Over the last six years many experiments have been done at the National Ignition Facility to measure the Hugoniot of materials, such as CH plastic at extreme pressures, up to 800 Mbar. The “Gbar” design employs a strong spherically converging shock launched through a solid ball of material using a hohlraum radiation drive. The shock front conditions are characterized using x-ray radiography. In this paper we examine the role of radiation in heating the unshocked material in front of the shock to understand the impact it has on equation of state measurements and how it drives the measured data off the theoretical Hugoniot curve. In particular, the two main sources of radiation heating are the preheating of the unshocked material by the high-energy kilo-electron-volt x-rays in the hohlraum and the heating of the material in front of the shock, as the shocked material becomes hot enough to radiate significantly. Using our model, we estimate that preheating can reach 4 eV in unshocked material, and that radiation heating can begin to drive data off the Hugoniot significantly, as pressures reach above 400 Mb.