Claas Falldorf

Digital holography and grating interferometry: a complementary approach

In this work we propose to extend the Grating Interferometry scheme by Digital Holography. The main advantages of this complementary approach are that no imaging lens is needed and that the reconstruction is not limited to the image plane as it is using a lens. Additionally, this technique provides sensitivity to both directions, normal and parallel to the surface under test. Because the grating is directly integrated into the surface, this allows measuring the displacement of that surface under long term conditions within the magnitude of the used wavelength. Good qualitative results are obtained using a single illumination direction. Quantitative results are obtained using multiple illumination directions and the in-plane sensitivity of the presented technique is shown to be equivalent to that of the Grating Interferometry.

2.5-dimensional polymer-based holograms with individually adjustable structure angle

We present polymer-based holograms with varying structure angle for each individual micrometer-sized pixel. The holograms are numerically calculated utilizing an iterative optimization procedure including a first-order Taylor series expansion to determine the slope of each pixel. The holograms are fabricated by 3D laser lithography. Due to the small pixel sizes and the individual slope of each pixel aliases can be avoided and the overall intensity of the desired projection is strongly increased compared to holograms consisting of larger and uniform pixels. Furthermore, the discrepancy between calculated and measured intensity distribution is strongly reduced.

Sampling the coherence function: a novel method for shape measurement

We present a method for shape measurements, which is based on sampling the temporal coherence function of the light using a shear interferometer. In contrast to standard White Light Interferometry our method has low demands regarding the spatial coherence of the light and is robust with respect to exterior disturbances. We sample the coherency function of the light with the help of of a test object with steps of a couple of micrometers. Due to its common path principle the system is intrinsically vibration tolerant.

Computational Wavefield Sensing

With the advent of faster computer processors, alternative methods of wavefield sensing have been developed throughout the past decades. In contrast to standard interferometry, these methods aim at solving an inverse problem , whereby the recorded intensities are interpreted as an effect caused by the underlying (unknown) wavefield when subjected to different manipulations. In contrast to holography and interferometry it is not possible to use film as a recording material and to optically reconstruct the wavefield. In fact, it is even pertinent to say that the numerical task of solving the inverse problem is an essential and integral part of the measurement process.

Digital Holographic Particle Sizing and Microscopy

While much of this book concentrates on the use of digital holography for vibration and stress analysis, contouring and metrology, the emphasis in this chapter is on the unique image forming characteristics of the hologram.

Fundamental Principles of Holography

The behaviour of light can be modelled either as a propagating electromagnetic (e-m) wave or as a stream of massless particles known as photons . Although the models are seemingly contradictory both are necessary to fully describe the full gamut of light phenomena. Whichever model is most appropriate depends on the phenomenon to be described or the experiment under investigation. For example, interaction of light with the atomic structure of matter is best described by the photon model: the theory of photon behaviour and its interactions is known as quantum optics. The phenomenon of refraction, diffraction and interference, however, are best described in terms of the wave model i.e. classical electromagnetism.

Digital Holographic Interferometry (DHI)

As we saw in Chap. 2, a conventional holographic interferogram recorded on photographic film is generated by superposition of two waves, which are scattered from an object in two different states of loading or excitation.

Robust digital speckle photography based on radon and Fourier-Mellin transforms

We present a new and robust method for determining in-plane displacements of an object from distorted Digital Speckle Photography (DSP) images. This new approach is designed particularly to facilitate accurate measurement of deformations of steel samples during a gas quenching heat treatment process, where rigid body motion and large deformations lead to unwanted image distortions. The new method allows the computation and correction of image rotation and magnification via Radon and Fourier-Mellin Transformations prior to calculations of in-plane displacements, thereby alleviating the inaccuracy that arises from the cross correlation of distorted images in conventional DSP. The method is validated through simulation and measurements with predefined deformations. Initial studies show that the new method is well suited for this application and that it enables measurement of displacements with high accuracy in the micrometer range.

Optical metrology and optical non-destructive testing from the perspective of object characteristics

High precision optical metrology may be viewed from the perspective of the relevant object properties for optical measurement such as microstructure, surface gradient and geometrical complexity. We discuss high precision measurement methods and compare their suitability with respect to these object properties. We emphasize reflectometry and shearography as examples of two interesting techniques particularly suited for high precision optical metrology and extend the discussion to optical non destructive testing (NDT). In this context, reflectometry and shearography appear to be interesting techniques suitable for both optical metrology and NDT. We finally discuss the unique features of laser ultrasonic for NDT.

Simultaneous recording of digital holograms using a two-wavelength femtosecond laser source

A novel scheme for a coherent multicolor femtosecond source, based on an adapted commercial Ti:sapphire amplifier system is presented. Its performance is demonstrated by simultaneously recording of two digital holograms using ultrashort pulses. The Ti:sapphire laser is modified to simultaneously generate two femtosecond pulses for 2-lambda contouring. For separating the two pulses, a specially designed half-wave retarder together with a polarizing beam splitter were employed in the Michelson-type interferometer.