However, the relative log values of the step tablet densities, which are plotted on the X-axis, are being used to determine the the configuration, or slope, of the curve on the y-axis. If the two units are not calibrated by measurment with the same mode, or color of light, it would appear to me that the CI of the curve would be either artificially expanded or contracted (slope increased or decreased), assuming of course a difference in measurement with different modes.
OK, I was very curious about this so I went back and compared results with an existing film tests, which was FP4+ in Pyrocat 1:1:100, using two different step tablet readings, both from the same step tablet but one made with Visual reading, the other with UV reading. The reading in Visual mode ranges from 0.05 at Step 1 to 3.05 at Step 21, and the UV reading ranges from 0.10 at Step 1 to 2.87 at Step 21.
As I suspected there was a significant different in the curves as plotted depending on which of the step table readings was used. The differences affected effective film speed, CI, and SBR values. To be precise, here is the difference when both calcualtions are made base on 10 minutes of development.
Step Tablet One, or the Default, made with Visual Reading: EFS=160, CI =.69, and SBR=8.3
Step Tablet Two, or the one made with UV reading: EFS=100, CI=.76, and SBR=7.5.
So my conclusion is that you do need to read the densities of the step tablet that will be used to expose your test negatives in the mode that will be used to measure these negaitves, and substitute these values in the WinPlotter program for the Default step tablet if the values are different.