Calorimeter to measure the 10.6 - µm absorption of metal substrate mirrors

An instrument to calorimetrically measure the 10.6-micron absorption of high-reflectivity metal-substrate mirrors has been designed, constructed, and tested. Optical, electrical, and thermal design considerations are discussed. The technique of absorption measurement, including the in situ heat capacities, the reflected laser light to an accuracy of plus or minus 4%, and the simplified data reduction technique are discussed. From an error analysis it is concluded that the accuracy is plus or minus 0.1 times the 10.6-micron absorption and has a precision of 0.0005. Comparative measurements on the same mirrors by different reflectivity and absorption devices and extensive absorption data on state-of-the-art 10.6-micron metal-substrate mirrors are presented. The best absorption that has been measured is 0.0010 for a gold-coated multidielectric enhanced mirror.