This analytical technique uses the light scattered from the vibrational energy modes of the sample.  Rayleigh scattering is a simple deflection of the incoming light, Raman scattering is an inelastic scattering of light.  There is an exchange of energy with the incoming photons resulting in a directional deviation of the light path. The resultant spectrograph is a ‘fingerprint’ of the substance being analyzed.  This incredibly powerful technique is used in a variety of applications from security inspection at airports to food safety.  Raman scattering is much weaker than Rayleigh scattering, and therefore requires much better filters to isolate the signal of interest – high transmission values, steep transitions and superior blocking to keep out unwanted photons.