A monochromator is an optical instrument used for separating and isolating light into its constituent wavelengths or colors. It typically uses a diffraction grating, which is a specially designed-surface with closely spaced lines or grooves. When light passes through the grating, it is dispersed into its spectral components due to diffraction.

The grating groove density represents the number of grooves or lines etched onto the grating surface within a specified length. It is usually often denoted as "N" and expressed in units of lines per millimeter, lines per centimeter, or lines per inch.

The grating groove density plays a crucial role in determining the efficiency with which a monochromator can disperse incoming light into its constituent wavelengths. Higher groove densities result in greater dispersion, which allows for finer wavelength resolution but may reduce the overall efficiency of the monochromator. Conversely, lower groove densities provide poor resolution but may yield higher efficiency.