the perceived brightness of light as a ratio of the total luminous flux to total radiant flux of the source; a measure of brightness obtained by dividing the source's luminous flux by the consumption of its energy.
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British Dictionary definitions for luminous efficiency
the efficiency of polychromatic radiation in producing a visual sensation. It is the radiant flux weighed according to the spectral luminous efficiencies of its constituent wavelengths divided by the corresponding radiant fluxSymbol: V
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012