Spatial filter

For example, an imperfect beam might form a bright spot surrounded by a series of concentric rings, as shown in the figure to the right.

Light in the very center of the transform pattern corresponds to a perfect, wide plane wave.

A pattern with very fine details will produce light very far from the transform plane's central spot.

In the example above, the large central spot and rings of light surrounding it are due to the structure resulting when the beam passed through a circular aperture.

In practice, the most commonly used configuration is to use a microscope objective lens for focusing the beam, and an aperture made by punching a small, precise, hole in a piece of thick metal foil.

A smaller aperture implements a closer approximation of a point source, which in turn produces a more nearly spherical wavefront.