Numerical aperature

Where R is the resolution limit and NA is the numerical aperature of the exposure tool. 

The denominator of Eq. 1 represents the numerical aperature (NA) of the objective lens, related to its light gathering ability. Other primary factors that influence the resolution of a lens is the wavelength of light used, the index of refraction tf) of the environment surrounding the lens (e.g., 1.00 for air), and the angle of illumination 6). 

A variety of apertures have been used to deliver nanometer-sized spots of light. While early NSOM tips were fabricated out of etched quartz crystals and micropipettes, tapered optical fibers with tip diameters of ca. 100 nm are now typically used. A metallic thin film such as aluminum is usually applied around the sides of the tapered region of the NSOM tip to focus the light toward the sample. For optical fibers, the numerical aperature is related to the difference in the indices of refraction of the cladding and core (Eq. 2) ... 

Numerical aperature The widest cone of light that can enter a lens. 

Numerical Aperature n The numerical aperture of a lens system (objective or condenser) is the sine of one-half the angular aperture times the refractive index of the medium (1.0 for air, 1.515 for Cargille immersion oil, etc.) between objective and specimen. The numerical aperture is a measure of the light gathering capacity of the lens system and determines its resolving power and depth of field. 

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