Christoph von Kopylow

Remote metrology by comparative digital holography

A method for the remote comparison of objects with regard to their shape or response to a load is presented. The method allows interferometric sensitivity for comparing objects with different microstructure. In contrast to the well-known incoherent techniques based on inverse fringe projection this new approach uses the coherent optical wave field of the master object as a mask for the illumination of the sample object. The coherent mask is created by digital holography to allow instant access to the complete optical information of the master object at any place desired. The mask is reconstructed by a spatial light modulator (SLM). The optical reconstruction of digital holograms with SLM technology allows modification of reconstructed wavefronts with respect to improvement of image quality, the skilled introduction of additional information about the object (augmented reality), and the alignment of the master and test object.

Phase retrieval by means of a spatial light modulator in the Fourier domain of an imaging system

We present an experimental configuration for phase retrieval from a set of intensity measurements. The key component is a spatial light modulator located in the Fourier domain of an imaging system. It performs a linear filter operation that is associated to the process of propagation in the image plane. In contrast to the state of the art, no mechanical adjustment is required during the recording process, thus reducing the measurement time considerably. The method is experimentally demonstrated by investigating a wave field scattered by a diffuser, and the results are verified by comparing them to those obtained from standard interferometry.

Evaluation methods for gradient measurement techniques

Many optical metrology methods deliver 2D fields of gradients, such as shearography, Shack-Hartmann sensors and the fringe reflection technique that produce gradients for deformation, wave-front shape and object shape, respectively. The evaluation for gradient data usually includes data processing, feature extraction and data visualization. The matters of this talk are optimized and robust processing methods to handle and prepare the measured gradients. Special attention was directed to the fact that optical measurements typically produce data far from ideal behavior and that parts of the measured area are usually absent or invalid. A robust evaluation must be capable to deliver reliable results with non perfect data and the evaluation speed should be sufficient high for industrial applications. Possible data analysis methods for gradients are differentiation and further integration as well as vector processing when orthogonal gradients are measured. Evaluation techniques were investigated and optimized (e.g. for effective bump and dent analysis). Key point of the talk will be the optimized data integration that delivers the potential of measured gradients. I.e. for the above mentioned examples: the deformation, wave-front and object shape are delivered by successful data integration. Local and global existing integration methods have been compared and the optimum techniques were combined and improved for an accelerated and robust integration technique that is able to deal with complicated data validity masks and noisy data with remaining vector rotation which normally defeats a successful integration. The evaluation techniques are compared, optimized and results are shown for data from shearography and the fringe reflection technique (, which is demonstrated in talk “High Resolution 3D Shape Measurement on Specular Surfaces by Fringe Reflection”).

Miniaturized digital holography sensor for distal three-dimensional endoscopy

A miniaturized sensor head for endoscopic measurements based on digital holography is described. The system was developed to measure the shape and the three-dimensional deformation of objects located at places to which there is no access by common measurement systems. A miniaturized optical sensor, including a complete digital holographic interferometer with a CCD camera, is placed at the end of a flexible endoscope. The diameter of the head is smaller than 10 mm. The system enables interferometric measurements to be made at speeds of as many as five reconstructions per second, and it can be used outside the laboratory under normal environmental conditions. Shape measurements are performed with two wavelengths for contouring, and the deformation is measured by digital holographic interferometry. To obtain full three-dimensional data in displacement measurements we illuminate the object sequentially from three different illumination directions. To increase the lateral resolution we use temporal phase shifting.

Shearing interferometer based on the birefringent properties of a spatial light modulator.

We present a shearing interferometer that is based on the birefringent properties of a phase-only spatial light modulator. The main advantages of this approach are the high flexibility, the robustness, and the light efficiency. In contrast to already existing methods the suppression of unwanted diffraction orders is not required.

Suppression of higher diffraction orders and intensity improvement of optically reconstructed holograms from a spatial light modulator

Spatial light modulators (SLMs) can be used to optically reconstruct the complex amplitude of a wavefield. The reconstructed wavefield in the far-field is corrupted by effects arising from the non-ideal modulation properties and the discrete nature of SLMs. These effects are regarded as drawbacks and particularly disturb the visual impression and reduce the light efficiency of the reconstruction in the region of interest. In this paper we present a novel method to improve the quality of the reconstructed light field by eliminating the higher diffraction orders and by modifying hologram data based on the characteristic modulation properties of the SLM. We achieve the elimination of the higher orders optically using only one lens in a 4f configuration with an appropriate amplitude mask inserted in the Fourier plane. For compensating the non-ideal modulation properties of the SLM we characterize the device to alter the digital hologram data in an appropriate way. The basic principle of this technique and its experimental verification are described.

Wave field sensing by means of computational shear interferometry

In this paper, we present a method to recover the complex amplitude of speckle fields from measurements performed by a shear interferometer. It is based on the optimization of an objective function using the steepest descent gradient technique in combination with a heuristic initial guess. In contrast to already existing methods, the algorithm finds a local minimum least-squares solution even in the presence of Poissonian and Gaussian noise.

Vision ray calibration for the quantitative geometric description of general imaging and projection optics in metrology

Exact geometric calibration of optical devices like projectors or cameras is the basis for utilizing them in quantitative metrological applications. The common state-of-the-art photogrammetric pinhole-imaging-based models with supplemental polynomial corrections fail in the presence of nonsymmetric or high-spatial-frequency distortions and in describing caustics efficiently. These problems are solved by our vision ray calibration (VRC), which is proposed in this paper. The VRC takes an optical mapping system modeled as a black box and directly delivers corresponding vision rays for each mapped pixel. The underlying model, the calibration process, and examples are visualized and reviewed, demonstrating the potential of the VRC.

Digital pre-filtering approach to improve optically reconstructed wavefields in opto-electronic holography

We present a method that significantly improves the quality of the optically reconstructed complex amplitude from a spatial light modulator (SLM). This method is based on applying a digital pre-filtering of the SLMs encoded distribution to compensate the signal distortion caused by the SLMs finite pixel size. In this approach the complex transmittance is changed in such a way that it compensates the modulation in the Fourier plane. The basic principle of this technique and its experimental verification are described.

Efficient reconstruction of spatially limited phase distributions from their sheared representation

We present a method that allows the reconstruction of smooth phase distributions from their laterally sheared representation. The proposed approach is efficient in the sense that only one sheared distribution is needed to completely restore the signal. A mandatory requirement is that the phase distribution is spatially limited. The method is exemplified by means of a synthetic signal, and in addition a practical algorithm is given. Finally, experimental results are presented. The deformation of a metallic surface is investigated by both speckle shearography and electronic speckle pattern interferometry (ESPI) respectively. To give proof of the proposed technique, the phase distribution reconstructed from the shearographic measurement is shown to match the results obtained by the ESPI.