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The use of a free surface boundary condition for radiation streaming into an optically thin exterior region of a plasma radiation source (PRS) is developed and tested within the Mach2 code. The matching problem for diffusive and free streaming radiation flux at an interface is solved in the presence of energy exchange between the diffusing radiation and the ambient electron fluid. It is found that the ratio of Planck to Rosseland opacity is a strong modifier of the radiation temperature profile required to meet the match condition. Moreover, as compared to an external radiation temperature boundary condition, a free surface boundary condition predicts a radiation fluence more compatible with a semithin emission regime assessed by the tabular CRE model. The free surface condition also removes the need to assess the radiation temperature at simulation boundaries far away from the PRS.