Quote Originally Posted by gainer
Not me. I still don't have a way to determine if my step wedge is neutral.
I found a few more sources and found mention of "Status V" densitometry - V for Visual. Of course, no one has gone into enough detail to actually describe the spectral characteristics of the Status V filter. But they did say that they were designed to replicate human vision for densitometric purposes. I suspect that the Gretag Visual filter that was discussed above would comform to this specification, as Gretag seems to know what they are doing when it comes to constructing densitmeters. But as we've seen, the filter used in the D-200 II densitometer, doesn't completely cover the range of human vision. But for densitometric purposes, it is probably sufficient. If more detail about the transmission characteristics of a material are needed, then Gretag and others will gladly sell us a spectrophotometer, which will be able to make density measurements on a wavelength by wavelength basis (or at least every 10 nm according to some of Gretag's brochures).

But along with the type of filters used are requirements on sensor position, light sources...

Quote Originally Posted by gainer
If it is not neutral then I have no way to compare, to within the proverbial gnat's behind, what I measure with what you measure. I know that some of my negs are not neutral, but neither of us will know by how much from either my measurements or yours, even if we use the same filter, unless we know the native spectral sensitivities of our densitometers as well as the transmission spectrum of our filters..
But if we both have densitometers, and the are both deisgned and contructed to conform to ANSI/ISO specifications that I assume are out there, and the filters used in them conform to the specifications as well, then we should get directly comparable results.

I would suggest that if we had the same filtration, that doing that would go a long way towards getting comparable results. At least we would be looking at the same band of wavelengths. And since most light sources used in densitometers are going to be incandescent, and even with the small range of color temps that those sources may have, they should be close as well. So that leaves detectors - I would be surprised if any modern densitometers use CdS cells (too sluggish and not much linear range) - I bet Si photodiodes are much more likely. Whether they go to the effort of using a Silicon Blue diode or not, I don't know. If they intend to do UV measurements, then it is probably not used as they drop off response faster than a regular Si photodiode.

I looked around the ISO site, and they do have several specs on densitometry, but at around $50 a standard, I have to pass on getting them. I'll see if I can find a library that has some of them around here...

Quote Originally Posted by gainer
It is true that when we are dealing with negatives that look neutral we can get a mutually agreeable measurement of contrast index, so that if I tell you that a certain film-developer combination developed under certain conditions gives a certain contrast index, you ought to come within experimental error of the same result in your darkroom.
I agree completely. Problems arise with materials that are not fairly neutral.

Quote Originally Posted by gainer
I think we stand a better chance of comparing certain results if we stick to narrow band filters.
I agree with you when you say we should get better agreeable results if we are both using narrow-band filters. In fact, I'll go further and claim that if we used filters that only transmitted very narrow bands of light, say like if we used a monochromator which could give us bands of light as narrow as a couple of wavelengths, then we should get identical results (within experimental error) as the response curve of the detector and the spectral distribution of the light source will not matter. There will essentially be no slope or curve to either one of those things to come into play. As you have pointed out before, we have reduced all the sources for error down to simply the measurements of the incident vs. the transmitted light.

PS - I like bouncing thoughts too!