Optical resolution

Optical resolution is the ability of an optical system (like a microscope or optical profiler) to distinguish two closely spaced points as separate features.
It defines the smallest lateral distance between two points that can still be distinguished. In practical terms, this is often evaluated using line/space patterns, where the resolution corresponds to the smallest spacing between lines that can still be clearly separated. Higher optical resolution means the system can reveal finer details on a surface or object.

Optical resolution is limited by the wave nature of light. When light passes through an optical system, it spreads out (diffraction), causing small details to blur.
As a result, if two features are too close together, they will appear as a single feature. Resolution mainly depends on the wavelength of light and the numerical aperture (NA).
This limitation is defined by the Rayleigh resolution limit, which gives the theoretical minimum distance at which two points can still be distinguished as separate.

Optical resolution specification is important in applications where very small features must be distinguished.
For example:

  Semiconductors: line/space and defects

  Optics and microlenses: surface quality and fine structures

  Medical imaging: cells and tissues

  Materials science: grains and microstructures

It ensures the system can resolve the features relevant to each application.