In the imaging optical facility, the light source, the spectroradiometer and the optical path are the three fundamental elements. Most of time the light source is not a dot source and the radiation flux received by the spectroradiometer doesn’t only obey the rule of the geometry optics. Experiment shows that the spectral irradiance does not obey the law of the inverse squares with the variation of the diameter of entrance aperture. Compared to the radiation flux calculated using geometry optics, there is diffraction loss due to diffraction. First, the spectroradiometer is calibrated at a fixed distance and diameter. After the calibration, the spectral irradiance is measured with the change of the two parameters. With a spectral irradiance standard lamp and spectroradiometer with double monochromator setup, the consistence of the spectral irradiance at the same condition can be higher than 0.1% easily, which makes the comparison between the experiment and the calculation feasible. From the experiment, it can be seen than diffraction effect does affect the measurement result.