Comparison between absolute thermal radiometers at 1300 and 1550 nm wavelengths

In 1991, the last international comparison of spectral responsivity with germanium detectors showed relative differences between national laboratories of up to 10⁻² at wavelengths of 1300 nm and 1550 nm. The 1997 EUROMET international comparison of optical-fibre power meters showed relative differences between European laboratories of 10⁻². The coincidence of the results from the two comparisons suggests that the radiometric standards used by these laboratories in the calibration of their power meters are the main cause of such differences, rather than the optical-fibre or transfer devices used in the comparisons. To test this assumption, three different electrical-substitution radiometers (ESRs), working at room temperature and used at the same wavelengths as radiometric standards in optical-fibre laboratories, were compared. The radiometers were an optical calorimeter, a pyroelectric radiometer and a thermopile. The relative differences obtained were within 5 × 10⁻³.