Monochromator is an important part of every spectrophotometer. Here you can learn how it is built and how it works.
Monochromator is an optical device based on a diffraction grating or a prism. The light enters the monochromator through an entrance slit and reflects on a mirror which directs the light beam onto the diffraction grating or the prism (Fig. 1). The light beam splits and depending on the position of diffraction grating/prism, only a selectable fraction of the spectrum transmits through the exit slit.
Fig. 1 Czerny–Turner monochromator scheme with diffraction grating
In spectrophotometers, the monochromator is usually placed between the sample and the detector. The spectrum of the measured sample splits in the monochromator and therefore the detector can measure the intensity of light for a specific wavelength.
An important property of the monochromator is spectral resolution.
The parameters of the monochromator need to be set before using the spectrophotometer and should be the same for all measurements in order to easily compare the results.
References:
1) https://en.wikipedia.org/wiki/Monochromator
3) https://www.nireos.com/how-does-a-monochromator-work/