ABSTRACT
This paper introduces a technique for freeform optical surface measurements using digital holography with multiple discrete wavelengths or wavelength scans. This experimental arrangement, i.e., a Mach-Zehnder holographic profiler, is optimized to achieve maximal theoretical precision and is capable of measuring freeform diffuse surfaces. Moreover, the approach can also be used for diagnostics of precise placement of elements within optical systems.
ABSTRACT
Digital holographic microscopy (DHM) is an effective method for the evaluation of surface topography. It combines the high lateral resolution of microscopy with the high axial resolution of interferometry. In this paper, DHM with subaperture stitching for tribology is presented. The developed approach allows large surface area inspection by stitching together multiple measurements, which brings a big advantage to the evaluation of tribological tests such as a tribological track on a thin layer. The whole track measurement provides additional parameters, which can offer more information on the result of the tribological test than the conventional four-profile measurement by a contact profilometer.
ABSTRACT
This paper describes the dependence of the precision of digital holographic methods on measurement parameters. The predominantly discussed parameters are illumination intensity and its homogeneity, surface microroughness, the influence of measurement geometry, as well as object shape, since most of them can be optimized by experimental arrangement. Frequency sweeping digital holography as well as dual-wavelength digital holography in the Fourier arrangement are tested and the results are discussed. It transpires that the methods are not very sensitive to object microroughness or overall reflectivity. Instead, it is the similarity of signal and reference waves that has the highest impact on measurement. After parameter optimization, the holographic methods can be advantageously used for ground surface measurements in optical workshops.
ABSTRACT
High-precision measurements of mechanical parts' surface topography represent an essential task in many industry sectors. Examples of such tasks are, e.g., precise alignments of opto-mechanical systems, large object deformation measurements, evaluation of object shape, and many others. Today, the standard method used for such measurements is based on use of coordinate measuring machines (CMMs). Unfortunately, CMMs have severe shortcomings: low measurement point density, long measurement time, risk of surface damage, etc. Indeed, the measurement time rapidly increases with the object complexity and with the density of measurement points. In this paper, we have developed a method for surface topography measurements called "frequency sweeping digital holography" (FSDH). Our developed FSDH method is based on the principles of wavelength scanning interferometry. It allows surface topography measurements of objects with a diameter of several hundred of mms and a high axial accuracy reaching 10 µm. The greatest advantage of the presented FSDH is the fact that the surface topology data are captured in a motionless manner by means of a relatively simple setup. This makes the FSDH method a suitable technique for topography measurements of objects with complex geometries made of common materials (such as metals, plastics, etc.), as well as for the characterization of complex composite structures such as acoustic metamaterials, active acoustic metasurfaces, etc. Measurement method principles, setup details, lateral resolution, and axial accuracy are discussed.
