This document specifies measurement procedures for the precise determination of the high reflectance or high transmittance (>99 %) of optical laser components. The methods given in this document are intended to be used for the testing and characterization of high reflectance of both concave and plane mirrors or high transmittance of plane windows used in laser systems and laser-based instruments. The reflectance of convex mirrors or transmittance of positive or negative lenses can also be tested by taking into consideration the radius of curvature of the mirror surface or the focal length of the lens. This document is complementary to ISO 15368 which specifies the measurement procedures for the determination of reflectance and transmittance of optical components with spectrophotometry. ISO 15368 is applicable to the measurements of reflectance and transmittance in the range from 0 % to 100 % with a typical accuracy of ±0,3 %, and is therefore not applicable to the precise measurements of reflectance and transmittance higher than 99,9 %.