When a sample is irradiated with an intense monochromatic light source (usually a laser), most of the radiation is scattered by the sample at the same wavelength as that of the incoming laser radiation in a process known as Rayleigh scattering. However, a small proportion of the incoming light – approximately one photon out of a million – is scattered at a wavelength that is shifted from the original laser wavelength. This shift is called the Raman shift. The difference in energy between the incident laser excitation and the Raman scattered light is equal to a vibrational transition energy of the molecule. By analyzing the inelastically scattered Raman light one can build up a Raman spectrum. The Raman spectrum is plotted as a Raman shift relative to the excitation wavelength.