  The luminosity constraint is a very precise relationship linking the power
released by the Sun as photons and the solar neutrino fluxes. Such a relation,
which is a direct consequence of the physical processes controlling the
production and the transport of energy in the solar interior, is of great
importance for the studies of solar neutrinos and has a special role for the
search of neutrinos from the CNO cycle, whose first detection with a 5$\sigma$
significance has been recently announced by the Borexino collaboration. Here we
revise the luminosity constraint, discussing and validating its underlying
hypotheses, in the light of latest solar neutrino and luminosity measurements.
We generalize the current formulation of the luminosity constraint relation so
that it can be easily used in future analysis of solar neutrino data, and we
provide a specific application showing the link between CNO and pp neutrino
fluxes.
