Originally Posted by Helen B
Forgive me if my explanations lack clarity but this is the first time I have considered this particular issue, and/or tried to explain it. My initial assumption would have been that the measurement of a neutral tone step wedge should be the same with both Visual and UV mode and in fact I don't fully understand why that is not the case.
However, as to which results are correct I again offer the suggestion that the log density units of measurments on the X-axis and y-axis need to be the same, i.e, taken with the same measuring instrument or mode of measurement. Otherwise the distance between equal units of log measurement would have to be different on the x and y axis. So you have apple to orange log units. In the case of the example cited the step tablet measured in UV mode had a total DR of 2.77 (2.98 - .10) in contrast to the DR of the Visual mode reading of 3.0 (3.05 - 0.05). But the actual range of 2.77 is being expanded to a physcial range on the X-axis of log 3.0 units. As one could predict this expansion of the X axis will result in a shallower slope, i.e. lower CI, if the plotting is based on the Visual mode reading.
With respect to your two questions.
"What is the purpose of the step wedge measurements - is it to tell the software what relative exposures the film had, or is it to calibrate the densitometer against the known UV transmission of a Stouffer step wedge?"
The purpose of the step wedge measurement is to establish a common unit of log density for the x and y axis.
"Can the software be used with other step wedges?"
Sure, but need to measure the step wedge with the same mode that you will measure the tests strips that are made from it.