As I see it, determining the real black point involves a series of measurements at low luminocities to find such L where readings between this L, a=0, b=0 differs from the reading of the same L with a/b changing to +/-2. It depends on the monitor and the instrument. I tried a different approach, extrapolating for the black point based on the measurements of the higher luminosity. It does not work well. -- Iliah Borg ib@xxxxxxxxxxx