A monochromator selects a narrow, adjustable wavelength interval from an optical spectrum. Its finite spectral bandwidth matters when calibrating a photodetector's spectral responsivity in calibrated radiant-flux measurement.
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A monochromator is an optical device that isolates a specific wavelength or narrow band of wavelengths from a broader spectrum of light. It typically consists of a light source, a dispersive element, and a detector. The fundamental purpose of a monochromator is to take incoming polychromatic light (light that contains multiple wavelengths) and separate it so that only the desired wavelength or range of wavelengths is transmitted to the output.