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.
Sorry, I don't undestand what you mean for "tone below white". May you explain?
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?
I have plotted the mean density of pure YMCKRGB squares of the cuprotype test against their absorption at 350, 370 and 380 nm. The observed densities of the test are in (almost) good agreement with the 380 nm absorption. I am waiting for the numerical results of the absorbance test on Pictorico from my friend, in order to perform more accurate calculations.
Light source and other chemicals in the sensitizer are ever interacting with the sensitive substance, so that the effective wavelength can be shifted from the optimal one (that is, the wavelength where the sensitive substance has its maximum quantum yield). In addition, the HSB test was printed with +10% inks because "normal" inking was not blocking enough.
As for the minimum printing time, for my light source cuprotype is about 30% less sensitive than cyanotype. Adding the long chemical processing, it is really a mess, but I am exploring it because I believe it has some potential about toning.