Patrick, I think we are thinking in different directions. I've been trying to discuss design considerations of a densitometer - for testing stained negs. A a color densitometer essentially, so that we can measure the density of negatives at various wavelengths. You seem to be telling me that it can't be done. I don't believe that you actually believe that.
There are certainly a lot of people that are selling color densitometers out there.
And I don't think you are trying to convince me that these instruments can't function. They certainly do. They are constructed with care being taken to the spectral properties of the light source, the optical pathway of the design, the spectral properties of the filters used, and the spectral response of the sensor. These manufacturers also supply calibration standards that have been measured under specified conditions. Calibration standards are needed because no matter how ideal our design, electronic parts, or the materials used to built the instrument, it will not have a perfect response. And since the standards have been measured under specified conditions, they should allow us to be able to exchange measurements and then hopefully have a reasonable expectation of comparable results.
You appear to be headed somewhere else - so I guess I'm not quite sure where you are going with the last post.