Numerical aperture Totally Explained

Everything about Numerical Aperture totally explained

In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. The exact definition of the term varies slightly between different areas of optics.

General optics

In most areas of optics, and especially in microscopy, the numerical aperture of an optical system such as an objective lens is defined by ť

mathrm,

where n₁ is the refractive index along the central axis of the fiber. Note that when this definition is used, the connection between the NA and the acceptance angle of the fiber becomes only an approximation. In particular, manufacturers often quote "NA" for single-mode fiber based on this formula, even though the acceptance angle for single-mode fiber is quite different and can't be determined from the indices of refraction alone.
The number of bound modes, the mode volume, is related to the normalized frequency and thus to the NA.
In multimode fibers, the term equilibrium numerical aperture is sometimes used. This refers to the numerical aperture with respect to the extreme exit angle of a ray emerging from a fiber in which equilibrium mode distribution has been established.

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