I work with .15 steps because i plan to introduce a total measurements option, meaning the user can input the X axis in luxs.
Could you elaborate on the calibration curve part ?
Since I mentioned speed and curve calibration I should answer both...
Right or wrong, this is how I declare the luxs that hits the film plane of the sensitometer:
Measuring luxs directly at the test exposure plane is hard. But indirectly is easy. Set up the "sensitometer" to be as consistent as possible. when you "find" a curve that fits ASA parameters (1.3 run 0.8 rise from the point 0.1 above base+fog) then it's fair to deduce that the film has reached its rated speed (assuming fresh film with good speed reputation and developed in standard developer). You can pin the luxs on that specific curve. From that day forward, that one curve is the luxs reference. So even though I determined the exposure using TMY2, today I used the same sensitometer exposure for TMY2, 5222 and Panatomic-X.
I would submit that standard curve for every run.
The calibration of each step then, comes from the measured densities of the step wedge.
When you get a step wedge (e.g., Stouffer T2115), the steps are never exactly 0.15 apart. Instead, you can measure the actual densities and consider each step to be attenuation of the luxs that hit the step of the Stouffer scale. The calibrated T2115C is an exception, they cherry pick good ones and read them with NIST-traceable densitometer and write down the measured densities and many of those steps are 0.15
Same as with the sensitometer exposure, I would use the densities of the Stouffer scale steps in every run.
To complicate things, I add a neutral density across half the scale with higher density (lower exposures). Nominally 0.6 but measures differently, for example 0.56. So although nominally the scale covers 0.0 to 3.0 in 0.15 steps, it's really about 0.05 to 3.05 in roughly 0.15 steps (and I stretch it out closer to 3.6)
