Abstract: How to reach a uniformly distributed temperature in a substrateradiatively heated in vacuum is one of the major issues in the thin filmdeposition. In this article, numerical calculations and infraredcolorimetric measurements of temperature are made to analyze a radiation heater usedin vacuum, named as IMCAS-VRH. IMCAS-VRH is used to heat a monocrystal siliconsubstrate with a diameter of 6 inches, under an electric power of3\,860\,W. The mean temperature over the substrate is 1\,093\,K, while thewhole temperaturevariation is within 6\,K. The calculated temperature distributions agree wellwith the measured data, and further computational analysis shows that the Mofilaments' shielding from radiation, the thermal insulation plates, andthe thermal conductivity of the radiation cavity all affectsignificantly either the mean temperature or the temperaturedistribution uniformity in the substrate.