Diffractionlimited

Diffractionlimited, often written as diffraction-limited, is an expression used in optics to describe a system whose imaging performance is limited only by diffraction of light through a finite aperture, rather than by optical aberrations or other imperfections. In such a system, the point-spread function and the resulting image quality are governed by fundamental diffraction effects rather than flaws in lens shape, alignment, or surface quality.

Key concepts include the Airy pattern and resolution limits. For a circular aperture of diameter D, the diffraction pattern has its first minimum at an angular radius of about 1.22 λ/D, where λ is the wavelength of light. The Rayleigh criterion uses this pattern to define the minimum angular separation at which two point sources can be resolved, approximately θR ≈ 0.61 λ/D. In imaging terms, a common practical expression for resolution is d ≈ λ/(2 NA), where NA is the numerical aperture of the system.

The Strehl ratio is another measure relevant to diffractionlimited performance, comparing the peak intensity of the actual image to that of an ideal diffraction-limited system. A Strehl ratio near 1 indicates a system that is close to diffraction-limited.

Achieving diffraction-limited performance requires high optical quality and stable operating conditions. In astronomy, atmospheric turbulence (seeing) often prevents telescopes from reaching the diffraction limit unless adaptive optics or space-based platforms are used. In microscopy, high-numerical-aperture objectives and careful correction of aberrations enable near-diffraction-limited imaging, while in laser focusing, diffraction sets the smallest achievable focal spot size.

Limitations include residual aberrations, misalignments, scattering, and polarization effects that can degrade performance away from the ideal diffraction-limited case.