1. The sine of the
vertex angle of the largest cone of meridional rays that can enter or leave an optical
system or element, multiplied by the
refractive index of the
medium in which the vertex of the cone is located. Note: The NA is generally measured with respect to an object or image point and will vary as that point is moved. 2. For an
optical fiber in which the refractive index decreases monotonically from n 1 on the axis to n 2 in the
cladding, an expression of the extent of the fiber's ability to
accept, in its bound modes, non-normal incident rays, given by NA = (n 12-n 22)½. Note: 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. (
188 ) 3. Colloquially, the sine of the radiation or
acceptance angle of an optical fiber, multiplied by the refractive index of the material in contact with the exit or entrance face. Note: This usage is approximate and imprecise, but is often encountered.
a measure of the acceptance angle of a lens. Higher numerical aperture means the lens gather more diffraction orders yielding higher resolution but at the expense of depth of focus.
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