Slit refers to one of the main components of the spectrometer. A slit is a narrow, elongated slit hole with adjustable width. There are fixed slits, unilaterally adjustable asymmetrical slits and bilaterally adjustable symmetrical slits. Each spectral line after the optical radiation is dispersed by the spectrometer is an image of the incident slit. The radiant energy entering or exiting the monochromator is adjusted by the slit width. In modern spectrometers the slit is coupled to the rotation of the grating and can be adjusted automatically. The unit of slit width is μm, and the maximum width is 2 mm. The spectrograph has only the entrance slit, the monochromator has two slits, the entrance and the exit, and the polychromator has several exit slits. The slits can be divided into incident slits and exit slits according to their functions. The exit incident and intermediate slits are important parts of a Raman spectrometer. The main function of the entrance and exit slits is to control the resolution of the instrument, and the middle slit is mainly used to suppress stray light. For a spectrometer, even if an absolutely monochromatic light is used to illuminate the slit, the outgoing light always has a spectral distribution with a width of Δυ. This is mainly caused by factors such as the grating of the instrument, the aberration of the optical system, parts processing and system adjustment, and thus determines the limit resolution of the instrument. In actual measurement, as the slit width increases, the resolution decreases linearly, which broadens the spectral line.