Achieving the requisite machining accuracy for freedom prisms to conform to design specifications is essential and necessitates the application of high-precision measurement techniques coupled with accurate compensation machining strategies. The incorporation of multiple freeform surfaces into a single optical element introduces a dual challenge in the machining process. This involves the precise compensation for individual surface errors and the resolution of interrelated positional accuracy errors among these surfaces. Failure to address these errors can significantly degrade the overall imaging performance of the optical system. To ensure the machining accuracy of freeform surface prisms meets design requirements, we propose a high-precision compensation machining method for freeform surface prisms, which combines in-situ and off-machine measurement techniques to achieve highly accurate measurement and precise compensation machining.

To address the machining error issues and enhance precision in the machining of freeform surface prisms, we introduce a novel high-precision compensation turning approach. This approach integrates both in-situ and off-machine measurement techniques, utilizing an analysis of machining errors on various surfaces to achieve effective error mitigation. By significantly reducing positional inaccuracies during the machining process, the proposed approach significantly elevates the imaging quality of the prisms. We initiate a detailed examination of the machining errors in infrared freeform surface prisms, leading to the development of a custom fixture tailored for single-point turning. The methodology incorporates three-coordinate off-machine measurement technology to assess angular and interfacial spacing discrepancies in prism blanks. The data is instrumental in establishing a model that facilitates compensation for the initial machining surfaces. Further, we contact the probe to fine-tune the initial turning orientation of the prism, ensuring precise machining of the initial surface through single-point planar turning. This is followed by the systematic machining of all surfaces, referencing the first machined surface for consistency. After the initial machining, the prism undergoes a three-coordinate off-machine inspection to assess angular and interface spacing errors. If specified standards are met, a secondary planar compensation turning is performed or, if normal standards are met, surface turning is conducted based on these results. The initial turning direction of the freeform surface prism is adjusted using dual optical probes to identify the rotation centers of each surface. Based on the remaining machining allowance, the surface turning of the initial surface is conducted until all surfaces are machined.

Customized design specifications are developed for freeform surface prisms, used for infrared detection targets (Table 1). The off-axis system designed for the infrared freeform surface prism utilizes an incremental optimization method that involves the eccentric and inclined machining of axisymmetric lenses (Fig. 1). At 25 ℃, the cutoff frequency of this system is 20 lp/mm, with a modulation transfer function (MTF) exceeding 0.84 (Fig. 1). Simulation analysis is carried out to assess the influence of different machining errors on the imaging quality of freeform surface prisms. To mitigate these errors, a specialized fixture for single-point turning is developed (Fig. 4). Through a high-precision compensation turning method integrated in-situ and off-machine measurement technologies, the maximum angular machining error of planar prisms is 0.03° (Table 5). Performance tests for both close-up and long-distance imaging are carried out. The results show consistent imaging quality in both edge and central areas within the field of view of infrared freeform surface prisms (Fig. 10). In conclusion, the results affirm the superior optical imaging performance of the freeform surface prism system and underscore the effectiveness of the high-precision compensation machining method.

To enhance the precision of collaborative machining of freeform surface prisms, we conduct a series of studies including tolerance analysis, tooling design, ultra-precision turning compensation, imaging experiments, and performance analysis. Initially, we analyze the influence of machining errors on the imaging performance of freeform surface prisms and develop specialized fixtures to control the center positions of each surface during machining. Then, we propose a high-precision compensation turning method for freeform surface prisms. This method integrates in-situ and off-machine measurement technologies to monitor and adjust errors in each machining stage. The precision and consistency of these measurement methods are verified by comparing in-situ techniques with off-machine techniques. This approach reveals the maximum angular machining error of planar prisms is 0.03°. Finally, imaging performance tests on the freeform surface prisms indicate that image quality is consistent both at the edges and in the central areas. These tests validate the precision and feasibility of the proposed high-precision compensation turning method, which utilizes both in-situ and off-machine measurement techniques.