1.2 - Absolute Mid-infrared Spectral Responsivity Scale Based on Thermal Detectors and the Cryogenic Radiometer

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).

Section: Introductionmentioning

confidence: 99%

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

Pohl

Meindl

Hollandt

et al. 2022

Absolute calibration of the spectral responsivity of thermal detectors in the near-infrared (NIR) and mid-infrared (MIR) regions by using blackbody radiation

Pohl

Meindl

Werner

et al. 2021

Abstract. The Physikalisch-Technische Bundesanstalt (PTB) has set up an additional measurement approach for the absolute calibration of the spectral responsivity of detectors in the near-infrared (NIR) and mid-infrared (MIR) spectral range. This alternative method uses the radiation of a blackbody operating at about 1200 K with a precision aperture. The blackbody radiation can be calculated by Planck's law and is additionally spectrally selected by accurately characterized optical bandpass filters. Thus, a calibration of the spectral responsivity of a detector with respect to irradiance can be achieved at the bandpass wavelength of the applied transmission filters. If the aperture of the detector is known, the spectral responsivity can also be calculated with respect to radiant power. Thermopile detectors with known aperture size were calibrated in terms of their spectral responsivity with several bandpass filters in the spectral range between 1.5 µm up to 14 µm with relative standard measurement uncertainties between 5 % and 19 %. The obtained results are consistent with previous calibrations at PTB's national primary detector standard. Therefore, this additional measurement approach is a further validation of the existing primary method which is based on a cryogenic radiometer and extends the usable wavelength range.

Measurement of the absolute spectral responsivity in the mid-infrared based on the cryogenic electrical substitution radiometer and an optimized thermopile detector

Pohl¹,

Meindl²,

Johannsen³

et al. 2019

Abstract. The Physikalisch-Technische Bundesanstalt (PTB) expanded its capabilities of the absolute measurement of radiant power to the spectral range of the mid-infrared (MIR) by implementing additional MIR laser radiation sources at one of the PTB's cryogenic electrical substitution radiometer facilities. This extension enables absolute calibrations of the spectral responsivity of detectors in the MIR traceable to the International System of Units (SI). The thermopile detector TS-76 was characterized and calibrated in view of its spectral responsivity s(λ) in the wavelength range between 1.5 and 10.6 µm at the expanded cryogenic electrical substitution radiometer facility. The relative standard measurement uncertainty was significantly reduced to 1.4 % by developing an optimized and thermally stabilized detector housing design. The TS-76 was established as a mid-infrared transfer detector for the SI traceable measurement of radiant power and the dissemination of the spectral responsivity s(λ) in the MIR.