Re: pyrogallol & pyrocatechin

From: "Gordon J. Holtslander" <holtsg@duke.usask.ca>
Subject: Re: pyrogallol & pyrocatechin
Date: Sat, 03 Apr 2004 11:00:46 -0600

> > From: "Gordon J. Holtslander" <holtsg@duke.usask.ca>
> > Subject: pyrogallol & pyrocatechin
> > Date: Fri, 02 Apr 2004 21:15:40 -0600

> > > I use a low pH phenidone ascorbic acid developer to produce
> > > continuous tone negatives from ortho lith film. It works well,
> > > but suffers from slow speed and poor shadow detail - its
> > > essential to pre-flash to get any shadow detail.

> I use Kodak Polychrome Graphics Camera 2000 CGP film. I don't know
> how it would compare to the above films.

I don't have experience with this film, but the spec sheet says it's
extremely high contrast and suitable for line application and RA 2000
developer. So I think the emulsion has rhodium dopants, or something
that works very similarly. It is basically a thresholding
mechanism. Say each grain has 5000 Rh(3+) ions. The grain won't be
developable until this grain gets at least 5020 photons (no latent
image center will be formed until all Rh(3+) combines with
photoelectrons to become Rh(2+), which is stable for thousands of
hours. So, this "thresholding" greatly reduces the speed (sensitized
grains need some 20 photons per grain to become developable) and
increase contrast. After all these developer discussions, I think
pre-flash exposure is the only effective way to deal with that kind of
films because you need to get rid of most of Rh(3+) before imagewise
exposure to register good shadow details. Because grains don't have
latent image, doing something radical to the developer can only
increase development fog, not useful shadow...

--
Ryuji Suzuki
"All the truth in the world adds up to one big lie." (Bob Dylan 2000)