The index of refraction of the ordinary ray in sapphire for temperatures from 24 to 1060°C and for wavelengths of 633 and 799 nm was found to be expressed to 0.02% (99% confidence level) by nS(T)633nm = 1.76565 + 1.258 × 10−5T + 4.06 × 10−9T2 and nS(T)799nm = 1.75991 + 1.229 × 10−5T + 3.10 × 10−9T2 where T is the temperature in degrees Celsius. These expressions were calculated from measurements of the relative change with temperature in the reflectance for a plane surface normal to the c axis of single-crystal sapphire.