Diffraction limit

The finite resolution imposed by wave diffraction and interference, which prevents an imaging system from distinguishing sufficiently close features. Its value depends on wavelength and the system’s aperture or numerical aperture.

Abbe diffraction limit: A resolution criterion relating the smallest distinguishable spacing to illumination wavelength and numerical aperture. Abbe’s formula gives the characteristic lateral resolution limit in microscopy.
Optical microscopy: The use of visible or near-visible light and lenses to form magnified images of small objects. Microscope resolution is commonly expressed through wavelength and numerical aperture.
Geometric optics: An approximation that models light as rays and neglects its wave behavior. Ray models omit diffraction and therefore cannot explain the resolution limit.
Ernst Abbe: A German physicist and optical scientist who developed foundational theories of microscope image formation and resolution. Abbe connected microscope resolution to wavelength and the angular range of collected light.
Near-field optics: Optical methods that detect or use evanescent fields close to a surface, before they decay with distance. Near-field measurements access fine spatial information unavailable to ordinary far-field imaging.
Rayleigh criterion: A rule for resolving two point sources, declaring them just resolved when one image’s peak meets the other’s first minimum. It translates diffraction patterns into a practical threshold for separating nearby sources.
Angular resolution: The smallest angular separation at which an instrument can distinguish two sources. For telescopes, diffraction limits how closely two stars can appear while remaining separable.
Aberration: An imaging error that prevents an optical system from bringing rays or wavefronts to an ideal focus. Aberrations degrade resolution but are distinct from the limit imposed by diffraction.
Lord Rayleigh: The title of John William Strutt, an English physicist whose work included optics, acoustics, and fluid dynamics. Rayleigh proposed a practical criterion for deciding when diffraction-blurred sources are resolved.
Evanescent wave: A field that decays exponentially away from an interface or region rather than propagating through space. These waves carry subwavelength detail but usually cannot reach a distant detector.

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