The numerical
aperture of an
optical system is given by
no sin uo
where no denotes the refraction index in the object domain and uo the angle of the narrowest cone entering the system and having the intersection of the optical axis and the object plane as its apex. This cone is defined by the aperture stop of the system, and numerical aperture generally characterizes the aperture of systems that are close to the object plane. It expresses the same idea as the aperture number.