We measured the optical characteristics of ultra low-loss and high reflectance mirrors. Reflectance was evaluatedby means of finesse measurement using the ring down method. The mirrors show a loss of 4.8 ppm and finesseof nearly equal to 105. Surface distribution of reflectance was measured for ultra-high quality mirrors fabricatedwith a RF-type Ion Beam Sputtering machine. The average reflectance was 99.9915% with a standard deviationof 8 ppm. The measured area was 5×5 mm by step of 0.5 mm. Two-dimensional mapping of mirror loss wasmeasured for the mirrors of a 20 m gravitational wave detection antenna using small laser spot. The mirrorloss coefficient was evaluated based on the transmission efficiency of the Fabry-Pérot cavity. The measuredarea was 7.5×5 mm. The average loss value was 40 ppm for the small laser spot measurement. The mirrorloss coefficient for a large beam was measured using a 20 m Fabry-Pérot cavity. The mirrors show a loss of 28ppm and a reflectance of 99.9875%.
