<P>Modern optical engineering一书中的定义:</P>
<P>The paraxial region of an optical system is a thin threadlike region about the optical axis which is so small that all the angles made by the rays (i.e., the slope angles and the angles of incidence and refraction) may be set equal to their sines and tangents.</P>
<P>即在光轴附近,倾角、入射和折射角可以近似等于其正弦和正切的光线</P>