Raman Spectroscopy:

A sample is normally illuminated with a laser beam in the ultraviolet (UV), visible (Vis) or near infrared (NIR) range. Scattered light is collected with a lens and is sent through interference filter or spectrophotometer to obtain Raman spectrum of a sample.

Since spontaneous Raman scattering is very weak due to the intense Rayleigh scattering, suitable filters are placed to remove the major components of elastic scattered light (Rayleigh scattering). The commercially available interference (notch) filters which cut-off spectral range of ± 80-120 cm^-1 from the laser line are used. While this method is efficient in stray light elimination, it does not allow detection of low-frequency Raman modes in the range below 100 cm^-1.

Stray light generated in the spectrometer due to light dispersion on gratings strongly depends on grating quality. As Raman spectrometers use holographic gratings, which normally have much less manufacturing defects in their structure then the ruled one, stray light produced by holographic gratings is less intense and hence does not affect. This can be reduced further by using multiple dispersion stages. For instance, the use of double and triple spectrometers allows taking Raman spectra without the use of notch filters, which allows Raman-active modes with frequencies as low as 3-5 cm^-1 to be efficiently detected.