U of S | Mailing List Archive | alt-photo-process-l | Re: again on color absorption

Re: again on color absorption

Alberto, I'm curious how you woud interpret the reintroduction of tone below white in this print. I know that in gum, there is a wide difference of opinion about what causes that, when it occurs; I wonder if there's a more straightforward explanation in metal processes.

Also, it's interesting to me that while the HSB printout for cuprotype follows the graph just above it to the extent that in both cases, a green color appears to give the best blocking (UV absorption), the other colors aren't quite so consistent from graph to HSB (for example, in UV absorption at 360-370 nm, cyan is next under green, but on the HSB array it appears to be no better at blocking than magenta. I'm curious what your thoughts are on that, is this a case where the light source and process could be interacting with the UV absorption so as to give a different result than the straight UV absorption?

Thanks for posting these; they are very helpful.

On May 11, 2009, at 1:01 AM, Alberto Novo wrote:

Just to get it right in my mind, when you use the word "absorption", your saying the light goes through the negative and not reflected?

Yes, this is applied to a transparency (first two graphs, where the first shows the Pictorico absorbance)
I'm trying to get a better understanding of your graphs.
Looking at the middle one with all the 2400 inks, at the 370nm point, I can see that the green ink is the highest on the graph. Meaning that the light at 370nm when hitting the green ink will reflect most of the light.

It is an absorption measure, hence the green ink is absorbing most of the light @ 370 nm
Therefore the magenta which is the lowest on the graph for the 370nm will "absorb" most of that light and penetrate the surface of the emulsion?
Am I correct in saying this?

No, that is not correct. Magenta is the less absorbing, so that most of the light passes through the ink and goes to the emulsion (which darkens).
What sort of tools were used to do this sort of measurement?

For the first two graphs, a spectrophotometre. See the thread on this argument started about two or three weeks ago.
The third figure is a negative with H=0 to 360 on the x axis and S=0 to 100, B=100 (upper part) and B=100-0, S=100 on the y axis. The dashed lines help to find the pure colors (B=100, S=100).
see http://www.inkjetnegative.com/images/RNP/rnp.htm