We can use the infrared radiation characteristics of a target for target recognition. Data on infrared radiation characteristics obtained by out-field infrared imaging equipment is significant in evaluating early warning, reconnaissance, and stealth effects. It is difficult to obtain the response coefficients of out-field infrared imaging equipment. We introduced and compared radiometric calibration methods using a collimator and an extended-area blackbody. We conducted experiments using different calibration methods and then provided response coefficients of the out-field infrared imaging equipment. The long-distance radiometric calibration results showed different response coefficients at different distances. An infrared imaging system conducted calibration experiments with different working times and fusions. The radiometric out-of-focus calibration results showed that diffusion is not the main factor influencing calibration. Calibration experiments for different working times also showed that the response coefficients remained unchanged. The factors affecting the radiometric calibration of the out-field infrared imaging equipment are environmental radiation, path radiation, and path transmission. Short-distance radiometric calibration using an extended-area blackbody is necessary to obtain the response coefficients of the out-field infrared imaging equipment. If the radiometric calibration distance is less than 10 m, the error between the short-and long-distance radiometric calibrations is approximately 5%. This research helps out-field radiometric calibration of ground-based infrared imaging equipment and designs a radiometric calibration–measuring system.