We have developed a means based on the properties of the nonlinear optical process of spontaneous parametric downconversion to provide an absolute calibration of the spectral irradiance of the output of a spectrophotometer [1].
1. A primary radiation standard based on quantum nonlinear optics, S. Lemieux, E. Giese, R. Fickler, M. V. Chekhova, and R. W. Boyd, Nature Physics 15, 529 (2019).