The working principle of the common grating is based on the formation principle of the physical moire fringe. When the line indicates the grating with the grating ruler on the line at an angle to place the grating two, will cause two line grating on the cross. Under the irradiation of light, small area near the intersection of the black line due to overlap, so the shading area and the lowest blocking effect is the weakest, the cumulative effect of light makes the area appear bright band. On the contrary, from the intersection of the far region, because of overlapping portions of the opaque grating two black lines are becoming less and less opaque area becomes larger, the shading area becomes larger, so that the light blocking effect becomes stronger, less light can pass through this area through the grating, making this region appear dark band. These lines and the grating almost vertical, with bright and dark bands of Moire fringe