An analysis of the optical flashes produced by Leonid meteoroids impacting the Moon in 1999 November is carried out in order to estimate the fraction of kinetic energy converted into radiation, the so-called luminous efficiency η. It is shown that the observational data are consistent with luminous efficiencies of 2×10-3 in the wavelength range of 400-900 nm with an uncertainty of about 1 order of magnitude. This experimental value of η is significantly larger than previous estimates for meteoroids of asteroidal composition based on numerical calculations and scaling laws from laboratory collisions. According to our results, the luminous efficiency might vary with mass, i.e., the smaller impactors converting less kinetic energy into light and vice versa. A comparison with recent numerical simulations for meteoroids of cometary composition is also carried out.