Noncontact Measuring of Aspheric Lenses
Hexapod positions test object and calibration sphere in an interferometer
For manufacturers, testing the aspheric shape accuracy is a considerable challenge: It requires measuring the tiniest deviations in shape in the nanometer range while making short measuring and setup times possible. For this reason, the metrology company Mahr developed a tilted wavefront interferometer which only needs 20 to 30 seconds to measure the entire surface. As part of the overall system, a hexapod from PI (Physik Instrumente) carries out several positioning tasks for calibrating and measuring.
Measuring and Referencing 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 50 nm.
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.
Mahr uses the H-824 hexapod from PI in order to meet the high demands on the positioning mechanism.
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.