No kinks in optics
With precise optics, light becomes a tool. This requires methods that are capable of measuring lenses in all sizes, shapes, and materials.
Nanometers count in high-precision optics. Accordingly, processes are needed that measure the surfaces of spherical and aspherical test objects in different sizes and materials. Each production step increases the value and sensitivity of the optics. The right measurement method at the right time is decisive.
During the grinding process, tactile methods are used. At later stages, non-contact optical methods are more suitable, such as interferometry. Based on the optical distance of light that is directed at and reflected by the test object, these methods determine height differences in the nanometer range.
Interferometry also for aspheres
Because lenses and other curved surfaces reflect light in many directions, null optics are used in this area. As an exact counterpart of the test object, they ensure that light meets the curved surface vertically and is also reflected vertically. If the lens is perfect, an absolutely even interferometric wavefront is produced. Since only deviations from this ideal have to be determined, the analysis is quick and extremely precise. However, the null optic must be exact.
Refractive lenses are available on the market for spherical lenses. However, individual null optics have to be made to test aspherical lenses. Computer-generated holograms are an alternative. Diffractive optical elements (DOEs) such as these are generated by introducing lattice structures into glass structures by means of lithography. Through diffraction, they adjust the light exactly to the surfaces of aspheric test objects. The manufacture of tailored DOEs is especially worthwhile where aspheres are needed in high numbers or with a very high degree of precision.
If even minimum production inaccuracies of DOEs or the deviations resulting from the test rig are too much, Dr. Klaus Mantel from the Max Planck Institute for the Science of Light in Erlangen, Germany advises using what are known as multiple wavefront DOEs. In addition to the aspherical wavefront that is directed at the test object, these also generate a spherical wavefront. With the help of special masks, the two fronts can be faded in and out to generate additional positions for the calibration. This stratagem makes up for the lack of symmetry of the test object during calibration. According to Mantel, deviations are systematically eliminated from interferometer measurements.
Flexible measurement with sub-nanometer resolution
Luphos GmbH, which was established 10 years ago, has already given important impetus to interferometric optical tests. "We market multi-wavelength interferometers that scan test objects with four wavelengths," explains development engineer Dr. Marc Wendel. In principle, four interferometers are combined in one device and software merges their measurements. In this way, Luphos aggregates the precision of interferometry and the flexibility of scanning measuring devices. The enormously increased ambiguity range of 1.25 mm is especially impressive. In the past, this was half a wavelength, in other words, about 300 µm. As a result, the measuring process is much more robust against dust, scratches and other production-related interferences.
Multi-wavelength interferometers are highly flexible measuring devices with which stepped, diffractive, or segmented lenses as well as lenses with saddle points or turning points can be measured. Surfaces and exact topographies with resolutions in the sub-nanometer ranges are captured within minutes. Elaborate calibration is, however, a requirement. “Many customers use our systems close to their production processes. In spite of their accuracy, they do not have to be operated in precision measurement rooms,” says Wendel. The strengths of the systems are in the interim and final checks.
Interferometry in lens production
But in general, interferometers are being used increasingly in production. For example, also with OptoTech Optikmaschinen GmbH, which manufactures production and measurement technology for precision optics and spectacle optics. According to Gerd Stach, the head of the Metrology Business Unit, production and measurement technology are becoming increasingly closer to each other.
Normally, lenses are pre-polished after grinding, during which about 20 µm is removed and slight corrections are made. Following this, tiny faults are eliminated with targeted local polishing. This is where the links between production and measurement technology are important. Interferometers determine exact 3D surface data, which is automatically passed on to a dwell-time controller in the polishing system. Because the material removal per time unit is defined, from the surface data it is possible to derive how long the polishing head has to remain at a specific point until the error is rectified. Another benefit of the closed loop: Since the measurements are transferred fully automatically, there is no risk of typing errors.
The problem with in-line measurements is that optics have to be cleaned for this. “Apart from a few exceptions, we measure close to production,” says Stach. It is only with the production of very heavy, up to two meter lenses that measurements are carried out in the machine. But even with production-related interferometry the industry would like to see more compatibility with the everyday situation in production processes. “Interferometers have very many setting options. That is practical to test individual test glasses but it is unnecessarily time-consuming in production,” is his criticism. A defined, software-based measurement program will provide a remedy. Based on digital measurement logs for the respective optics, customers should also be able to reproduce measurement data. “Our goal is to put quality control on a more objective basis,” explains Stach.
The bayern photonics network offers a workshop on "Metrology for modern optics production". Speakers from industry and science will explain the methods that are used and the areas in which they are used.
Dr. Horst Sickinger
Phone: +49 8153 953687