An incidence model is formulated from the basic principles of radiation heat transfer to predict the light intensity profile inside a photocatalytic monolith reactor, where a lightabsorbing and light-reflecting catalytically active thin film is coated on the inner walls of monolith channels of either circular or square cross section. A diffuse external light source and diffusely reflecting wall coatings were assumed. The mathematical model representation of the light flux distribution to the monolith wall and through the cross section of the monolith channel take the form of integral equations. In dimensionless form, these equations reveal that, for a given channel type, light intensity profiles are controlled by channel aspect ratio and film reflectivity. The equations were solved numerically using Gauss-Legendre quadrature to give quantitative estimates of the radiation intensity profile down the length of the monolith channel for a specified incident light intensity distribution at the entrance of the channel and an assumed thin film reflectivity. Experimental cross-sectional light intensity profiles for square channeled, uncoated ceramic monoliths with two different cell densities confirmed the prediction of the model that dimensionless profiles are not dependent on absolute channel dimensions, but rather are uniquely determined by the channel aspect ratio. Experimental intensity data for titania-coated monoliths were well described by model predicted profiles assuming an average reflectivity of 40%. For identical aspect ratio channels, model predictions reveal that the light intensity profiles for square and circular channels are quite similar. Model predictions indicate that radiation field gradients are large, with relatively little light penetrating beyond a length equivalent to three channel widths. This prediction implies that, for monoliths with typical commercial aspect ratios, a large fraction of the coated channel wall is not effectively irradiated.