing them through. Which color (orange or yellow-green)
> is closest to the exact complement of peak sensitivity? And my more
> fundamental question is, how does one determine the color complement of
> wavelengths that aren't on the color wheel? Sorry if this seems like a
> stupid question, but I'm having trouble wrapping my mind around this
> whole spectral density idea.
>
> And are the sensitizers used in platinum and gum sensitive to the same
> UV wavelengths or to different wavelengths; my further question being
> does it make sense to use the same colors for the different processes?
>
> Thanks for any "light" anyone can shed on this for me.
> Katharine Thayer
>
>
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