Measuring in 3-D

Today, complex optical free-form geometries are used primarily in car headlamps and in optics for cameras and digital projectors. These optical components are expensive to manufacture and to test.

“The lenses used in many optical components today – for instance in car headlamps or in digital projectors or cameras – are no longer spherical, but have free-form geometries,” says Dr. Gunther Notni of the Fraunhofer IOF.

“Free-form geometries are not rotationally symmetrical, but may be surfaces of any shape. This makes them expensive to manufacture, and the conventional methods used so far have not allowed the lenses and mirrors to be tested thoroughly enough. Until now, it has taken over an hour to measure the aspherical lenses using high-precision coordinate measuring devices.” This was reason enough to develop a new measuring system for the expensive lenses in a collaborative project.

The optical 3-D scan system enabled the scientists to increase the speed significantly: The measuring process now takes a mere 15 minutes. Since the surface is not touched by a scanner, scratches are avoided and the process can immediately be corrected with the aid of the data obtained. Notni explains: “We project fringe patterns onto the free form with a digital projector. These are recorded with a CCD camera from different directions. We then analyze the fringe bending on the computer, using a special mathematical method. Deviations from the reference values of as little as one micron can be quickly and easily identified. What makes the new method unique is that the data obtained can also be used for the subsequent grinding process, and these rounds off the correction cycle.”; Source: Fraunhofer-Gesellschaft