Fraunhofer Group for Light & Surfaces
Interferometric and holographic measuring technology makes it possible to measure geometrical forms precisely, at sub-micrometer accuracy. In this process, differences in the paths of the measuring and reference beams in the wavelength range are compared with one another and measured by means of interference. The lateral accuracy only depends on the optics used, the coherence of the beam source and the measuring distance.
For this purpose, Fraunhofer ILT has developed an absolute measuring interferometric sensor system that can measure the distances to technical objects in a measuring range of 8 mm (with typical working distances of 50 to 200 mm) at an accuracy of < 200 nm. This sensor system – »bd-1« – has a compact fiber-coupled measuring head with bidirectional beam guidance. Forward and backward rays propagate along the same line. Thanks to its submicrometer accuracy, the system can even determine the roughness characteristics of glossy, polished or rough technical surfaces.
Due to their compact design and their low weight, »bd-1« measuring heads can easily be integrated into processing and testing devices. They open up a new class of applications which are not available with conventional triangulation methods: the linearity error is lower by a factor of 10, the construction volume of the measuring head is smaller by a factor of 40, and the bidirectional concept significantly improves the accessibility to the place of measurement.
Applications can be found in high-precision thickness measurement during cold rolling, the testing of camshafts or measurement tasks for in-line monitoring of laser machining processes: laser welding, laser deposition welding, selective laser melting (SLM), or surface microstructuring with ultrafast lasers. For these applications, the measuring beam of »bd-1« is coaxially superimposed on the processing radiation. For multi-point measurement, the conceptual design was expanded so as to make highly precise distance measurements with up to 16 measuring beams at different points of an object.
Holographic measurement technology
The holographic measurement technology represents a further embodiment of interferometric measurement technology. For this purpose, Fraunhofer ILT uses special computer-generated holograms (CGH) that permit the high-precision measurement of optical freeforms. The CGHs are manufactured at Fraunhofer IOF and are used in corresponding interferometric measuring systems.
The digital holographic 3D measuring technology pursued at Fraunhofer IPM enables the fast (sub-second range) and, at the same time, high-precision (μm range) measurement of 3D geometrical shapes, typically of mechanical components. Interferometric recording and subsequent digital reconstruction allow the surface to be measured three-dimensionally.
With electronic speckle-pattern interferometry (ESPI), minimal changes in the topography can be detected very quickly, flatly and down to the nanometer range – even in the case of fast production sequences. The great advantages of this measurement are the imaging and the accuracy in the nm range in the direction of observation.
Inline Microscopy
An important field of application of inline microscopes is production monitoring in medical technology and biotechnology. Here, special production conditions and high demands on precision and speed require specially designed microscopy systems. Fully automated microscope systems from Fraunhofer IPM monitor the production of micromechanical medical devices and enable a 100-percent quality control.