Characterisation and uncertainty of measurement analysis of a detector spectral power responsivity measurement system

Abstract

The accurate measurement of spectral power responsivity of detectors is required in several applications, ranging from industries such as aerospace, defence and manufacturing to traceability for other optical measurements, such as the realisation of the SI unit, candela. At NMISA, the spectral power responsivity measurement capability is maintained on national measurement standards, published in the Government Gazette, to support such applications. In this study, a new measurement system for spectral power responsivity of detectors was implemented and characterised. This measurement system consists of a scanning double monochromator and other components selected based on system requirements, including sources of optical radiation, order sorting filters diffraction gratings, imaging optics and detectors. The first step of characterisation was to perform the wavelength calibration of the monochromator. This was done for the wavelength regions of 200 nm to 400 nm and 600 nm to 1 100 nm and produced the wavelength calibration equations used in the spectral power responsivity calibrations performed. Uncertainty of measurement analyses were performed for these calibrations and the results were used in the uncertainty of measurement determination for the spectral power responsivity calibrations. The second step of characterisation was to perform spectral power responsivity calibrations of detectors. In the UV region, 260 nm to 400 nm, the spectral power responsivity of a PtSi photodiode detector was determined through calibration against a UV-enhanced Si photodiode detector, previously calibrated by the National Physical Laboratory of the United Kingdom for spectral power responsivity, using the substitution method. An uncertainty of measurement analysis was performed for this calibration as a function of wavelength. The measurement results obtained were successfully verified through comparison against a calibration performed by Physikalisch-Technische Bundesanstalt in Germany for the same PtSi photodiode detector for spectral power responsivity. In the visible to near-IR region, 600 nm to 1 100 nm, the spectral power responsivity of a Si photodiode detector was determined. This was done through calibration for absolute power responsivity at 632,8 nm against a Si-trap detector using a stabilised HeNe laser source and extending this absolute calibration to the wavelength region of 600 nm to 1 100 nm, using a pyroelectric detector with known spectral absorptance, on the implemented measurement system. An uncertainty of measurement analysis was performed for this calibration. The absolute tie point calibration of the Si photodiode detector at 632,8 nm was successfully verified through calibration against a second Si-trap detector as verification standard and through comparison against a previous calibration. The spectral power responsivity of a Si photodiode detector determined on the implemented measurement system was successfully verified through comparison with its previous calibration. As a second verification, the Si photodiode detector was also calibrated for spectral power responsivity against the UV-enhanced Si photodiode detector on the implemented measurement system in the 600 nm to 1 100 nm region and the uncertainty of measurement was determined as a function of wavelength. Using this uncertainty of measurement as a function of wavelength for comparison, the verification proved to be unsuccessful at some wavelengths between 650 nm and 720 nm and 1 060 nm to 1 100 nm. However, since the measurement results were within the stated uncertainty of the Si photodiode detector calibration against the pyroelectric detector and Si-trap detector between 600 nm and 1 050 nm, with the exception of 660 nm, the spectral power responsivity of the Si photodiode detector was considered successfully verified between 600 nm and 1 050 nm. Following the implementation and characterisation of the new measurement system, the spectral power responsivity measurement capability at NMISA was assessed by SANAS for accreditation, which was obtained for the wavelength regions of 260 nm to 400 nm and 600 nm and 1 050 nm. Future research will include the establishment of the spectral power responsivity measurement capability in the wavelength regions of 200 nm to 260 nm and 400 nm to 600 nm.