Measurements of losses in high reflectance mirrors coated for λ = 514.5 nm

We have measured the losses of high reflectance mirrors coated for λ = 514.5 nm using the decay time of light in a Fabry Perot cavity. Currently the lowest losses we have measured in air, for substrates polished by General Optics and coated by British Aerospace using the ion beam sputtering technique, are 80±20 ppm. As expected, both the quality of the substrate and the method of deposition of the coating are important for achieving the best results.