highnumericalaperture

High numerical aperture refers to optical systems with a large numerical aperture, a dimensionless parameter that measures light-gathering ability and resolving power. Numerical aperture is defined as NA = n sin θ, where n is the refractive index of the medium between the objective and the specimen and θ is the half-angle of the maximum cone of light that enters or exits the lens. In practice, high-NA is achieved with immersion objectives using oil or water between the lens and specimen; typical oil immersion objectives reach NA around 1.3–1.4, which enables superior resolution and brightness compared to air objectives (NA < 1.0). In the literature this concept is usually denoted as high-NA.

High-NA systems improve lateral resolution roughly according to the Abbe limit, with d ≈ λ/(2 NA). They also collect more emitted light, boosting signal-to-noise in fluorescence imaging. However, higher NA reduces depth of field and working distance, and increases sensitivity to refractive-index mismatches and aberrations, especially for thicker samples. To realize the benefits of high NA, careful matching of immersion medium and objective design is required, along with precise cover-glass thickness and alignment. Spherical aberrations become more pronounced if the sample’s refractive index differs from the immersion medium.

Beyond microscopy, numerical aperture is also a key parameter in fiber optics, where NA characterizes a fiber’s ability to couple and guide light. High-NA fibers accept light over a larger angle but may support multiple propagation modes, affecting bandwidth. In all contexts, high numerical aperture is a trade-off among resolution, brightness, depth of field, and practical implementation constraints.