With the understanding of aspheric lens manufacturing provided in this article, designers must have the tools to optimize their aspheres; the next step is to understand how to specify and tolerance the asphere so it can meet its application’s requirements. Every manufacturing process needs metrology, whether to aid in an iterative process such as magnetorheological finishing (MRF) or simply to ensure the part meets its design requirements.
Testing accurately is not a simple endeavor. Asphere metrology technology has made great advances in the last decade in the form of new options, increased flexibility of surfaces that can be tested, and improved measurement accuracy, time and cost.
Three subtractive methods polish the optics with small-surface-area tools:
- Computer Numerically Controlled (CNC) grinding and polishing process uses precisely controlled tools
- MRF uses a ribbon of magnetically responsive fluid to polish the surface
- diamond turning uses even smaller single-point tools to shape the optical surface
Molding methods produce surfaces without removing material. Both precision glass molding and plastic injection molding can be used to produce aspheres. Precision glass molding heats glass to its softening point and compresses it between two molds; plastic injection molding forces liquid plastic into a mold then cools it to a solid.
No matter which manufacturing method is used, proper metrology to the necessary accuracy will ensure the parts meet required surface accuracies. This is even more important when using an iterative process such as MRF, which requires accurate surface data to adjust removal. Molding processes require less frequent metrology; however, it is still necessary to validate the production and ensure continued performance as tools wear down and are corrected.
The most common measurement techniques fall into two categories:
Interferometry measures the difference of a reference wavefront and the wavefront either reflected off a surface or transmitted through the optic. Profilometry measures the changes in height of a part under test by moving a probe across the part; typically this is done in slices or spirals across the surface to build either a cross section or surface map of the surface heights. Profilometry tends to be simpler to set up and more flexible in the shapes it can measure but also tends to be less accurate. Using slices is generally faster but does not provide full surface information and can miss non-rotationally symmetric errors.
Knowledge of manufacturing and metrology methods will help you properly tolerance and specify your aspheric surfaces. The background given in this and the previous article provides you with a foundation from which to build your own component designs. Perhaps the most important piece of advice, though, is to select a fabrication partner that is both experienced and innovative, and will help you select or define the proper components to fit your needs. Look for suppliers with a wide range of stock aspheres and the ability to produce custom parts in the volumes you require. Then be certain your manufacturing partner has access to options for fabricating and measuring your components.