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.