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The letters NA, a lens's light-gathering cone.
Also known as: NA, Numerical aperture, Acceptance cone measure
Numerical aperture measures the widest cone of light a lens can accept, and it sets the resolution limit of a microscope: finer detail needs a larger NA, which is why oil-immersion objectives exist at all — the oil raises the refractive index in the formula.
Denotes numerical aperture: the refractive index of the medium times the sine of the half-angle of the accepted cone.
Contexts: Laboratory
Sets the diffraction-limited resolution of a microscope objective, which is why immersion oil increases resolution by raising the index term.
Contexts: Laboratory
Two capital letters N and A set together in roman type.