A variety of techniques have been developed to measure the three-dimensional shape of an object using structured illumination. The measurement of objects with diffusely reflecting surfaces by means of projected patterns can be considered a standard technique. Also more recently, methods for the measurement of specularly reflecting or transparent surfaces by evaluation of the images of regular patterns have been published. In this paper, we suggest a systematic treatment of all of these methods as generalized active or passive triangulation techniques.
With deflectometric measurement methods there are powerful systems available today that are capable of measuring the
geometry of specular surfaces or the power distribution of refractive optics in a fast and flexible manner without
influencing the measurement object by tactile probing. They are based on the general principle to image a known
spatially coded reference structure via the unknown measurement object onto an optically calibrated camera. As a
representation of the reference structure LC-displays are very suitable as they provide a high flexibility in the generation
of spatial coding patterns like sinusoidal fringes. As the characteristics of the reference structure have a huge impact on
the resolution, the accuracy and the measurement range of the whole system, in this work two displays with different
LCD technologies are analysed, compared and evaluated especially for deflectometric applications. The main focus is on
the quality of gray-value rendering and the dependency between the characteristic curve and the observation angle. The
experimental results corroborate the theoretical finding that IPS-technology is superior to TN- and MVA-displays in
terms of an observation-angle independent shape of the grayscale-characteristic curve. So IPS should be the technologyof-
choice when selecting a LC-display for a deflectometric measurement system.
For fast, accurate and robust shape measurement of specular surfaces there are several powerful measurement techniques based on deflectometry. For boundary surfaces of transparent objects like refractive optics, on the contrary, deflectometry is so far limited to the measurement of only one surface in reflection. This is unsatisfying from a metrological point of view as the geometrical relation between both surfaces, which substantially defines the optical function, is lost. In this work a new deflectometric approach is presented that works in transmission and allows the simultaneous measurement of both surfaces of refractive optics. The basic idea of the approach is calculating the unknown surface geometry of a transparent object by iteratively adapting a surface model to the observed light ray deflections. The main problem in this case is the ambiguity of the refraction at the two boundary surfaces of a lens, as there are multiple possible solutions that produce the same measurement results. This is solved by combining four different views on the object under test, which allows to find an unambiguous solution. An experimental measurement setup is presented and results of different simulations and tests are discussed in this paper.
The Portevin-Le Châtelier (PLC) effect is a case of plastic instability that may occur during the deformation of ductile
alloys. The phenomenon is usually investigated by means of tensile tests, in which, for certain ranges of temperature and
strain rate, the plastic deformation of the material intermittently loses the uniformity and concentrates in narrow regions.
These regions are referred to as PLC bands. In the recent years, great attention is given to the characteristics of these
bands, which are often studied with optical methods. With the aim of improving the resolution of these observations, the
employed camera should frame a small region of the material, but this leads to the drawback that the rest of the specimen
is not considered. To overcome this limitation, the propagating nature of the bands can be profitable since one can
predict the position and the time of the next event and then move the camera accordingly. In this paper an optical
measuring system is presented, permitting the tracking and observation of a certain type of PLC bands. The system
consists of two cameras, one for a global observation of the specimen and a real time detection of the emerging bands,
and a second one for the detailed observation with a high resolution. This last camera is moved along the specimen in
order to continuously image the propagation of the band.