Reference luminous solar constant and solar luminance for illuminance calculations

A recently proposed extraterrestrial solar spectrum is used to derive reference values of the luminous solar constant. Two slightly different photopic curves, CIE's V(λ) and VM(λ), are used for the needed convolution with the spectrum. A historical perspective about these two curves is presented, with the conclusion that both curves can be useful, depending on application. Two reference values (one for each photopic curve) for the luminous solar constant, the Sun's luminous efficacy and the average Sun's luminance are proposed, consistent with the most recent determination of the solar constant. Comparisons are made with earlier determinations of these quantities, as well as of the average Sun's radiance and the Sun's effective temperature.