Several methods have been established for checking the shape of aspherical lenses for accuracy: for example, interferometers with computer-generated holograms (CGHs) generate an aspherical wavefront in the desired shape and therefore make it possible to determine the deviation of the lens. The CGHs need to be created individually for each test object shape and are therefore only economical for series production. Interferometric measuring of aspheres in circular subsections is another possibility. Finally, each partial measurement is combined to a full-surface interferogram. The process is very flexible compared to CGHs and is also suitable for the production of prototypes and smaller series. However, “stitching” the circular rings is often very time consuming, as in the case of steeper optics, only smaller circular interference pattern rings can be captured and therefore many interference patterns have to be stitched together.
A new approach: tilted wavefront technology
For this reason, the metrology company Mahr developed a new instrument for precise, fast, flexible measuring of different aspheres directly on the production line, without CGH, classical stitching or tactile contacting. In contrast to existing systems that need several minutes to do the measuring, this tilted wavefront interferometer (TWI) needs only 20 to 30 seconds to measure the entire surface. The next test object can be already measured while the previous one is being evaluated.
Measuring and referencing aspheric lenses process
The new measuring system captures the test object in several subapertures. The individual interference patterns are combined to form the topography of the test object’s surface and the deviation of the test object’s actual shape is determined from the nominal shape. The new method allows measuring of individual surface shapes with high lateral resolution and measuring uncertainties under 50nm.
For calibration and referencing, a highly accurate sphere of known geometrical specifications is moved to a specific position for each subaperture and then measured with this subaperture. As all kind of positioning errors of the calibration sphere affect the correction algorithm of the respective subaperture, the calibration sphere needs to be positioned very exactly. A maximum lateral position error of 5 µm with a repeatability of less than 0.5 µm is required.
Hexapod positions calibration sphere and test object
To meet the high demands on the positioning mechanism in the TWI and after careful testing, Mahr finally made the decision to use the H-824 hexapod from PI (Physik Instrumente). The parallel-kinematic system positions both in five degrees of freedom, the calibration sphere and, prior to the actual measuring process, the test object. In doing so, the target and actual position must be matched very precisely. For example, deviations in tilt may not exceed 60 µrad.
The high-performance C-887 digital controller takes care of controlling the hexapod that, thanks to the user-friendly software, enables easy commanding. The positions are specified in Cartesian coordinates, and all transformations to the individual drives are done inside the controller.
Doris Knauer is with PI