BCD - Chalonge and Divan - 1952

Three quantities (D, 1 and b) are defined over the 3300-4600 Å portion of photographic prism spectra.
D measures the magintude of the Balmer discontinuity, 1 indicates the effective location of the Balmer discontinuity in the spectrum and b is related to the slope of the continuum in the blue region of the spectrum longward of the Balmer discontinuity ( 3800 - 4800 Å ) [65] .