A spectral model of scalar irradiance with depth is applied to calculations of photosynthetically available radiation for a vertically homogeneous water column. The model runs more than 14,000 times faster than the full Hydrolight code, while it limits the percentage error to 2.20% and the maximum error to less than 4.78%. The distribution of incident sky radiance and the effects of a wind-roughened surface are integrated into this model. It can be applied to case 1 waters as well as to case 2 waters that happen to be gelbstoff rich, and the volume-scattering phase function can be generated dynamically based on the backscatter fraction. This new model is both fast and accurate and is, therefore, suitable for use interactively in models of the oceanic system, such as biogeochemical models or the heat budget part of global circulation models. It can also be applied by use of remote-sensing data to improve light-field calculations as a function of depth, which is needed for the estimation of global ocean carbon production and the ocean heat budget.