When manufacturing precision optical surfaces of relatively larger sizes it is critical to understand the thermal stability of the substrate material. The material properties associated with thermal homogenization are commonly reviewed and soak schedules are created. These schedules ensure a surface under test is in a stable state and is ready for wavefront measurement with an interferometer. However with some materials such as N-BK7, standard soak schedules may not be enough. This paper shows the thermal challenges associated with manufacturing precision optical surfaces when the substrate material is N-BK7, and how the issue can be easily confused with poor metrology. Throughout the manufacturing of precision optical surfaces, the substrates are exposed to varying heat sources and loads. During the manufacturing of lenses greater than 4 inches in diameter we have observed permanent deformation of the optical surface as a result of exposure to temperatures well below the glass strain point. While the reasons why the change occurs is not yet well understood, the result is well documented and was recently observed during the manufacturing of a 15 inch diameter spherical mirror. We use this lens as a case study highlighting the challenges associated with this phenomenon.
With the ongoing advancements in aspheric manufacturing and metrology, companies have to overcome processing challenges and from time to time learn costly lessons along the way. Optimax Systems, Inc., a leader in quick delivery prototype optics, has been manufacturing aspheric lenses for over 20 years. Along the way, we have learned many lessons, some the hard way. In this paper, I will share a few stories of how aspheres have humbled us, how we overcame the problem, and provide takeaways for other manufactures and designers.
Freeform optical shapes or optical surfaces that are designed with non-symmetric features are gaining popularity with lens designers and optical system integrators. Tolerances on a freeform optical design influence the optical fabrication process. Case studies and soft tolerance limits for easier fabrication will be discussed. This paper will also give a high level overview of a freeform optical fabrication process that includes generation, high speed VIBE polishing, sub-aperture figure correction and testing of freeform surfaces.