Don't quote me on this empip but i believe the source point is the intersection where the light beam is inverted by the lens otherwise the lamps filament is a good starting point because of light scatter, reflector and lens influences and physical properties it's probably hard to accurately determine the exact point source in practice and probably doesn't make a lot of difference unless the lamp is being used for calibration purposes.empip said:It looks like the distance is measured from somewhere in or between the reflector and the front lense.
