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luminous efficacy

British  

noun

  1.  K.  the quotient of the luminous flux of a radiation and its corresponding radiant flux

  2.  ηv.   Φv.  the quotient of the luminous flux emitted by a source of radiation and the power it consumes. It is measured in lumens per watt

"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