I have a Heiland black and white transmission/reflection densitometer. I'd like to "adapt" it a little for doing some admittedly imperfect readings of Pyro negatives. On a colour densitometer I'd be using visual, blue and green channels. Since some models (eg Macbeth) do colour readings by rotating Wratten filters into the light path, I thought I could do ok using Kodak Wratten gels. I have a few of these: #47, #47b, #58. I've tried this before using the #47 (and #58) and got readings that at least "made sense".
However I thought I remembered something about the Macbeth from Richard Henry's book so I went back to check and sure enough... For measuring reflection densities the Wratten filters are fairly standard (#25, #58, #47) but for transmission they are #92 (red), #93 (green) and #94 (blue). Why would the peak transmittances, cutoffs etc need to be different for transmission and reflection measurements?
Second question, if I use the gel filters, should they be placed between the light source and negative or between the negative and the reading head? I've noticed a difference of a few density points depending on this. I don't know why that would be.
I'd track down the sensitivity spectrum of the paper you intend to use and find a filter that is compliment of that. I'd look on the Roscoe site, they list the spectral transmission of all their filters. Here is an example. I'd put the filter wherever is convenient, but be consistent.
Very good point. I mostly use Ilford MGIV FB. I have a list of all the Wratten transmittances but I will look at Rosco and Lee as well (probably a lot easier and cheaper to get than Kodak/Tiffen). I'm still wondering though if in general the measurements I get with the #47 and #58 would be reasonable averages.
There are actually 3 sets of filters. We called them Status filters and they were:
Status A : Positive films
Status M: Negative films
Status D: Reflection print materials
Ok, now why without technical details....
The reversal films are designed to be viewed by the eye as a reflected image from a screen through projection. The Status A filters are designed to look at reversal films in such a way as to pretty much duplicate the response as you would see it.
The color negative film is designed to be printed to form a reversal image on film or paper. The filters therefore mimic the response of the print material to the dyes.
The color papers cause some degree of "distortion" of the dye image due to the reflective back and the internal reflections. Therefore, the filter set is designed to "see" what the human would see due to the distortion.
You get quite different curves from each filter set, but if you get used to using a "wrong" set, you can make do quite easily, but there is a limit if you are doing ultra critical work. In some cases, you can see a neutral but the densitometer will not read it as a neutral - or vice versa.
Thanks, PE. It should have occurred to me you'd need different filters for colour negatives and papers. I was thinking too "B&W". I guess ideally I'd use a filter that matches the sensitivity of the paper as closely as possible. Difficult with VC paper as I'd need a blue-green colour. I've designed a few experiments and we'll see what happens.
To use a gel filter with a B&W densitometer, I assume I should zero it with the filter first?
Any idea why it might make a difference of a few density points if the filter is placed between the light source and the emulsion side of the negative (ie below the negative) vs between the base side of the negative and the reading head (ie above the negative)? I'm assuming it must have something to do with small differences in reflection/transmission of light depending on where the gel filter is in the light path, but not sure how. I'm just trying to get it as comparable as possible to the measurements made without the filter.
Of course I'm also assuming a white light reading of a stained negative gives an accurate measurement of silver-only density. A lot of assumptions here...
The instructions from my densitometer tell to put the negative emulsion-up when reading. I believe it relates more to the definition of "diffuse" reading, and affects the scattering of light, more of which is then collected by the photocell if the scattering occurs up into space, than into the base and light-piped elsewhere.
I figure a blue filter would give different readings than the visual filter, and the difference may imply "density" as might be attributed to stain. Don't know that it would accurately measure it, but at least it would detect yellow.
I cannot answer the questions here. I can only refer you to the experts.
I have the very same densitometre as you. Before I picked up a colour densitometre, I used a #47 filter when reading pyrcat negs. Worked well.
I guess I'd ask how do you know it worked well? Even with a colour densitometer, densitometry with stained negatives is quite complex. We can only generate approximate curves, especially if we're using VC papers. In the end the only way to really do it is to print the results and see how the paper actually "sees" the silver and stain.
From my testing of pyro stained negs, I'd suggest measuring with a red filter to get just the density for the silver portion of the neg. Then when you compare red to blue and green density, you can better see the amount of density from the stain.
Be aware that pyrogallol stain is different than pyrocatechol stain, and you can see that in the ratio of blue to red vs green to red.