A laser is used to illuminate a grid pattern followed by a converging lens one focal length away. The result of this arrangement is the Fourier transform of the grid imaged at the plane one focal length downstream from the converging lens. A second lens may be used to image this plane and the original grid on a projection screen or wall as shown in the second and third images above. An adjustable slit placed at the Fourier transform plane can be used to filter spatial frequencies. In the fourth image above, a vertical slit has been used to filter out all but the zeroth of the horizontal frequencies in the image of the grid.

L0, L1

PIRA DCS 6Q20.10