We propose a space benchmark sensor with onboard SI (Système International) traceability by means of quantum optical radiometry. Correlated photon pairs generated by spontaneous parametric down-conversion (SPDC) in nonlinear crystals are used to calibrate the absolute responsivity of a solar observing radiometer. The calibration is systematic, insensitive to degradation and independent of external radiometric standards. Solar spectral irradiance at 380–2500 nm is traceable to the photon rate and Planck’s constant with an expected uncertainty of about 0.35%. The principle of SPDC calibration and a prototype design of the solar radiometer are introduced. The uncertainty budget is analyzed in consideration of errors arising from calibration and observation modes.

In order to establish a transferring chain from a photon flux of a single-photon source in quantum radiometry, the nonlinearity of the photodetector needs to be accurately measured. Using the flux superposition method, a nonlinearity measurement setup has been designed. The measurement setup consists of two tungsten halogen lamps, parent–child integrating spheres, an adjustable aperture, a diaphragm tube, and an optical filter. It has the advantage of low polarization error, low interference error, and low stray light effect. The Si photodiode to be measured is cooled to −40 °C to obtain a low noise level for low-flux radiation measurement. The nonlinearity of the Si photodetector is measured for photocurrent ranges from 10⁻¹² A~10⁻⁶ A level, with a relative standard uncertainty from 0.0092~0.023%. The relative standard uncertainty of the nonlinearity correction factor ranged from 0.023~0.049%.

In order to meet the high uniformity calibration requirements for scientific-grade, large-size space detectors used in the CHES Extrasolar Planet Exploration Mission, this paper presents the design of a wide dynamic range, high uniformity spectral irradiance source (WHUIS). Utilizing a cascade integrating sphere design, and optimizing the overlapping area radiant flux adjustment structure and illumination light path, we achieve a wide dynamic range and high uniformity irradiance output. We established an irradiance transmission model based on the new assumption and analyzed the influence of factors such as illumination distance, stray light, and non-uniform radiance on the uniformity of irradiance output. The model is then validated by building experimental equipment. The findings show that in a circular area of 40 mm, the irradiance uniformity of our light source system exceeds 99.9%, and constant color temperature is adjustable within six orders of magnitude, consistent with the uniformity level predicted by the model.