An accelerated weathering test apparatus and method for calibration and operation thereof including a spectroradiometer for monitoring a full spectrum power distribution of a light source. Calibration includes a calibration light source in a factory test apparatus operated at a fixed power level and collecting the full spectrum power distribution of the calibration light source to generate a first data set. The calibration light source is then installed in a client test apparatus and operated at the fixed power level in order to collect the full spectrum power distribution and generate a second data set. The first and second data sets are filtered and aligned to determine a system response factor of the client test apparatus so that the irradiance level control may be calibrated.