Contrast control with carbon, was Re: replenishing dichromate for pt developer


Sandy King (sanking@hubcap.clemson.edu)
Tue, 16 Mar 1999 20:02:42 -0400


Luis Nadeau wrote:
>

>>I am currently printing with 350na BL fluorescent tubes. For a couple of
>>years I used GE Daylight tubes, which required an approximate 2X increase
>>in sensitizer strength to give the same contrast.
>
>With the same strength it provided more or less contrast?
>
>What about the speed? What distance away from the printing frame?
>

Luis,

The GE Daylight tubes, which produce most of their energy above 450na, gave
images of greater contrast than the 350na tubes with the same strength
sensitizer. As for speed of printing, there was very little difference with
a given negative once the sensitizer was adjusted to the exposing light
source. That is, a negative printed approximately the same (contrast and
speed) with a tissue sensitized at 1/2% and exposed with the BL tubes as
one sensitized at 1% and exposed with the Daylight tubes. I believe this
information is fairly consistent with the data Kosar gives re: the speed
and contrast of dichromated colloids with sensitizers of a given strength.

My light source consists of 14 (fourteen) 24" tubes spaced as close
together as physically possible, so close that they can only be removed by
starting at one of the sides. This allows me to expose with the printing
frame very close to the lights, approximately 1 1/2" to 1 3/4". Exposures
tend to be rather short. With this system a negative with a DR of 1.25
(Dmin of about .25 and a Dmax of 1.5) needs an exposure of around 3
minutes, with a tissue sensitized at 1/4% or 1/2%. For color printing with
separation negatives of DR of 1.3-1.6, the cyan, magenta and yellow tissue
exposures are typically less than 1.5 minutes for each color.

I suspect that the reason for the dilute sensitizer solutions is very short
spacing between the light source and printing frame, but would certainly
appreciate your thoughts on the subject.

Sandy King



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