菲涅尔透镜是具有阶梯表面的光学元件(图67)。相邻阶梯之间的距离愈小,则当透镜厚度较小,越能准确满足减小剩余象差的条件。阶梯之间所能达到的最小间距为0.05mm。这些阶梯可能是被一些同心的、螺旋形的或平行的小槽所分隔。当阶梯被同心的或螺旋形小槽所分隔时,阶梯是锥形曲 The Fresnel lens is an optical element with a stepped surface (FIG. 67). The smaller the distance between adjacent steps is, the smaller the lens thickness is, the more accurately the condition for reducing the residual aberration can be satisfied. The smallest distance that can be achieved between the steps is 0.05 mm. These steps may be separated by some concentric, spiral or parallel small grooves. When the steps are separated by concentric or spiral grooves, the steps are conical