Accurate and rapid IR metrology for the manufacture of freeform optics

The freeform revolution continues to redefine mainstream optics fields. Specific applications of freeform optics are currently prevented, however, because there is a lack of accurate metrology methods to guide efficient fabrication. The wide and flexible design space of freeform optics allows the design of new cutting-edge optical system concepts and form factors. Some next-generation astronomy telescopes use these systems to achieve improved performance. For instance, the Giant Magellan Telescope (GMT) combines multiple highly aspheric freeform primary mirror segments.1 A rapid precision manufacturing technology is thus one of the remaining pieces that prevents full realization of a freeform age, which will make complex tailored optics a tangible reality. To achieve a rapid optics manufacturing process, it is highly desirable (because of the high material removal rate) to have precise surface figuring during the grinding phase. A surface error that requires one minute of grinding, for instance, can easily take more than 1000 minutes if it is removed during the polishing phase. Such grinding processes, however, are mostly limited by available metrology techniques. Current grinding phase metrology options include the use of a laser tracker2 or an IR interferometer, but these are often limited by coarse spatial resolution, low accuracy, or low dynamic range. Additionally the IR interferometer requires an expensive custom null IR optics system to measure freeform surfaces. We have developed a new IR deflectometry (the deflection of light from a known source is measured, and the profile of the surface with which the light interacted is determined via back calculations) system (see Figure 1) that overcomes the limitations of existing metrology approaches.3, 4 We use deflectometry Figure 1. Photograph of the Scanning Long-wave Optical Test System (SLOTS). The long-wave IR (LWIR) camera (red arrow) and the tungsten ribbon IR source (blue arrow) are indicated.3