A method for calibrating spectral bidirectional reflectance in any illumination and viewing geometrics is proposed. The absolute bidirectional reflectance factor of a standard sample made by sintered polyte trafluoroethylene (PTFE) is determined in the range of 400-2500 nm. The reference standard is direction hemisphere reflectance factor transferred by the National Institute of Measurement. The absolute bidirectional reflectance factor can be measured on the instrument of measuring bidirectional reflectance factor can be measured on the instrument of measuring bidirectional reflectance-distribution function at nearly all the illumination and viewing geometrics.