[pure-silver] Re: multigrade paper is amazing --- see attachment
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sat, 10 Nov 2007 12:21:52 -0800
----- Original Message -----
From: "Dave V" <DValvo@xxxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Saturday, November 10, 2007 6:18 AM
Subject: [pure-silver] Re: multigrade paper is amazing ---
see attachment
Good question Claudio,
The silver grains are "sensitized" not "filtered". This
means that where a
grain used to be "sensitive" to blue light it is now
"sensitive" to green
light when the dye is added. And the grain becomes low
contrast as well.
The blue sensitive emulsion had no sensitivity to green
light and
"filtering" it with a green filter would do nothing.
Sorry for using
emulsion chemistry terms.
Now I have attached the spectral sensitivity curves for
Polymax paper to
show everyone exactly what is happening. The three
curves are density
curves. 0.3 and 1.6 are the furthest ones and show the
contrast. When the
curves are close together the contrast is high. ...when
far.... the
contrast is low. Expose the paper with the color of
light shown on the
horizontal axis and you get the contrast at that light
wavelength. So you
can get variable contrast in your printing by ratioing the
blue and green
light exposures OR using filters that let certain ratios
of blue and green
light through.
By the way, even though the curves stop at about 550 nm
the sensitivity
continues into the red but at a lower level than shown.
Just put a straight
ruler on the slope of the line and extend it down....a
foot or two.
Dave
An interesting set of curves. Without actually plotting
the difference the minimum contrast appears to be at about
430nm and the maximum at about 460nm.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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