Numerical Reconstruction Of Holograms Research Articles

Digital Holographic Interferometry for Wholefield NDT Applications

Holographic interferometry is a non-contact whole field optical technique used as a powerful NDT tool. This technique is highly sensitive and maps the object surface deformation as holographic fringe pattern. Conventional holography technique has practical difficulties in recording and reconstructing holograms. With the development in imaging sensor technology and fast algorithms in image processing, digital holography has been established. Digital holography uses high-resolution digital camera for recording holograms and does reconstruction of holograms numerically using a computer. This has enhanced the scope and application of holography for NDT many fold. In this paper application of digital holography to non-destructive testing is presented. The basic principle of digital holography and digital holographic interferometry in obtaining double exposure hologram for NDT is described. The software developed in house for numerical reconstruction of holograms is also explained. The advantages of digital holography over conventional holography in NDT applications are highlighted. Extensive experiments were carried out to prove the detectability of defects in both metallic and non-metallic materials and the results are presented. It is shown that digital holography is able to identify defects in a wide range of materials with great ease because of its high sensitivity and whole field nature.

Size determination of mixed liquid and frozen water droplets using interferometric out-of-focus imaging

We record simultaneously interferometric out-of-focus images and digital in-line holograms of liquid and frozen water droplets. We show that the analysis of speckle-like out-of-focus images allows a quantitative estimation of the size of the particles which is corroborated by numerical reconstruction of holograms recorded simultaneously. Interferometric out-of-focus imaging could be extended to the characterization of ice in clouds in the atmosphere.

Three-dimensional instantaneous velocity field measurement using digital holography microscope

In the present study, a digital holography microscope has been developed to study instantaneous 3D velocity field in a square channel of 1000 × 1000 μm2 cross-section. The flow field is seeded with polystyrene microspheres of size dp = 2.1 μm. The volumetric flow rate is set equal to 20 μl/min. The instantaneous 3D velocity field is obtained by correlating the particles obtained from the 3D numerical reconstruction of holograms using particle tracking velocimetry (PTV).

Real-Time Digital Holographic Microscopy for Phase Contrast 3D Imaging of Dynamic Phenomena

Digital holography is a very effective tool for three- dimensional imaging of objects, since it yields the phase information directly without the need for additional optics. Another advantage is that of numerical focusing, allowing one to focus on to any desired plane. It has been used for microscopy with high effectiveness. One of the important features of digital holographic microscopy (DHM) is that it can be used for phase contrast imaging of otherwise transparent objects. This aspect of DHM is utilized here to study the dynamics of phase objects. Angular spectrum propagation (ASP) approach to scalar diffraction theory is used for numerical reconstruction of holograms.

Mapping the transient concentration field within the diffusion layer by use of the digital holographic reconstruction

The digital holographic reconstruction was employed in studies of dynamical changes of the concentration and diffusion layer at the Cu/CuSO 4 solution interface during the galvanostatic electrodeposition of copper in 0.24 mol dm −3 CuSO 4 solution. During electrochemical reaction processes, changes of solution’s refractive index, which were caused by changes of the concentration, were recorded in situ and were shown in holograms. Changes in the phase of the object beam were caused by changes in the refractive index when they passed through the solution. The distribution of the phase difference (the phase change) was obtained by numerical reconstruction of holograms to reflect concentration changes. The process of numerical reconstruction of holograms is described in detail, and the two-dimensional concentration distribution is presented in illustration of the diffusion layer and concentration changes.

Whole-field NDT of thermal protection system using digital holography

In the development of re-entry space vehicles, thermal protection systems (TPS), which protect the vehicle from the high thermal environments during re-entry, constitute one of the most critical systems. Both metallic and non-metallic materials are candidates for TPS. Generally, the TPS materials are bonded to the structure in vacuum under load. For the successful performance of the vehicle, the TPS should be intact to the vehicle without any debond and hence the detection of debonds in TPS is very important. Holographic interferometry is established as a reliable NDT tool for detecting debonds. Digital holography is a technique that uses digital recording and numerical reconstruction of holograms, which does not require photographic films, wet processing and laser reconstruction. In this paper the application of the digital holographic interferometry technique in detecting debonds in different types of thermal protection systems is presented. The results of the tests on a few samples made of ablative and non-ablative TPS materials with debond are presented. The paper also describes the experimental details and stressing techniques used for the detection of debonds in TPS. The practical application of this technique for the NDT of TPS is explored.

Talbot self-image effect in digital holography and its application to spectrometry.

