In this article, we study an optimized radiative cooling geometry for ion projection lithography masks. In this design, a polished infrared mirror guides radiation from the mask to a high-emissivity cooled-cylinder positioned about 0.5 m away. Previous finite element modeling of this system predicts thermal distortion below I nm for practical mask designs. This is the first experimental study of the design. We show, by measuring the mean mask stress as a function of cooled cylinder temperature, that radiative cooling balances beam heating, with a power density of 2.15 mW/cm(2), when the cooled cylinder is held at 15 degreesC and the temperature of the mirror and mask frame are at 25 degreesC. The results agree with finite element modeling.