  The spectrum of vacuum fluctuations of the electromagnetic field is
determined solely from first physical principles and can be seen as a
fundamental property that qualifies as a primary radiation standard. We
demonstrate that the amplitude of these quantum fluctuations triggering
nonlinear optical processes can be used as a reference for radiometry. In the
spontaneous regime of photon pair generation, the shape of the emitted spectrum
is nearly independent of laboratory parameters. In the high-gain regime, where
spontaneous emission turns to stimulated emission, the shape of the frequency
spectrum is uniquely determined by the number of created photons. Both aspects
allow us to determine the quantum efficiency of a spectrometer over a broad
range of wavelengths without the need of any external calibrated source or
detector.
