What are holographic algorithms
The digital-holographic 3D measurement technology enables fast (sub-seconds) and at the same time highly precise (µm range) measurement of 3D geometries, typically of technical components the size of a matchbox. The basic principle of holography goes back to an invention by Dennis Gabor in 1948, for which he was awarded the Nobel Prize in Physics in 1971.
In contrast to photography, in which the spatial distribution of the light intensity is stored, holography also uses the recording of the phase information. The prerequisite for this is a coherent light source - typically a laser. If the surface of a test object is illuminated with laser light, the shape of the test object is stored in the phase distribution of the backscattered light wave. Through the interferometric recording and subsequent digital reconstruction, this information is accessible and used to measure surfaces in three dimensions.
Measuring with several wavelengths on technical surfaces
By using several narrow-band lasers, different synthetic wavelengths are generated. Thanks to these different wavelengths, a wide measurement spectrum is available, depending on the roughness of the surface, from the (sub) micrometer to the millimeter range. The resolution and reproducibility of the measurements depend on the distance between the individual wavelengths and the surface properties and can be flexibly adapted to the respective application through a clever choice of light sources.
In contrast to classic interferometry or holography with only one laser wavelength, optically rough surfaces can be measured with multi-wavelength holography. The speckle noise that occurs on rough surfaces, which normally makes quantitative phase evaluations for topography determination impossible, is eliminated by the numerical reconstruction at different wavelengths. This creates a phase map at the beat frequency of the individual wavelengths, which contains the information about the topography of the illuminated object and can be quantitatively evaluated.
Accuracy and speed for the most demanding inline inspection tasks
The computationally expensive digital-holographic reconstruction of the complex-valued wave fields in which the test object topography is stored is carried out on modern graphics cards and has been accelerated by several orders of magnitude in recent years. The “HoloTop” 3D sensor developed by Fraunhofer IPM evaluates more than 100 million measuring points per second and is therefore unique in terms of accuracy and speed.
Fraunhofer IPM has a key patent (DE102008020584B3) for self-calibration of the synthetic wavelengths, the length of which represents the measuring standard of the process.
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