In this paper we extend our radiation field model previously developed to predict the monochromatic light intensity profile inside a photocatalytic square-channeled monolith reactor to predict the integral average polychromatic light intensity profile. In this extended polychromatic formulation, the mathematical representation of the model accounts for a source that emits a distribution of wavelengths and for wavelength-dependent optical properties (reflectivity) of the active photocatalytic thin film coated on the inner walls of the monolith channels. In the sense that these wavelength dependent properties of a given photocatalytic reactor system can be independently measured, the polychromatic radiation field model contains no adjustable parameters. Model predictions for the integral average light intensity transported through square monolith channels of varying length are in excellent quantitative agreement with experimental measurements employing Degussa P25 titania-coated monolith channels and near ultraviolet lamps emitting light primarily in the 300-400 nm range. The model reveals that axial light intensity gradients are severe, with the light intensity in the portion of the monolith channel beyond about 3-4 aspect ratios in length falling to below 1% of the incident light intensity. At a given axial distance down the monolith channel, the effective light flux that reaches the catalytic wall is approximately a factor of two lower than the total light flux through the channel cross section near the channel entrance, and an order of magnitude lower than the total cross section light flux near the channel exit.