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The penetration of natural and artificial UV-BBE (Biologically Effective UV-B, Caldwell 1971) and PAR (400–700 nm) in a grass canopy with increasing LAI was followed during 2 months. Overall, the transmission of UV-BBE sunlight is significantly higher than of PAR sunlight. This is mainly due to the higher proportion of diffuse light in the UV-B. Under cloudy conditions no difference between UV-BBE and PAR could be found. Sun angle and intensity of the radiation were less important in determining the penetration of light. Artificial light penetrates much more through the canopy, resulting in higher irradiation levels in the lower part of the canopy, but a lower UV-BBE/PAR ratio (since UV-B transmittance of the leaves is lower). The UV-BBE/PAR ratio reaching the leaves was influenced by LAI, sun angle, percent diffuse light and leaf angle. The large differences in UV-BBE/PAR ratio per unit leaf area under natural and artificial light conditions are important in understanding the influence of UV-B on plants.