同心扫描法制作凹球面等距网栅的误差分析

Error analysis of fabricating isometric mesh on the concave of a spherical substrate by concentric optical scanning

纬线与纬线相交可形成球面等距网栅,而采用“同心扫描”的运动方式可在凹球面上生成纬线。“同心扫描”要求直写物镜光轴、直写物镜水平转轴、工件回转轴和工件分度轴四轴交于工件凹面球心。采用两两相交误差几何分析的方法,得到了同心误差与离焦量和栅距误差的变化关系:随着同心误差增加,系统离焦量和栅距误差也增加,且二者的变化趋势基本一致。因为网栅线宽一般要求达到微米量级,与之匹配的栅距则往往为几百微米,所以栅距误差与栅距相比可以忽略,系统离焦量对网栅线宽的影响却必须认真考虑。采用几何光学基本公式对物镜系统的进一步分析表明,为了制作细且均匀的网栅线条,必须采用小光斑,而小光斑对应的焦深也小,从而要求离焦量小,对同心误差要求严格;反之,则可放宽对同心误差的要求。

An isometric mesh on the concave of a spherical substrate was formed by latitude line intersection each other, in which latitude lines was generated by a concentric optical scanning method. An optical axis, erection turning spindle, horizontal dividing spindle and a pitching dividing spindle were positioned on the concave center of the spherical substrate in concentric scanning. The concentric error between these spindles with the concave center may bring on the defocus of optical system and the error of the mesh period, but because the ratio of the period error to the period is very small, the latter may be ignored. The theoretical analytical result shows that the line width of mesh is restricted by the dimension of focal spot and the defocus of optical system determines the smallest dimension of focal spot, so the line width of mesh is restricted by the concentric error,i, e. the smaller the concentric error is, the smaller the line width is also.