Particularities of pyroelectric detectors in absolute measurements of chopped radiation shown for the example of a spectral responsivity calibration in the near- and mid-infrared spectral range at two primary radiometric standards

Abstract. The Physikalisch-Technische Bundesanstalt (PTB) expanded its capabilities for the calibration of the spectral responsivity s(λ) in the spectral range between 1.5 µm and 14 µm, traceable to the International System of Units (SI), with pyroelectric detectors as transfer standards. The pyroelectric transfer standards were calibrated absolutely against two independent primary radiometric standards, regarding their spectral responsivity s(λ). The first approach uses infrared laser sources at one of the PTB's cryogenic substitution radiometer facilities, which is a primary detector standard for the measurement of radiant power. The second approach uses a blackbody radiator with a temperature of about 1200 K, whose radiation can be calculated by Planck's law and is, in addition, spectrally selected by accurately characterized optical bandpass filters. Due to their measurement principle, pyroelectric detectors can only measure temporal changes in the input radiant power and are, therefore, operated with a chopper wheel to chop the incident radiation. The detector signal, which is typically measured with a lock-in amplifier, depends not only on the amplitude but also on the temporal shape of the chopped radiant power. It is shown that the calculation of the radiant power used for the determination of the spectral responsivity must be based on an accurate approximation of the temporal shape of the chopped radiant flux at the detector. This shape is different for both applied primary methods. It is further shown that the particularities of the lock-in-technique have to be considered in the calculation of the spectral responsivity, including the correct calculation of the detector signal. The results of the calibration with both approaches are consistent, and the realized measurement uncertainty is in the range between 1 % and 14 %. The pyroelectric detectors were thereby established as transfer detectors for the SI traceable measurement of radiant power in the near-infrared (NIR) and mid-infrared (MIR).