Fluorescence and Raman (affectionately known as COORS or Common Old Ordinary Raman Spectroscopy) spectroscopies are also four wave mixing nonlinear spectroscopies but they are incoherent, i.e. there is no phase relationships between excited molecules in different parts of a sample. The WMEL diagrams for Raman and fluorescence spectroscopy are still represented by figures 2c and d, respectively. However, instead of the ω_S excitation being caused by a real excitation beam, it is instead caused by the vacuum fluctuations which have no long range phase relationships so the coherences they stimulate are incoherent and therefore not directional. The emission intensity then scales as the concentration N.