End to end calibration of a radiometer at high irradiance levels

In this contribution we present a method to calibrate a radiometer at high irradiance levels (18 kW/m2). The method has been developed in the frame of the ESA Technology Research Programme. The radiometer under test is used as a reference detector in solar simulators. In order to underpin that the irradiance level that is generated in the solar simulator is within the required levels of uncertainty, the performance of the reference radiometer has been validated. The concept of the calibration of the radiometer is that the radiometer cavity is exposed to a known optical flux, provided by a laser. Taking into account the area of the cavity aperture, which is measured independently with a coordinate measurement machine, the average irradiance level is determined. The reading of the radiometer is then compared to the applied irradiance level. For this calibration the challenge is not only to achieve SI traceability at high irradiance levels, but also to meet other requirements like not to exceed a pre-determined level of irradiance at any position of the radiometer cavity, to avoid potential damage. The measurement uncertainty for the calibration of the instrument is <1%. Results show that the irradiance provided by the radiometer agrees within the measurement uncertainty with the known applied irradiance over the tested calibration range (0.04 W/cm2 - 1.8 W/cm2).