Abstract: The Kirchhoff＇s diffraction formula is used to draw the diffraction luminous intensity distribution of an ideal slit in the different regimes through numerical calculations.The numerical results are compared with the one by the Fresnel and Fraunhofer diffraction formula,the application range of the results is also discussed in detail.

Key words: Kirchhoff＇s diffraction, Fresnel＇s diffraction, Fraunhofe＇s diffraction