For the first time to the authors' knowledge, the Talbot effect has been observed and investigated in digital holography. By numerical reconstruction of holograms, the Talbot self-imaging phenomenon is observed by reconstruction of the amplitude of the image at different distances and (or) wavelengths. A simple spectrometer based on Talbot self-imaging in digital holography is proposed and demonstrated.

Digital recording and numerical reconstruction of holograms: an optical diagnostic for combustion.

Holographic interferometry (HI) has proved to be a useful tool for nonintrusive temperature measurements in flames (and thereafter for inference of the local composition based on the state relationship approach) with high spatial and temporal resolution. Digital holographic interferometry (DHI) is a relatively new imaging and measurement technique that electronically records a hologram (e.g., on a CCD) and reconstructs it by a numerical method. Cumbersome chemical processing of the hologram is avoided in DHI, which thereby provides greater flexibility, speed, and the potential for real-time processing. In conventional holography, fringes that are neither bright nor dark on a hologram cannot be accurately resolved. The DHI technique has not yet to our knowledge been used for combustion applications. Herein we evaluate its efficacy for making temperature measurements in flames and assess its applicability through a simulation. Each part of a double exposure associated with the holographic technique is considered to be recorded by a hypothetical CCD sensor at a separate time from the other part. We applied the principles of Fourier optics to develop two numerical methods for hologram reconstruction, and we show that both methods provide an accurate reconstruction of the phase image associated with a flame. Because of the periodic nature of the wave function, the reconstructed phase values are limited to the interval [-pi/2, pi/2]. Thus an unwrapping algorithm is provided that produces a continuous phase distribution based on the condition that the reconstructed phase is jumped by a value of -pi or pi. We have also developed an iterative calculation method to adjust the value of the digital reference wave parameters that determines the phase imaging reconstruction in DHI.

Optimization of the low energy electron point source microscope: imaging of macromolecules

Optimal conditions for low energy electron point source microscopy are investigated by the simulation and numerical reconstruction of holograms of phthalocyaninato polysiloxane, PcPS, a rod-like macromolecule. The effects of the electron energy, width of the electron beam and the detector size on the spatial resolution in the reconstructed images are modeled. We find that for electron energies around 200 eV, with the specimen 0.1 μm from the source, a screen recording the image in a cone of at least 15° half angle (7 cm lateral dimension at 10 cm from the source) with at least 512×512 pixel and 8-bit resolution will result in near atomic resolution.

Digital recording and numerical reconstruction of holograms: reduction of the spatial frequency spectrum

Direct recording of Fresnel holograms on a charge-coupled device and their numerical reconstruction is possible if the maximum spatial frequency of the holographic microstructure is adapted to the spatial resolution of the detector array. The maximum spatial frequency is determined by the angle between the interfering waves. For standard CCDs with spatial resolutions of ;100 lines/mm, the angle between ref- erence and object wave is limited to a few degrees. This limits the size of the objects to be recorded or requires a great distance between object and CCD target. A method is described in which the primary object angle is optically reduced so that objects with larger dimensions can be re- corded. The principle is demonstrated for the example of deformation analysis. Two Fresnel holograms, which represent the undeformed and the deformed states of the object, are generated on a CCD target, stored electronically, and the wave fields are reconstructed numerically. The interference phase can be calculated directly from the digital holograms, without generating an interference pattern. As an application of this method, we present the transient deformation field of a plate that is loaded by an impact. © 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: digital holography; hologram interferometry; numerical hologram reconstruction; charge-coupled devices.

Digital recording and numerical reconstruction of holograms: a new method for displaying light in flight

We present a new method for displaying light in flight. Fresnel holograms are recorded directly on a CCD sensor, electronically stored, and numerically reconstructed. Experimental results are shown. From different parts of a single holographic recording, different views of a wave front can be reconstructed. This means that the temporal evolution of a wave front can be observed by numerical methods.

Apodization in numerical holography

Apodization, a process in which the energy in a diffraction pattern is redistributed among the main lobes and the side lobes, has proved its usefulness in optical imaging systems and in linear sonar arrays. It can also make the numerical reconstruction of holograms more adaptable to varying requirements. To demonstrate, the hologram of a rectangular slot is first reconstructed numerically in the conventional manner. It is then reconstructed after apodizing the diffraction pattern contained in the hologram. One type of apodization led to a reconstructed image that accentuated the discontinuity at the edge of the slot. Another type gave an image with a concentration of energy at the center of the object, thus minimizing interference from nearby objects. It is expected that the method can be extended to other objects by expressing them as summations of rectangular slots.