Comparison of absolute values of diffuse reflection spectral factor obtained by various methods

In the USSR there are two absolute methods of measuring the diffuse reflection spectral factors realized in two high-precision ethalon devices. Hence the unity of measurements requires the comparison of the measurement data obtained from these two devices. This work presents the results of comparisons of these two ethalon devices. We also describe the peculiarity of other main methods for absolute measurements of the diffuse reflection spectral factor.