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Photon-counting imagers such as photonic cameras and imaging spectroradiometers have been improved rapidly for cutting-edge applications such as multi-photon microscopy, super-spectral imaging, hyper-spectral LIDAR, and so on. At low light level, the optical radiant power (in watts) can be interpreted as photon rate (in photons per second) and this would make more sense especially when the photonic nature of individual photon is crucial for in-depth analyses. Similarly, the spectral irradiance and radiance can be formulated into spectral photon irradiance (or spectral photon flux) and spectral photon radiance, respectively, for few photon applications. At the National Institute of Metrology of China, the calibration facility of the spectral quantum efficiency for the photon-counting detectors has been established with uncertainties of <0.5%, traceable to both the classical absolute cryogenic radiometer and the calibration facility based on correlated photons, with the calibration using these two methods agreed within 0.3% @ 633 nm. However, the three major kinds of photoncounting detectors including silicon avalanche photodiodes (Si-APDs), photomultipliers tubes (PMTs) and superconducting single photon detectors (SSPDs) can be well calibrated for spectral photon rate measurements but not spectral photon irradiance (or spectral photon flux) or spectral photon radiance. For instance, Si-APDs usually have very small effective sensing sizes, PMTs have significant non-uniformity over the sensing area, and SSPDs need bulky cooling systems. Photon counting detectors with mm-sized uniform sensing areas can be developed based on photomultiplier tubes and the non-uniformity of the quantum efficiency was measured to be better than 3% at 633nm. The spectral quantum efficiency of the photon counting detectors can be further determined over the 300 nm ~ 1000 nm spectral range and the non-uniformities are less than 5%. Precision apertures with their areas carefully measured can be installed before these photon counting detectors for spectral photon irradiance (or spectral photon flux) measurements. For instance, the photon counting detectors with precision apertures can be applied to measure the spectral photon irradiance (or spectral photon flux) of an integrating sphere illuminated with a wavelength-tunable monochromatic light when placed at certain distance from the exit of the integrating sphere. The spectral photon radiance can also be evaluated when a precision aperture is installed at the exit of the integrating sphere. The calibrated integrating sphere source can then be used as standard spectral photon irradiance (or spectral photon flux) and spectral photon radiance sources for photon-counting imager calibration.
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