Progresses in microsystem technology promise a lot of new applications in industry and research. However, the increased complexity of the microsystems demand sensitive and robust measurement techniques. Fullfield and non invasive methods are desirable to get access to spatially resolved material properties and parameters. This contribution describes a simple and fast interferometric method for the analysis of shape and deformation of small objects by optical means. These quantities together with a well defined loading of the components can be the starting point for a big variety of interesting material characteristics which can be evaluated by the use of physical models of the objects. Holographic interferometry and multiple wavelength contouring as well as multiple source point contouring are precise enough to fulfill the requests for precision and resolution in microsystem technology even on complex shaped structures with steps or gaps. A new adaptive, iterative algorithm is developed and applied to the measured results that allows the numerical evaluation of the phase data to get absolute shape and deformation information in Cartesian coordinates. Surfaces with holes, gaps and steps can be registered without any ambiguities. Digital holography as the underlying holographic recording mechanism is extremely suitable for small objects and lead to simple and compact setups in which the objects’ shape as well as their deformation behavior can be recorded. Experiments are described to show the great potential of these fast and robust measurement techniques.
The comparison of two objects is of great importance in the industrial production process. Especially comparing the shape is of particular interest for maintaining calibration tools or controlling the tolerance in the deviation between a sample and a master. Outsourcing and globalization of production places can result in large distances between co-operating partners and might cause problems for maintaining quality standards. Consequently new challenges arise for optical measurement techniques especially in the field of industrial shape control. In this paper we describe the progress of implementing a novel technique for comparing directly two objects with different microstructure. The technique is based on the combination of comparative holography and digital holography. Comparing the objects can be done in two ways. One is the digital comparison in the computer and the other way is by using the analogue reconstruction of a master hologram with a spatial light modulator (SLM) as coherent mask for illuminating the test object. Since this mask is stored digitally it can be transmitted via telecommunication networks and this enables the access to the full optical information of the master object at any place wanted. Beside the basic principle of comparative digital holography (CDH), we will show in this paper the set-up for doing the analogue comparison of two objects with increased sensitivity in comparison to former measurements and the calibration of the SLM that is used for the experiments. We will give examples for the digital and the analogue comparison of objects including a verification of our results by another optical measurement technique.
Electronically addressed spatial light modulators (SLMs) are key elements for the reconstruction of digital holograms. Reflective liquid-crystal-on-silicon displays (LCOS) have great potential to fulfill this task due to their high fill factors of over 90% and their small pixel sizes of less than 15 μm. In order to obtain maximum diffraction efficiency of the holographic reconstruction, analog phase holograms have to be implemented making a maximum phase shift of 2π in each LCOS pixel necessary. Therefore, each LCOS display has to be thoroughly characterized prior to its use as a holographic element. In this publication, we report on a specially designed LCOS test bench. Here, displays can be characterized with respect to their phase and amplitude modulation (i.e. the complex transmittance) under a varying angle of the incident linearly polarized light. Additionally, the Jones matrix of the displays can be measured, which allows computation of the response of the displays to light of arbitrary polarization. The measurement of panel flatness is also possible which is necessary to compensate wave front aberrations. Results of measurements of two LCOS dis-plays are presented and a comparison to other measurement methods is given.
With increasing globalization many enterprises decide to produce the components of their products at different locations all over the world. Consequently, new technologies and strategies for quality control are required. In this context the remote comparison of objects with regard to their shape or response on certain loads is getting more and more important for a variety of applications. For such a task the novel method of comparative digital holography is a suitable tool with interferometric sensitivity. With this technique the comparison in shape or deformation of two objects does not require the presence of both objects at the same place. In contrast to the well known incoherent techniques based on inverse fringe projection this new approach uses a coherent mask for the illumination of the sample object. The coherent mask is created by digital holography to enable the instant access to the complete optical information of the master object at any wanted place. The reconstruction of the mask is done by a spatial light modulator (SLM). The transmission of the digital master hologram to the place of comparison can be done via digital telecommunication networks. Contrary to other interferometric techniques this method enables the comparison of objects with different microstructure. In continuation of earlier reports our investigations are focused here on the analysis of the constraints of the setup with respect to the quality of the hologram reconstruction with a spatial light modulator. For successful measurements the selection of the appropriate reconstruction method and the adequate optical set-up is mandatory. In addition, the use of a SLM for the reconstruction requires the knowledge of its properties for the accomplishment of this method. The investigation results for the display properties such as display curvature, phase shift and the consequences for the technique will be presented. The optimization and the calibration of the set-up and its components lead to improved results in comparative digital holography with respect to the resolution. Examples of measurements before and after the optimization and calibration will be presented.
