Abstract: Method and apparatus for expressing as a number the relative brightness of artificial illumination as it is perceived by the average observer. There is extracted (i.e., detected) from the illumination to be measured, a blue-appearing narrow band, a green-appearing narrow band, a yellow-appearing narrow band and a red-orange-appearing narrow band. From these extracted bands are generated six different signals: a signal expressed as (aB) where (a) is about 0.23 and (B) is the watts of energy in the blue band, a signal expressed as (bG) where (b) is about 0.97 and (G) is the watts of energy in the green band, a signal expressed as (cR) where (c) is about 0.33 and (R) is the watts of energy in the red-orange band, a signal expressed as (dY) where (d) is about 0.19 and (Y) is the watts of energy in the yellow band, a signal expressed as (eG) where (e) is about 0.25 and (G) is the watts of energy in the green band, and a signal expressed as (fR) where (f) is about 0.