A radio-frequency transparent window having internal conduits for the passage of cooling fluid is configured for simulating a highly uniform thermal environment for testing a device intended for use in space. The device to be tested is placed within a chamber in which a vacuum condition is maintained by a radio-frequency transparent pressure window under a pressure seal. Within the chamber, the thermal window is positioned adjacent, but not in contact with, the pressure window. A radio frequency signal is capable of passing directly through both the thermal window and the pressure window to permit communication with the device being tested within the housing. The thermal window is not in contact with the device so there in no conduction of heat from the device. Radiant heat transfer may occur from the device to the thermal window.