Monochromators and Spectrographs are optical devices that disperse light into its spectral wavelengths. These devices form the heart of any optical spectrometric system. More specifically, a monochromator disperses light and isolates one spectral wavelength to viewed or measured or otherwise used (hence the term monochromator, for monochromatic light). When a wider range of the dispersed wavelengths are used or measured at the same time the same instrument is called a spectrograph.
A scanning monochromator, with either a manual or computer controlled motorized turret, scans a selected spectral region one wavelength at a time so that all wavelengths are covered in the end. Scanning monochromators are often used as tunable wavelength light sources or fluorescence excitation sources where monochromatic light is required for illumination. They can also be mated to a single channel detector to measure the spectral content of the input light one wavelength at a time.