Compared to spherical and aspherical lenses freeform shapes are much more challenging for the measurement required in the production process. A lot of metrology systems available for measuring spheres and aspheres are not capable of measuring freeforms. In addition lenses with a freeform surface may also contain reference elements whose position has to be determined. However, there are a few types of instruments which can in principle perform measurements of freeforms. Challenges of measuring freeforms are discussed and different types of potential measuring instruments and their capabilities are presented. This is demonstrated providing measuring results from different samples measured with different instruments.
A single aspheric or freeform lens contains the elements front and backside surface, edges and maybe other reference surfaces or structures. In this paper is demonstrated how a high precision form tester is capable of measuring these elements utilizing an optical and a tactile probe system. The measurements can be used to determine the orientation or alignment of the different surfaces and structures. Different measuring examples are presented and the influence of the measuring uncertainty from the instrument and the measuring strategy are shown. This information may be useful for tolerancing of lenses and an optimization of the production process.
In addition to the aspheric or freeform surface a lens or mirror also contains other structures such as edges and fiducials. These surfaces are important for the production process including the assembly of the optical system. It is demonstrated how these structures can be measured with a high precision cylindrical coordinate measuring system involving an optical and a tactile probe system. The measurements reveal a reference coordinate system which defines the position and orientation of the freeform surface which is also a necessary requirement for the measurement of the freeform surface.
The cylindrical coordinate measuring machine MarForm MFU200 can measure not only rotationally symmetric aspheric samples but also nonrotationally symmetric freeform surfaces. Applying both an optical and a tactile probe system, the measuring processes of the optical freeform surface and fiducials can be combined in a very flexible way. A strategy to measure freeforms including the determination of reference coordinate systems, the measuring process, and the analysis are discussed. In this process, fiducials defining a reference coordinate system are of fundamental importance. It is shown how different positions of fiducials can be measured.
The strategy for measuring and analyzing freeforms with a new high precision cylinder coordinate measuring instrument equipped with an optical point sensor is presented. As freeforms compared to aspheres are not rotationally symmetric considering outline and shape the measuring process has to be designed in new ways. In addition fiducials on the sample or fixture have to be measured to determine position and orientation, i.e. a coordinate system, of the sample. In the following analysis process this coordinate system has to be taken into account. The performance of the measuring instrument is demonstrated and measuring results of different samples are shown.
Two techniques for the measurement of aspheres as well as freeforms are presented. The first method, the tilted wave interferometer, is a full aperture interferometric measurement method without any moving parts during the measurement. The second method applies an optical single point sensor in conjunction with two translational and one rotational axes. Both techniques are compared by measuring a selected asphere and a special freeform surface. Differences between both measurement principles are discussed.
A new type of machine setup combining a profilometer with a rotational measuring axis to measure aspheric lenses is
presented. The instrument is very flexible, as it does not need a specific hologram for each type of asphere. In general,
the setup is also capable to measure freeforms. Metrology strategies and first results are presented.
The production process of aspheric lenses relies on the measurement results from interferometers and profilometers. A
new type of machine setup combining a profilometer with a rotational measuring axis is presented. It allows to measure
lenses in all production stages, i.e. from rough grinded surfaces, which can not be measured with interferometers up to
polished surfaces. The setup is very flexible, as it does not need a specific hologram for each type of asphere. These
properties provide new options to optimize the production process.