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We calculate numerically the spectral power distributions (SPDs) which maximize luminous efficacies of radiation (LERs) for white light of particular color rendering indices (Ra's) and color temperatures (CTs). We find that, except for the very highest color rendering indices, the spectra are spiky rather than continuous. We present a useful analytic expression for the dependences of the maximum white luminous efficacy of radiation (MWLER) on Ra and CT and discuss these dependences. We propose that, for any white light source of a given Ra and CT, its absolute spectral efficiency is simply the ratio of its LER to the MWLER at that same Ra and CT. We discuss the absolute spectral efficiency, defined in this way, of various lighting technologies: incandescent, fluorescent, high-intensity discharge, and solid-state lighting. Finally, we discuss the possibility of alternative MWLERs based on alternative indices for color rendering quality.