I have recently purchased an Xrite 310 densitometer. This is a color densitometer and for that reason has the capability to measure the effects of pyro stain on the blue channel, magenta/green channel (for VC materials), and visual channel (reading without stain effects factored in).
For those who may be using staining developers such as ABC, Pyrocat, or PMK and want to have an accurate reading of the density spread of your negatives (contrast range), I will read these for you at no charge if you will send them to me (maximum two negatives) and include a self addressed and stamped return envelope.
Additionally, this densitometer does have capability of reading a prints reflection density. If you have a desire to know your print's Dmin and Dmax I will read a representative print's values if you include one as well.
My address is Donald Miller Photography, P.O. Box 48094,,Wichita,,,Kansas 67201
I used to use my color densitometer for that purpose, too. But I found out that these readings are not always useful. The dye color works in combination with the spectral sensitivity of the paper. You will get different results when using VC or graded paper or different brands. Usually, one cannot judge from the stain readings to effective contrast without applying a paper dependend and emperically determined correction factor.
I think that I understand what you are saying. My purposes in measuring the Blue and Magenta/Green channel and comparing them to the visual reading is not to measure the effects of a color addition brought about by the stain. It is instead to measure the effects of that stain on actual negative density as that would impact upon the light spectrum that is exposing a paper.
In the case of Azo, which exposes primarily to blue and upper band UV, the effects of stain upon the density presented to the passage of that quality of light must be a valid comparator. At least I have found it to be true in my experience. The addition of a value of .30 pyro stain density onto a visual density of 1.10 would be a sizable effect in the case of Azo. In fact if I were relying on a visual channel reading of 1.60 to achieve a 1.30 net contrast range (subtracting .30 as the actual low density). I would be dismayed to find that the negatives were in fact printing with a density range of 1.60 and be of too extreme a contrast to print on Azo. When one is shooting 8X10 and 12X20 negatives the cost of these errors would be regretful.
Now on to the matter of green/magenta on VC materials. I think that we can agree that green or magenta are the colors that affect the higher contrast emulsion on VC materials. It would appear to me that any effect of staining density upon the transmission of those colors would be affective of that high contrast emulsion. While my tests indicate that Pyrocat and ABC pyro are not as influential on the magenta/green channel as upon the blue channel, there is indication that the pyro stain does in fact affect density as it affects the transmission of the magenta/green spectrum.
I would appreciate any further thoughts that you have on this matter.
Thilo is right to the extent that a density reading with a color densitometer, presumbably through the blue channel, will not give you a exact equivalent of the actual effective printing density range of the negative. That is because, as he rightly points out, the actual spectral sensitivty of VC papers and graded papers is different, and in fact even varies by manufacturer. And the matter is much more complicated for VC papers than for graded papers.
Experienced BTZS users who print in AZO know that readings of stained negatives with the blue channel will not give an exact effective printing. However, it will bring you very close and with some modification of develoment times based on actual field tests you can establish effective N, N+ and N- times for a variety of shooting situatioins a lot faster than with other testing methods.
As for alternative processes that depend primarily on UV light, the actual effective printing density of a stained negative is very, very close to the density range I get from a UV reading at 373 nanometers. This has proven true for me in carbon, kallitype and palladium printing.
I have only very little expericence with AZO. But what you are saying makes sense to me.
Since AZO is only sensitive to blue light, your blue channel reading may give direct results. The problem is, that you must use the blue channel to measure yellow/brown stain even if your paper is sensitve to other colors as well. Your lucky case is that the blue channel actucally "sees" like the AZO paper does. But this is not true for most papers, including graded.
Since the stain is actually a color density (that's why you are using a color densitometer to determine it), one usually cannot ignore the color issue. One must look at it with the paper's eyes.