Fresnel number

The Fresnel number is a dimensionless ratio of an aperture’s squared characteristic size to wavelength times propagation distance. It indicates whether diffraction is predominantly near-field or far-field.

Fresnel diffraction: The near-field diffraction regime in which wavefront curvature affects the observed field. The Fresnel number indicates when this regime’s finite-distance effects matter.
Diffraction: The spreading and redistribution of waves when they encounter apertures, obstacles, or spatial variations. The Fresnel number classifies diffraction according to distance and aperture scale.
Fourier optics: The study of optical systems using Fourier transforms to describe fields and image formation. The Fresnel number helps determine when Fourier-based far-field methods apply.
F-number: The ratio of a camera lens’s focal length to its entrance-pupil diameter. It also involves aperture geometry, but describes image brightness and depth of field rather than propagation regime.
Huygens–Fresnel principle: A wave-propagation principle that treats each point on a wavefront as a source of secondary spherical waves. The Fresnel number helps simplify the secondary-wave contributions across an aperture.
Wavelength: The spatial period of a wave, measured between successive points in the same phase. It sets the scale against which aperture size and propagation distance are compared.
Computational diffraction: Numerical calculation of wave fields diffracted by apertures, obstacles, or optical elements. Its value helps select a propagation model and sampling scale.
Numerical aperture: A dimensionless measure of an optical system’s ability to accept or emit light over a range of angles. It characterizes angular collection, unlike the Fresnel number’s distance-dependent diffraction scale.
Fresnel integral: An integral used to calculate wave propagation in the Fresnel diffraction regime. It describes the field whose geometric scaling is summarized by the Fresnel number.
Aperture (optics): An opening or bounded transmitting region that limits the spatial extent of an optical wave. Its characteristic size is squared in the Fresnel-number definition.

Linked from 4 pages

Rayleigh lengthCompared with: It characterizes a propagation regime rather than a Gaussian beam’s waist-relative range.