Classical spectrometry derives the instrumental response function for any spectrometer by propagating a single-frequency monochromatic continuous wave. The only available source of light for spectrometry in the old days was spontaneously emitting sources. A. Michelson’s Fourier transforms spectrometry that requires complex data processing of the fringe intensity variation with delay to reconstruct the source spectrum. Michelson’s Fourier transform spectroscopy is a major tool both in academia and in industry with highly sophisticated instruments and texts available off the shelf. Heterodyne spectrometry, by mixing a known reference frequency with an unknown source, provides another way of achieving high-resolution spectrometry under suitable conditions. In a grating spectrometer, the detection plane is indicated by the x-axis, which traditionally is in the front focal plane of a convergent lens or a convergent mirror, while the grating is placed at the back focal plane. The physical frequency content has already been set by the original source.