"point light source"as provided by Durst/Omega/Beseler uses small, clear 12 V DC bulbs (literally common automotive dome lamps). A large directional source is by definition not a point light source, especially if it's diffused.
Calibration is only calibration. My shoes are calibrated (size 11, US). We can calibrate just anything, but it's not necessarily part of the process. My shoe calibration may be helpful when I'm shopping for shoes, but it's only a fascinating topic for conversation otherwise. I can't tell you how many buxom young blondes I've picked up with scintillating shoe-size conversation.
My electric train transformer put out 12V, precisely. The rheostat allowed zero to 12. There are little marks that serve to calibrate, to the extent that's helpful. This calibration is simply an alternative control for exposure (like f-stops) if one decides to standardize on the same exposure time (eg 20 sec). Color temp does vary with voltage, that might have an influence on variable contrast paper, but this isn't a significant issue using graded paper.
And, in any case, the rheostat control makes a toy train transformer into a better tool than the fixed voltage that's traditionally used in point light source.
