A monochrometer is frequently used to measure the spectral response of a light source. The purpose of a monochrometer is to convert a broad light spectral input into a narrow spectral output. Light enters the monochrometer through a slit that directs the input light to the first mirror inside the monochrometer. The firs lens (mirror) collimates the entering light and reflects it into a grating. The groves or line on the grating have a specific spacing that produces constructing and destructing interference of the reflected light from the grating. This process called dispersion separates the different colors of light (wavelengths) in space, reflecting each color at slightly different angles. The second lens (mirror) reflects the disperse light and focus a given light color into the outlet slit, allowing only a narrow band of light to escape the monochrometer. The following diagram illustrates this principle.
It is important to note that the resolution of the monochrometer is dependent on the grating and the outlet slit. The grating geometry and configuration limits the accuracy at which the individual wavelength can be separated in space. The outlet slit limits the how narrow of a band is allowed to escape the monochrometer.
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