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• NA
• nephelometer
• NIR
• NM
• NN
• NSOM
• numerical aperture

Numerical aperture (NA). It determines the rate of convergence of a focused light beam or of a field of view of a lens (Figure 1). For an ideal lens, imaging a collimated light beam, NA is determined by the following equation (for example, Wyatt CL 1991):

NA = m' sinα

(1)

where m' is the real part of the refractive index and α is the half-angle of the convergence cone of the light beam or the field of view. The maximum NA equals m' (at α = 90°). For a lens in air (m' ~ 1), the maximum NA ~ 1.

 
Fig. 1. Numerical aperture / f-number geometry. Notation: pp - principal plane of the lens, f - focal plane of the lens. The real part of the refractive index of the medium neighbouring the lens at the right side is m'.

The numerical aperture of a lens is closely related to the f-number of the lens. [MJ]