If only the first order correction to the scalar paraxial field is concerted, then the field can be expressed as =[ε x,0,ε z] and the longitudinal field energy E z is proved to be constantly independent of the position z of the beam cross section.
对于一个傍轴标量光场ε＝［εｘ，０，０］，考虑其一阶纵[向]场修正项的影响时，光场的纵[向]场能量Ｅｚ是一个不随光束截面位置ｚ变化的守恒量，其值完全决定于傍轴场（εｘ）的分布情况。
