The advent of stimulated Raman scattering (SRS) microscopy has launched a rapidly growing field in chemical imaging with broad impact. Although the physical picture seems to be well understood from classical models, prediction of absolute SRS signals remains a challenge. Here, we present a quantum electrodynamics treatment of the newly introduced stimulated Raman cross section. The resulting formula for calculating the absolute SRS signal is simple and differs from the commonly used relations by only one factor. We demonstrate the utility of this formula in a broad range of crucial applications of SRS microscopy, including stimulated Raman enhancement factor (>10 times), signal-to-noise ratio (SNR) of typical imaging experiments, population saturation under high power excitation, and energy deposition during stimulated Raman photothermal microscopy. In particular, the theory predicts that SRS microscopy is almost always more sensitive than spontaneous Raman microscopy for chemical imaging.
