Technique to estimate the reflectance of a high-reflectance dielectric multilayer coating mirror using incident beam angular dependence of its transmittance

The reflectance of a high-reflectance dielectric muitilayer coating minor tuned to a monochromatic laser wavelength was estimated from its transmittance which was measured with 0.4 ppm accuracy. The transmittance was calculated from its theoretical formula including three parameters (refractive indexes of low and high refractive films (n(L), n(H)) and a number of layer units composed of a pair of one low and one high refractive film (N)) which were estimated from the incident beam angular dependence curve of the transmittance. This calculated transmittance agreed with that measured as a ratio of the laser beam power before and after the mirror with 6 ppm difference.