The grating in a spectrometer disperses the signal onto the CCD detector by deflecting each wavelength at a different angle. The number of grooves per millimeter determines the dispersion characteristics. A high number of grooves/mm (lines/mm) results in a high dispersion and thus a high resolution, by distributing the signal over a larger number of CCD pixels.
To optimize efficiency, gratings are typically “blazed” for a certain wavelength. This means that the grooves are angled so that the grating efficiency can reach 80% for the first diffraction order. The grating efficiency sets the upper limit of the throughput of the spectrometer.
It is often useful to switch between multiple gratings: one that covers the full Raman spectrum (-100 to 3600 cm-1) and a high-resolution grating that delivers approximately 1 cm-1 spectral resolution.