A radiant source with a large aperture at 5-95 Celsius degree in the wavelength bands of 8-12 'mu'm for calibrating infrared imaging systems has been designed. The effective emissivity of its flat bottom with concentric V-grooves was evaluated by the Monte-Carlo method whose correctness was tested and accuracy was discussed. The structure of the source was completed by incorporating the simulation results with the blackbody cavity effect. The source was certificated via an optical measurement system. The source can provide a consistent radiant flux with temperature uniformity of +-0.1 Celsius degree over an area of diameter of 80 mm.