numericalaperture

Numerical aperture, abbreviated NA, is a dimensionless quantity that characterizes the light-gathering ability and angular acceptance of an optical system, especially microscope objectives and condensers. For a given medium, NA equals n sin(theta), where n is the refractive index of the medium between the specimen and the objective, and theta is the half-angle of the maximum cone of light that can enter or exit the lens.

NA determines resolving power and brightness; higher NA allows finer detail to be resolved and more light to be collected. In diffraction-limited optics, the lateral resolution d is approximately λ/(2 NA) for visible light, and the axial resolution depends on NA as well. Therefore NA is a key spec for determining performance, alongside wavelength and detector characteristics.

NA depends on the immersion medium and the optics. With air as the surrounding medium (n ≈ 1.0), the practical NA is about 0.95 for the best objective lenses. Oil-immersion objectives use a medium with n ≈ 1.515, enabling NAs around 1.4 to 1.5. Water-immersion objectives operate with n ≈ 1.33 and typically reach 1.1–1.3. NA is limited by the refractive index of the medium; it cannot exceed n.

Use in practice: NA is specified for microscope objectives and often also for condensers. In fluorescence microscopy, higher NA improves excitation light collection and emitted fluorescence detection, but at the cost of shorter working distance and more stringent alignment. The concept also applies to other optical systems, such as fiber optics and laser scanning microscopes, where NA governs coupling efficiency and resolution.