Remote and virtual metrology using comparative digital holography offers the ability to compare two nominally identical but physically different objects, e.g., master and sample, in industrial inspection processes, even if master and sample, or engineer and production facility are located in far apart regions of the world. Remote metrology advances beyond the mere control of the physical set-up or the transmission of the digitally recorded data. It encompasses such concepts as advanced documentation of the experiment, efficient methods for metadata storage, the possibility for remote reviewing of experimental results, the adding of real experiments to publications by providing remote access to the metadata and to the experimental setup via the Internet, the presentation of complex experiments in classrooms and lecture halls, the sharing of expensive and complex infrastructure within international collaborations, or remote testing of the performance of a new device in a realistic setup Internet.
Figure 1: Schematic architecture for the remote experimental system
This provides instant, global access to the complete optical information of the master object. For comparison, the master hologram is either optically reconstructed using a spatial light modulator and projected onto a sample under inspection, resulting in interferometric patterns that can be analyzed to retrieve the difference between the master and the test object, or the phase of both the master and the sample hologram are reconstructed numerically and the difference in optical pathlength is calculated to measure the deformation of the sample.
The system architecture for the remote lab is shown schematically in Fig. 1. At the heart of the architecture is the laboratory with the respective remote experiment (e.g. the digital holographic microscopic system). The experiment is hidden behind a proxy server and can be accessed directly only by an operator at the institute. The computer running the software necessary for controlling the physical experiment is invisible from the outside. All outside contact is handled by the proxy server, using an SSH tunnel for encrypted, secure data exchange. Users access the experiment through the BW-eLabs portal, which authenticates against an eSciDoc user data base. On successful authentication, an SSH tunnel is opened to the SSH server running on the proxy, with authentication passed on using PAM (Pluggable Authentication Modules). The open source research environment eSciDoc also provides storage and access to experimental data, passing data for automatic configuration of the experiment, and access to the publication infrastructure of OPUS. From the user's perspective, the functionality of eSciDoc is mostly transparent, working automatically in the background. The coordinator has to provide a script defining the data, the format and the metadata to be stored. It is very important to state that the so-called metadata, that is data providing information about one or more aspects of the data (means of the creation of the data, purpose of the data, time and date of creation, author, location on a computer, used standard, ...) are recorded automatically with the system. The actual storage process and the corresponding retrieval process is fully symmetrical, allowing not only access to raw data for analysis, but also to restore the complete state of the experimental setup (e.g. in the case of digital holographic microscopy: the object under investigation, its position, the focusing of the microscope, the parameters required in the reconstruction of the hologram, ...). eSciDoc is accessible by third party users, providing search functionality based on metadata generated during the experiment. The roles and rights of users in eSciDoc are rather complex and can be set individually for each experimental set-up and each set of data (if desired), protecting against undesired third party access while enabling collaboration between privileged partners.
Fig. 2: Schematics of the experimental holografic microscope
Fig. 3: Photo of the digital setup for the digital holographic microscopic system in the laboratory
A prototype of a digital holographic microscope has been build, including remote access, authentication and control, and the storage of all relevant metadata. The setup allows the choice of transmissive or reflective illumination, micrometer-precision positioning and focusing, 3D numerical reconstruction by phase unwrapping. The system stores the actual holograms as well as all parameters required for the reconstruction of the object and the precise state of the system during the measurement process, including information about the laser, the position of the object, a description of the object and of the algorithms used in the reconstruction.
- Wilke, M., Alekseenko, I., Situ, G., Sarker, K., Riedel, M., Pedrini, G., et al. (2011). Remote Laboratory for Digital Holographic Microscopy. Proc SPIE.8082. SPIE