光学平面面形的绝对检验技术规避了干涉仪参考面形精度的制约，能够有效提高平晶面形的检测精度。采用N次图像旋转法的两平晶三面互检的绝对检验技术，求解待测平晶的三维绝对面形分布，结果中包含了中频波段的信息。利用递推公式构造旋转变化项的N次虚拟旋转结果，求和平均后得到旋转变化项，叠加旋转不变项结果后得到待测波面面形。推导了算法的理论误差，针对旋转角度进行优化并增加虚拟旋转次数，提高了算法精度，优化后的仿真结果的残差波面的均方根值精度为0.14 nm。对150 mm口径平晶进行两平晶三面互检实验，并将实验结果与传统三面互检法结果进行比对，均方根值偏差小于0.5 nm，验证了算法的准确性。

The absolute test technology of optical surface shape avoids the limitation of reference surface shape accuracy of interferometer and can effectively improve the detection accuracy of flat-crystal surface shape. The absolute test technology of two-flat crystal three-sided mutual test of N-position image rotation method was adopted to solve the three-dimensional absolute surface shape distribution of tested flat-crystal, and the results included the information in medium frequency band. The recursion formula was used to construct the N-position virtual rotation results of rotational change item. After summing and averaging, the rotational change item was obtained, and the tested wave surface shape was obtained after superposition of the rotational invariant item. The theoretical error of the algorithm was derived, the rotation angle was optimized and the number of virtual rotation was increased, and the accuracy of the algorithm was improved. After optimization, the simulation results show that the RMS value accuracy of residual wave surface is 0.14 nm. The two-flat crystal three-sided mutual test of flat-crystal with 150 mm caliber was carried out, and the experimental results were compared with those of the traditional three-sided mutual test method. The RMS value deviation is less than 0.5 nm, which verifies the accuracy of the algorithm.