  The formal link between magnitudes per square arcsecond and luminance is
discussed in this paper. Directly related to the human visual system, luminance
is defined in terms of the spectral radiance of the source, weighted by the CIE
V($\lambda$) luminous efficiency function, and scaled by the 683 lm/W luminous
efficacy constant. In consequence, any exact and spectrum-independent
relationship between luminance and magnitudes per square arcsecond requires
that the latter be measured precisely in the CIE V($\lambda$) band. The
luminance value corresponding to m_VC=0 (zero-point of the CIE V($\lambda$)
magnitude scale) depends on the reference source chosen for the definition of
the magnitude system. Using absolute AB magnitudes, the zero-point luminance of
the CIE V($\lambda$) photometric band is 10.96 x 10^4 cd/m^2.
