Spectral irradiance and spectral actinic flux in the troposphere are calculated by a matrix operator model (Nakajima and Tanaka, 1986), with respect to solar zenith angle, height, and wavelength. Different surface albedos, aerosol, and air density distributions are considered. The actinic flux and the irradiance are split into a diffuse and a direct part. The ratio of diffuse irradiance to diffuse actinic flux rd is calculated and discussed. The calculations have shown that the approximation diffuse ratio rd=0.5 results in an error up to over 20% for calculated photolysis frequencies. However, no simple expression for the diffuse ratio can be given. The diffuse ratio is not constant with height and depends on the solar zenith angle and wavelength. ¿American Geophysical Union 1993