highNA

HighNA, or high numerical aperture, refers to optical systems whose numerical aperture is large, enabling more efficient light collection and higher angular resolution. Numerical aperture is defined as NA = n sin α, where n is the refractive index of the surrounding medium and α is the half-angle of the maximum cone of light that can enter or exit the objective. Higher NA allows finer detail to be resolved and more light to be collected.

In microscopy, higher NA improves lateral and axial resolution. The approximate lateral resolution for incoherent imaging is d ≈ 0.61 λ / NA, while the Abbe limit d ≈ λ / (2 NA) is often cited. Axial resolution scales roughly with 1/NA^2 and with the refractive index. Realizing high NA typically requires immersion objectives (oil, water, or glycerol) to increase n and thus NA. High NA also reduces depth of field and increases sensitivity to aberrations and refractive-index mismatches, particularly with thick or heterogeneous samples.

Applications include confocal and widefield fluorescence microscopy, super-resolution techniques such as STED and SIM, and single-molecule experiments where light collection efficiency is critical. Design and use considerations include correcting for cover slip thickness with correction collars, maintaining immersion medium, and balancing NA against working distance, field of view, and sampling. The term 'high-NA' is generally context-dependent but denotes systems achieving NA approaching or exceeding ~1.2–1.4 in modern oil-immersion objectives.