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The spectrum of vacuum fluctuations of the electromagnetic field isdetermined solely from first physical principles and can be seen as afundamental property that qualifies as a primary radiation standard. Wedemonstrate that the amplitude of these quantum fluctuations triggeringnonlinear optical processes can be used as a reference for radiometry. In thespontaneous regime of photon pair generation, the shape of the emitted spectrumis nearly independent of laboratory parameters. In the high-gain regime, wherespontaneous emission turns to stimulated emission, the shape of the frequencyspectrum is uniquely determined by the number of created photons. Both aspectsallow us to determine the quantum efficiency of a spectrometer over a broadrange of wavelengths without the need of any external calibrated source ordetector.

A Primary Radiation Standard Based on Quantum Nonlinear Optics