G. von Bally

Automated Phase-measuring Profilometry Using Defocused Projection of a Ronchi Grating

Abstract A new method is presented for automatic phase measuring profilometry (PMP), in which a defocused image of the projected square-wave Ronchi grating and a phase shifting technique are chosen for the optical phase evaluation. This makes it possible to design all-electronic LCTV-based PMP systems. In this paper a detailed analysis of the applied phase evaluation algorithm as function of the number of steps, N, is given. The alternating feature of the phase measurement precision ϵ(N), which lead to the obvious choice of seven or five steps, is clearly demonstrated. Then an analytical description is presented of the low-pass optical filtering performed by defocused imaging of the projected and video-frame-grabbed Ronchi grating. It is followed by an analytical description of the additional low-pass digital filtering of the same signal due to the inherent properties of the N-step phase shifting algorithm. The figure of merits are estimated in comparison to the simulated perfect imaging of a sinusoidal grating. The results are verified experimentally by a relief determination of a typical object. For this purpose the five accurately shifted defocused projections of the original Ronchi grating, spread over the object, were captured by a CCD based video-digitizing system and evaluated with a PC.

Phase-stepping grating profilometry : utilization of intensity modulation analysis in complex objects evaluation

Abstract A new method for phase-stepping grating profilometry of complex objects is proposed and verified by experiments. Both, the discrete phase distribution and the modulation depth distribution are calculated. To construct binary control masks the discrete values and the histogram of the modulation intensities are inspected first. Then the blocking lines, which are connecting the pairs of opposite poles, are traced down through the local minimums of the modulation intensities. The control masks are used to identify valid and invalid discrete phase distributions during phase unwrapping and phase interpolation, and to control the path of phase unwrapping. The method could equally well be used in grating profilometry or in interferometry.

Holographic double-exposure interferometry in near real time with photorefractive crystals

A holographic interferometric technique based on fast sequences of double exposures in photorefractive crystals for the dynamic analysis of reflection-type objects is presented. Methods for optimizing the diffraction efficiency and the decay time of the holographic gratings recorded at short exposure times are discussed. Special attention is paid to the use of external electrical fields. The maximization of fringe contrast in double-exposed interferograms is investigated both theoretically and experimentally. Finally, the automatic evaluation of interferograms is demonstrated by use of a sample sequence of interferograms.

Lensless Fourier holography for digital holographic interferometry on biological samples

Abstract Digital holography as a tool for highly sensitive, interferometric non-destructive testing has several advantages compared to holographic measurements based on conventional storage media like an all-digital processing and a direct access to the phase of the object wave. Experimental results of interferometric investigations of heart valve bio-prostheses with a setup for lensless Fourier holography are presented which demonstrate that this technique is applicable to such biological samples with their wet and unstable surfaces. Limitations on size and resolution of the reconstructed object caused by the properties of the CCD sensor are discussed.

Gradient-index optical systems in holographic endoscopy

Small limiting apertures of endoscopes cause gross speckles when using coherent light sources in holographic endoscopy. Thus, for a high optical signal-to-noise ratio, imaging systems with high numerical apertures and low f/Nos. are required, which under given geometrical constraints can be provided in particular by gradient-index rod lenses. For additional speckle reduction the limiting aperture has to be enlarged within the given outer diameter of the instrument. This can be achieved by using the same gradient-index rod lens for both imaging and illumination, so that no separate illuminating fiber bundle is necessary. Holographic recording problems due to reflections of the illuminating object beam from the end faces can be solved by appropriate positioning and shaping of the rod as well as by holographic subtraction.

Phase shifting holographic double exposure interferometry with fast photorefractive crystals

Abstract A technique for quasi real-time holographic interferometry with reflection type objects is presented. It is based on fast sequences of holographic double exposure interferograms recorded in sillenite-type photorefractive BTO crystals. The application of the phase shifting method for automatic quantitative evaluation of the obtained interferograms under the special constraint of short storage times in fast photorefractive crystals is investigated. Approaches to overcome this difficulty by applying a time dependent external electric field are discussed. Experimental results for dynamic analysis of reflecting type objects are presented.

Quantitative determination of out-of-plane displacements by endoscopic electronic-speckle-pattern interferometry

The combination of endoscopes and electronic-speckle-pattern interferometry requires a more detailed consideration of their imaging properties with respect to interferometric fringe formation, if a quantitative analysis of the observed deformation is desired. Due to a relatively small distance between the illuminating and the imaging optical elements and to strongly divergent beams, the sensitivity vector may not be regarded as constant, and image distortions caused by the endoscope optics must be taken into account. A simplified model that deals with the situation of a plane object perpendicular to the optical axes of the endoscope is presented and verified by experimental results.

Modulation analysis of phase-shifted holographic interferograms

Abstract A new evaluation technique for phase-shifted holographic interferograms is developed and verified by experiments. In evaluating the interferograms, both the discrete phase distribution and the intensity modulation of interference fringes are calculated and a binary control mask is generated by analysing histograms and tracing local minima of the intensity modulation distribution. The control mask is used to identify valid and invalid areas of the discrete phase distribution in subsequent phase unwrapping and phase interpolation, and to control the path of phase unwrapping. The evaluation of experimentally obtained phase-shifted holographic interferograms shows that this method can be successfully applied for the processing of complex interference patterns, even if the interferograms have fringe discontinuities and shaded areas.

Utilization of photorefractive crystals for holographic double-exposure interferometry with nanosecond laser pulses

A holographic double-exposure interferometer for deformation and vibration analysis of diffusely reflecting objects in quasi real-time is presented. Writing of holographic gratings with 3 ns laser pulses in photorefractive sillenite-type crystals enables recording of sequences of double-exposure interferograms. A repetition rate of 12.5 Hz is achieved with the set-up used. Automatic phase evaluation of the interferograms is performed with a spatial-heterodyne technique. Photorefractive recording in a BTO:Cd crystal with pulsed illumination is investigated and discussed in view of the application in double-exposure interferometry. The performance of the interferometer is investigated by observing deformations of a vibrating test object.

Diffraction efficiency enhancement of holographic gratings in Bi 12 Ti 0.76 V 0.24 O 20 crystals after recording

An enhancement of the diffraction efficiency of holographic gratings in Bi12Ti0.76V0.24O20 crystals by a factor of almost 40 is obtained after recording by application of an external dc electric field and illumination with a spatially homogeneous light beam. The enhancement increases with increasing applied field, larger spatial frequency, and longer writing time of the holographic grating, and with intensity of the homogeneous illumination. A theoretical description of the enhancement effect is proposed, assuming a modulated photoconductivity, which is generated by a strongly modulated concentration of photorefractive traps or donors.