The standard "GB/T 40279-2021 Optical inspection reflection method for testing film thickness on the surface of silicon wafers" specifies the method for measuring the film thickness of silicon dioxide film and polysilicon film on the surface of silicon wafers using Optical inspection reflection method. It is suitable for testing smooth, transparent or translucent films with film thickness ranging from 15 nm to 105 nm, such as amorphous silicon, silicon nitride and photoresist, etc. The standard describes in detail the interaction of the incident ray with the film interface, including reflection, refraction and interference principles. By analyzing the wavelength of light and the optical path difference of the film, it is possible to accurately measure the film thickness of the film. Optical inspection is particularly suitable for films with stable characteristics and low absorption coefficient.