Numerical aperture (NA)

Numerical aperture is a dimensionless quantity describing the range of angles over which an optical system or fiber can accept or emit light, directly related to its light-collecting ability and resolution.

How is numerical aperture calculated?

For an optical fiber, NA is determined from the difference in refractive index between the core and cladding — the greater the difference, the wider the light-acceptance angle and the more modes are guided. For imaging systems, NA depends on the lens aperture and its distance from the focus, with a higher NA meaning better resolving power at the cost of shallower depth of field.

Applications

NA is a key parameter when choosing an optical fiber for coupling to a light source and the required number of modes, and in microscopy and lithography — the parameter that determines a system's ultimate optical resolution.

Related terms

Optical waveguide

TIR

MTF