Dr.-Ing. Jürgen Schweizer, product management marketing, Mahr, and Dipl. Geogr. Doris Knauer, global campaign manager industrial automation, Physik Instrumente (PI)
Aspherical lenses have rotationally symmetrical optics around the optical axis, whose radius of curvature changes with their distance from the centre. This allows optical systems to achieve high image quality, and at the same time, the number of elements they require decreases, saving weight and costs. However, testing the aspheric shape accuracy—which affects the quality of this type of lens—is a considerable challenge for the manufacturer. It requires measurement of the tiniest shape deviations in the nanometre range, with as short as possible measuring and setup times. A solution is to use Mahr’s MarOpto TWI 60 tilted wave interferometer. As part of this interferometer, PI’s H-824 six-axis hexapod is responsible for several positioning functions.
Several interferometry 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. However, 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, each partial measurement being combined to a full-surface interferogram. The process is very flexible compared with 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 where only smaller circular interference pattern rings can be captured and therefore many interference patterns have to be stitched together.
As well as this non-contact measuring method, tactile and quasi-tactile measuring is possible (similar to a scanning probe microscope). In this case, the surface is only scanned point-to-point and with gaps instead of homogeneous coverage. However, the use of tactile measuring methods is not the best choice for polished surfaces because of the risk of scratching.
Tilted wavefront technology
Mahr—a Germany-based manufacturer of production metrology equipment—developed the MarOpto TWI 60 tilted wave interferometer (image 1) for fast, flexible and precise measuring of different aspheres directly on the production line, without CGH, classical stitching or tactile contacting. In contrast with traditional systems, which typically take several minutes to perform the measuring, the MarOpto TWI 60 takes 20 to 30 seconds to measure the entire surface. The next test object can be measured while the previous one is being evaluated, which usually takes around two minutes.
Image 1: the MarOpto TWI 60 tilted wave interferometer for fast, flexible and precise measuring of different aspheres directly on the production line, without CGH, classical stitching or tactile contacting.
In addition to short measuring and evaluation times, the system distinguishes itself by its flexibility. It is not only possible to measure aspheres but other optics that have geometries deviating from standard shapes, so-called freeforms. The system is exceptionally robust and can therefore be situated in production areas.
The MarOpto TWI 60 does not immediately acquire the entire test object optically in one single image but in several subapertures that are active at different times. In the case of optics that have steep surfaces—such as aspheres and freeforms—acquiring the test object at once would cause the interference patterns to converge and it would not be possible to resolve them afterwards. The individual subapertures are spread out and actively switched. This allows the various tilted wavefronts to hit the inspection optics so that the resulting interference patterns do not overlap (image 2). An undisturbed interference pattern of a local part of the test object surface is obtained from each subaperture and the entire surface of the test object can be measured in a short time.
Image 2: The individual subapertures are spread out and actively switched. This allows the various tilted wavefronts to hit the inspection optics so that the resulting interference patterns do not overlap.
Finally, the individual interference patterns are combined to form a single pattern of the test object’s surface (image 3a). This represents the surface of the (aspherical) test object and can then be evaluated accordingly (image 3b). The deviation of the test object’s actual shape from the nominal shape is important for the user. The design of the MarOpto TWI 60 also makes it very flexible with respect to the surface geometry of the test object. This means that each test object can have an individual surface shape without the need to change the setup of the interferometer or even interrupt the production process at all. Furthermore, segmented and off-axis aspheric, toroid and freeform optics can also be measured quickly at high lateral resolution and measuring uncertainties under 50 nm.
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Images 3a/b: The individual interference patterns are combined to form a single pattern of the test object’s surface (3a). This represents the surface of the (aspherical) test object (3b) and can then be evaluated accordingly.
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Images 3a/b: The individual interference patterns are combined to form a single pattern of the test object’s surface (3a). This represents the surface of the (aspherical) test object (3b) and can then be evaluated accordingly.
The referencing process
The MarOpto TWI 60 needs to be referenced and calibrated just like any other measuring device. For this purpose, a highly accurate sphere with known geometrical specifications is moved to a specific position for each subaperture and then measured by that subaperture. Due to the complex optical beam path, and in contrast with conventional systems, these types of subaperture interferograms are more sophisticated (images 4a/b). The wavefronts generated from the individual subapertures are combined to form an overall wavefront. Finally, all measurements are evaluated and an algorithm is used to correct the systematic measurement deviations across all subapertures.
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Images 4a/b: Due to the complex optical beam path, and in contrast with conventional systems, these types of subaperture interferograms are more sophisticated.
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Images 4a/b: Due to the complex optical beam path, and in contrast with conventional systems, these types of subaperture interferograms are more sophisticated.
As all kinds 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.
To meet the high demands on the positioning mechanism and after careful testing, Mahr made the decision to invest in the H-824 six-axis hexapod from PI (Physik Instrumente) (image 5). This hexapod positions the calibration sphere as well as the test object in five degrees of freedom before the actual measuring process begins. Both the nominal and actual position need to be matched exactly since, for example, deviations in tilt may not exceed 60 µrad.
Image 5: The H-824 hexapod positions the calibration sphere as well as the test object in five degrees of freedom before the actual measuring process begins.
Advantages of the parallel-kinematic positioning system
Hexapods—also known as parallel-kinematic positioning systems—are able to position in six degrees of freedom with high accuracy and travel along trajectories with high precision. As opposed to serial kinematics systems, all six actuators of hexapods act directly on the same platform (image 6). This allows for a more compact design than the stacked arrangement of serial kinematics systems as well as improved path accuracy, repeatability and flatness. In addition, because hexapods move only one platform, the overall mass is less, resulting in high dynamics in all motion axes.
Image 6: As opposed to serial kinematics systems, all six actuators of hexapods act directly on the same platform.
Another essential characteristic of hexapods is the freely definable rotation or pivot point, which means it is possible to define various coordinate systems that, for example, refer to the position of the workpiece or tool.
The C-887 hexapod controller and user-friendly software allow for easy commanding of the hexapod (figure 7). The positions are specified in Cartesian coordinates, and all transformations to the individual drives are performed inside the controller.
Image 7: The C-887 hexapod controller and user-friendly software allow for easy commanding of the hexapod.
The TWI 60 system is being used by the Physikalisch-Technische Bundesanstalt (PTB)—the national metrology institute of Germany—as well as a number of well-known manufacturers of aspheric precision optics.
Mahr
Physik Instrumente