Numerical aperture

Japanese: 開口数 - かいこうすう（英語表記）numerical aperture

A number that represents the resolution of lenses in optical instruments, mainly microscopes, and the brightness of optical images. It is abbreviated as NA. It is engraved on the tube of the objective lens, and the higher the number, the better the performance of the optical instrument. In general, when the angle at which light from an object point on the optical axis in a medium with a refractive index of n is incident on the effective radius of the objective lens is θ, the numerical aperture is expressed as NA = n sin θ. When the medium is air, n = 1, so NA = sin θ. In order to increase the numerical aperture in microscopes, the immersion method is used, in which the space between the objective lens and the cover glass of the object is immersed in a liquid with a high refractive index.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

光学器械，おもに顕微鏡のレンズの分解能や光像の明るさを表わす数値。略号 NAを用いる。対物レンズの鏡筒に刻んであり，数値が大きいほど光学器械の性能がよい。一般に，屈折率 n の媒質中にある光軸上の物点からの光が，対物レンズの有効半径に対して入射する角を θ とするとき，開口数は NA＝n sin θ で表わされる。媒質が空気のときは，n＝1 であるから NA＝ sin θ になる。顕微鏡では開口数を増すために，対物レンズと物体のカバーガラスとの間を，屈折率の大きい液体で浸す液浸法が使われる。

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