Rayleigh length

The Rayleigh length is the distance along the optical axis requires a laser beam until its cross-sectional area, starting doubled from the beam waist. The radius is greater by a factor therein. The Rayleigh length is thus, after which route before or after a focus, the area illuminated by the light beam area has doubled. If the - for laser usually permissible - considered approximation of a Gaussian beam, so can the Rayleigh length expressed as follows:

Wherein the radius of the beam at the focus and the wavelength of light is used.

Taking into account the beam quality expressed by the factor, the formula changes to:

So this means that if red light (eg 650 nm ) of ultraviolet light (eg 325 nm) is focused on the same surface in the focus of a lens or a parabolic mirror, the red light after only half the distance of the ultraviolet has already spread as far back that it illuminates the dual output surface.