This paper outlines the scientific methods that have been employed at different times to compare the efficiencies of light sources. The historical methods of estimating and the values obtained for "luminous efficiency" and for the "mechanical equivalent of light" are noted briefly to show how they are inadequate for our more exacting present needs, and how they have evolved into the current definition of efficiency. The object of this paper is to aid in clearing up the confusion that exists at present, and to bring to the solution of the scientific side of the problem some pieces of work which have only recently become available, or whose availability has not heretofore been realized. The discussion centers on four topics: (1) "Radiant Luminous Efficiency," the most frequently used basis in comparison of light sources
(2) "Total Luminous Efficiency"
(3) "The Mechanical Equivalent of Light"
(4) "Reduced Luminous Efficiency," the term applied to the more rational and exact basis of comparison that it is the object of this paper to present and emphasize.