Numerical aperture

Numerical aperture measures how much light an optical system can accept or emit and, in imaging, how finely it can resolve detail. It depends on the refractive index of the surrounding medium and the system’s acceptance angle.

Acceptance angle: The largest angle at which light can enter an optical system and still be transmitted or collected. The acceptance angle sets the angular part of numerical aperture.
Microscope objective: A microscope’s lens assembly nearest the specimen, forming the first magnified image. Its numerical aperture is a standard specification for light collection and resolution.
Snell's law: The law relating the angles of incidence and refraction to the refractive indices of two media. It governs how rays bend at the interfaces surrounding an optical system.
F-number: The ratio of a lens’s focal length to its entrance-pupil diameter. F-number and numerical aperture both relate aperture to light gathering, but use different geometry and conventions.
Refractive index: A dimensionless measure of how much light slows and bends in a material. The surrounding medium’s refractive index scales numerical aperture.
Oil immersion microscopy: Microscopy that places immersion oil between the specimen and objective to reduce refractive-index mismatch. Oil immersion raises the attainable objective numerical aperture.
Solid angle: A measure of the angular area subtended by a region as seen from a point. The accepted cone of rays can also be described by its solid angle.
Working numerical aperture: The effective numerical aperture of an optical system at its actual operating distance or configuration. It distinguishes practical performance from a nominal aperture specification.
Abbe sine condition: A condition relating object and image angles in an optical system that preserves image quality. It explains why well-corrected systems use sine-based angular measures.
Optical fiber: A flexible transparent waveguide that carries light through a core surrounded by lower-index cladding. Fiber numerical aperture describes the range of entering rays it can guide.

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