Quote Originally Posted by Bill Burk View Post
There's meter spectral response, human eye spectral response, film spectral response and the spectral distribution of light and the "color" of the test target. Lots of variables.

Many people inadvertently obtain a "Tungsten" speed rating by their tests without realizing it.

Others put an 80b filter over the light or lens to (at least partially) simulate daylight.

More important in my mind is to stick with a test plan where you understand it's limitations, and strive for consistency. Yes it's significant in your case, but if your test-to-test result stayed within 2/3 stop (or 2/3 N step), then you can use your test to control your processes.
Thanks again Bill. I agree with what you wrote above. I might perform one more test before deciding on what process control test I will regularly perform. That test I'll do is to repeat my testing of photographing a transmission step tablet under particular controlled conditions. That is to stick the tablet onto a piece of white translucent perspex, masking off all areas outside the tablet as seen by the camera then affixing that assembly to an outside facing window, and finally draping a black cloth between the mask and the camera to ensure no additional light can enter. This is about as controlled as I can get the conditions when including the camera in the testing process.

regards
Peter