Therefore it is possible to use the same grating for different optical arrangements as long as the four previously mentioned parameters are the same. A spectroscopic instrument consists generally of an entrance slit, a collimator, a dispersive element, focusing optics, and sometimes an exit slit.
Radiation entering the entrance slit is collected by the collimator, generally a concave mirror.
The dispersive element, in this case a grating, deviates the radiation in a direction which depends on the wavelength. Optical layout for three monochromators. In a monochromator there is an exit slit, which transmits a narrow portion of the spectrum. The entrance and exit slits are fixed, and the spectrum is scanned by rotating the grating. The grating thus works with a constant angular deviation between the incident and diffracted light.