When laser light interacts with a molecule, the vast majority of photons scatter elastically meaning they bounce off without any change in energy. This is called Rayleigh scattering. However, a tiny fraction of photons interact differently: they either gain or lose energy during the collision with the molecule. This inelastic scattering is the Raman effect, and it is the foundation of the entire technique.

A Raman spectrometer is built around four key components. First, a laser provides the excitation light. Common wavelengths include 532 nm, 785 nm, and 1064 nm, with the choice depending on the sample; shorter wavelengths produce stronger signals but risk fluorescence interference that can mask results.

Second, optical filters clean up the signal. Bandpass filters ensure only the correct laser wavelength reaches the sample, while long pass filters block the dominant Rayleigh scatter so the much weaker Raman signal is not overwhelmed.

Third, a spectrometer disperses the collected light, separating it by wavelength so the different scattering components can be individually resolved.

Fourth, a detector commonly a back-thinned CCD captures the dispersed light with high sensitivity, sometimes reaching quantum efficiencies of around 90%, which is critical given how little Raman signal is actually generated.