-_nən(t)s noun
( -s )
Etymology: lumin- + -ance
1. : the quality or state of being luminous
2. : the luminous intensity of a surface in a given direction per unit of projected area
as the word reflectance refers to the effective physical reflectance of a surface for a given quality of light, so the word luminance refers to the effectiveness of a given light on the eye, regardless of its origin — R.M.Evans
the photographic quantities that are of interest to the photographer are luminance of the object photographed and illuminance of the sensitive plate or film — A.R.Greenleaf