The spectral distribution and time correlation of spontaneous parametric down-conversion were measured, and a radiometer based on spontaneous parametric down-conversion calibration was established. Based on the detection efficiency of spontaneous parametric down-conversion calibration, a radiance measurement scheme in terms of photon count was proposed, the radiometer can correct the degradation of its own response and observe the radiance of the target in real time. By using the experimental technique of inserting optical attenuator and changing detectors, the radiometer can correct its own degradation if observed radiance measurement results under the condition of optical degradation and electronics degradation. The results show that the consistency of observed radiance can meet 0.4%.The measurement results provide experimental basis for the application of spontaneous parametric down-conversion calibration to space platforms.