The photopic luminous efficiency function (V(λ)) is used internationally for the definition of light output from electric light sources (lumens) and thereby for regulating minimum luminous efficacy requirements (lumens per watt) for the manufacture and sale of light sources. V(λ) has, however, a long-wavelength spectral bias with respect to the overall spectral sensitivity of the human retina. When used in luminous efficacy regulations, the long-wavelength spectral bias of V(λ) effectively penalizes many of the benefits expected from lighting that can be provided by short-wavelength light (e.g. scene brightness, colour rendering, circadian regulation and off-axis detection). Regulators must use ad hoc reductions to luminous efficacy requirements for ‘cool’ light sources to ensure that lighting benefits can be provided to society. These reductions would be unnecessary if a luminous efficiency function representing the overall spectral sensitivity of the human retina were used instead of V(λ) in luminous efficacy regulations. The universal luminous efficiency function (U(λ)) is proposed as a replacement. Utilization of U(λ) would obviate ad hoc adjustments to regulations for ‘cool’ light sources, minimize wasted electric power imposed by regulations based upon V(λ), and perhaps most importantly, encourage manufacturers to produce light sources that efficiently provide multiple benefits to users.
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