 Most commonly, the unit chosen for expressing wavenumber in Raman spectra is inverse centimeters (cm−1).Since wavelength is often expressed in units of nanometers (nm), the formula above can scale for this unit conversion explicitly, giving

Modern Raman spectroscopy nearly always involves the use of lasers as excitation light sources.Because lasers were not available until more than three decades after the discovery of the effect, Raman and Krishnan used a mercury lamp and photographic plates to record spectra.