光學系統的數值孔徑（NA）是一個無因次的數，用以衡量該系統能夠收集的光的角度範圍。在光學的不同領域，數值孔徑的精確定義略有不同。在光學顯微鏡領域，數值孔徑描述了物鏡收光錐角的大小，而後者決定了顯微鏡收光能力和空間解析度；在光纖領域，數值孔徑則描述了光進出光纖時的錐角大小。參考文獻，維基百科。

 

In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the property that it is constant for a beam as it goes from one material to another, provided there is no refractive power at the interface. The exact definition of the term varies slightly between different areas of optics. Numerical aperture is commonly used in microscopy to describe the acceptance cone of an objective (and hence its light-gathering ability and resolution), and in fiber optics, in which it describes the range of angles within which light that is incident on the fiber will be transmitted along it.