We study the principle and method of obtaining cylindrical lens type directional backlight flattop beam by using a butterfly liquid crystal switch.Based on the generalized Huygens-Fresnel diffraction integral theory,we deduce the diffraction field distribution expression of beam after passing through cylindrical lens,and the butterfly liquid crystal switch unit is the light source of the beam.Placing the butterfly liquid crystal switch unit on the cube focal plane of the cylindrical lens,we simulate transve...