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Photonics Dictionary

numerical aperture

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. Generally measured with respect to an object or image point, and will vary as that point is moved. The numerical aperture of an optical system is critical in determining the resolution limits along with the diffraction limited spot size of a given optical system.
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