Quote Originally Posted by Jorge
It has to do with the thickness of the material and being able to read it with the densitometer probe. If the material is too thick you will have extraneous light going into the probe, plus the aligment of the sensor will not be the same as if it was a thin material, for which these instruments are designed.

Suppose you turn off all the lights to read the glass or acrylic filters, then at the same time you have light from the densitometer bulb escaping due to thickness.
I can understand what you are saying. I am not sure that it is applicable and I am not sure that it is not applicable. The factor that I see is that the sensing unit in the head of the densitometer is still reading only the known light as presented on the light source side of the material. Extraneous light or the absence of it would seem to be of no effect since the sensing unit is flat against the material. I have tried to read densities of samples of the material that I want to use for filters and the readings seem to be reasonably accurate (at least at this juncture). I will try the readings in actual field conditions and see how they hold up. Thanks for your thoughts.