Digital multi-wavelength holography – from the laboratory to production

Digital holography

Digital holographic 3D measurement technology enables the fast (sub-second), and at the same time, highly precise (micrometer range) measurement of 3D geometries of typically matchbox-sized technical components. The basic principle of holography dates back to an invention (1948) by Dennis Gabor, for which he was awarded the Nobel Prize in Physics in the year 1971.

Unlike photography, in which the spatial distribution of light intensity is stored, holography also utilizes recording of the phase information. This necessitates a coherent light source – typically a laser. If the surface of a test specimen is then illuminated with laser light, the shape of the test specimen is stored in the phase distribution of the backscattered light wave. Interferometric recording and subsequent digital reconstruction make this information accessible and it is used to measure surfaces three-dimensionally.

Measurement with Multiple Wavelengths on Technical Surfaces

Using multiple narrow-band lasers, various synthetic wavelengths are generated. These differing wavelengths facilitate a broad measuring range from the (sub) micrometer range to the millimeter range, depending on surface roughness. Resolution and reproducibility of the measurements depend on the distance between the individual wavelengths and on surface properties and can be flexibly adapted to the relevant application by the skillful choice of light sources.

Unlike classic interferometry or holography with only one laser wavelength, multi-wavelength holography allows the measurement of optically rough surfaces. The speckle noise produced on rough surfaces which normally renders quantitative phase evaluations for determining topography impossible is eliminated by numerical reconstruction at different wavelengths. This results in a phase map at the beat frequency of the individual wavelengths which contains the information regarding the topography of the illuminated object and which can be evaluated quantitatively.

Accuracy and Speed with Regard to Extremely Demanding Inline Inspection Tasks

The CPU-intensive digital-holographic reconstruction of the complex-valued wavefields in which the test specimen topography is stored is performed on state-of-the-art graphics adapters and this process has been accelerated by several orders of magnitude in recent years. The »HoloTop« 3D sensor developed by Fraunhofer IPM evaluates over 100 million measuring points per second and is thus unique as regards accuracy and speed.

Fraunhofer IPM has a key patent (DE102008020584B3) for the self-calibration of the synthetic wavelengths, the length of which represents the measurement standard of the procedure.