In order to assess the solar radiation on the complex geometry of Solar Tower receivers, usually detailed maps of the flux distribution are generated using optical simulations based on ray tracing techniques. Transient modeling including a large set of such simulations implies very high computational effort. A new methodology is presented which allows for transient assessment of the flux distribution based on a comparatively small set of optical simulations and subsequent interpolations. For this purpose, two different discretization grids are used: a set of uniformly distributed solar vector nodes on the sky hemisphere and a set of different heliostat field fractions being on focus. For the interpolation in the sky discretization, three different techniques are introduced and compared in terms of accuracy. Partly defocused heliostat fields as well as complex aiming strategies can be readily taken into account by the presented approach. The methodology is validated by means of two exemplary test setups (PS10 and Gemasolar). The accuracy of the different interpolation techniques depending on the refinement of the discretization grids is assessed with appropriate error measures. Depending on the temporal resolution of the transient application, the computational effort can be reduced by several orders of magnitude compared to a direct simulation of the flux distribution for every time step. In addition to the quantitative validation, the use of the developed methodology in conjunction with a thermo-hydraulic simulation is demonstrated by means of the PS10 setup.