Shearography is an approved and powerful tool for the non-destructive inspection of industrial components with respect to material faults and technical imperfections. An application field of high interest is the in-service inspection of aircraft and automotive components. However, the non-cooperative character of the surface of various technical components has to be taken into account carefully. This paper describes a complete test facility consisting of a shearographic sensor, adapted loading equipment for thermal and mechanical stressing and a new evaluation software ensuring a high sensitivity for fault detection. To increase the performance of the system with respect to industrial applications new components and procedures were implemented and tested recently. To them belong a CMOS-camera to increase the dynamic range of the image sensor, a multiband light source to test the coherence requirements of a shearography system and tunable thermal loading equipment to improve the identification of material faults within components having a bigger wall thickness.
Modern production requires more and more effective methods for the inspection and quality control at the production place. Outsourcing and globalization result in possible large distances between co-operating partners. This may cause serious problems with respect to the just-in-time exchange of information and the response to possible violations of quality standards. Consequently new challenges arise for optical measurement techniques especially in the field of industrial shape control. A possible solution for these problems can be delivered by a technique that stores optically the full 3D information of the objects to be compared and where the data can be transported over large distances. In this paper we describe the progress in implementing a new technique for the direct comparison of the shape and deformation of two objects with different microstructure where it is not necessary that both samples are located at the same place. This is done by creating a coherent mask for the illumination of the sample object. The coherent mask is created by Digital Holography to enable the instant access to the complete optical information of the master object at any wanted place. The transmission of the digital master holograms to this place can be done via digital telecommunication networks. The comparison can be done in a digital or analogue way. Both methods result in a disappearance of the object shape and the appearance of the shape or deformation difference between the two objects only. The analogue reconstruction of the holograms with a liquid crystal spatial light modulator can be done by using the light modulator as an intensity modulator or as an phase modulator. The reconstruction technique and the space bandwidth of the light modulator will influence the quality of the result. Therefore the paper describes the progress in applying modern spatial light modulators and digital cameras for the effective storage and optical reconstruction of coherent masks.
In this paper we describe a new sensor for the direct holographic comparison of the shape of two objects where it is not necessary that both samples are located at the same place. In contrast to the wellknown incoherent techniques based on inverse fringe projection this new approach prefers a coherent mask that is overlayed to the sample object having different microstructure. The coherent mask is created by digital holography to enable the instant access to the complete optical information of the master object at any wanted place. The transmission of the digital master holograms to the relevant places is done with a broadband digital telecommunication network such as the Internet. The availability of the complete optical information of the master object in the form of its digital hologram offers two ways with respect to the comparison of its shape and/or deformation with those of a sample object having different microstructure: digital comparison of the respective phase distributions of the relevant digital holograms and analogue comparison of both holograms/interferograms by digital comparative holography.
Digital Holography makes it possible to reconstruct the phase distribution of wavefields directly. By application of interferometric technics the observed interference phase contains the information about the shape of the object under test and/or its deformation after loading. These data can be used to investigate the materials' behavior of microcomponents. However, the observed mod2(pi) -interference phase must be unwrapped and the absolute phase values have to be transformed into 3D-coordinates and displacement components. To this purpose a multi-wavelength procedure was developed that avoids the complicated spatial unwrapping procedure. Moreover an adapted calibration technique is used to calculate the metrological data from the distorted phase field. In combination with special loading techniques and physical models of the loading behavior of components with beam geometry some important material parameters such as the Young's modulus, the Poisson ratio and the thermal expansion coefficient of microcomponents can be measured. The paper describes the measuring technology and shows some examples of microcomponent testing.
The growing development of modern microcomponents, structures and systems, makes it necessary to qualify large-scale approved measurement methods for the investigation in micro- scales. New materials and structural design are employed whose behavior cannot be easily predicted by FE-methods. Material properties and boundary conditions which are known form large- scale investigations may differ considerably for components with micro-scale dimensions. Other problems are due to the high aspect ratio of the micro structures which makes it necessary to consider the real shape of the component especially for reliability studies. Consequently a wide field has been opened for optical and dimensional metrology with respect to the investigation of microcomponents. Many methods which were tested successfully at big components can be used for microcomponents, too. But new ways must be followed especially in the field of loading, handling and observing micro-scale components. The article deals with optical techniques which have proven to be useful for the investigation of large-scale components and their qualification for the investigation of microcomponents. For the discussion the 3 problem classes mentioned above are taken into account.