Digital Hologram Processing Research Articles

Increasing the Interferogram Sensitivity by Digital Holography

This paper presents the results of experimental studies and digital methods for recording of holograms using spatial light modulators. The use of holographic interference methods for measuring nanoscale irregularities is described for the case of a nanostep (70 nm in height and 30 $$\mu$$ m in width). Examples are given of the method of increased sensitivity for nanostep measurements and the method of hooks using a digital technique. Correction of internal system distortions in the interferometer has been shown to be a workable method. The holographic interference methods with digital processing of holograms allows the sensitivity of interferograms to be increased without loss of basic information about the object and without increasing the measurement accuracy.

Quasi-common path three-wavelength holographic interferometer based on Wollaston prisms

This article presents a digital three-wavelength holographic interferometer based on the use of two Wollaston prisms. This provides an in-line setup with quasi common-path and as a consequence there is no additional independent reference wave to be added. Thus, immunity to external perturbations such as vibrations or thermal perturbations is achieved. Furthermore, the set-up exhibits a single shot and real time capability which is very useful to study dynamic events. By using the two Wollaston prisms in an astigmatic configuration, spatial carrier frequencies can be adjusted both in amplitude and orientation. The digital hologram processing is based on Fourier processing and filtering around the carrier spatial frequency so that phase shifting is not required. The use of three wavelengths leads to visualizing directly the zero order fringe and regions for which there is no air density variation in a dynamic flow. Experimental proof of concept is demonstrated with a supersonic jet when the injection pressure varies.

Review on theory and applications of wavefront recording plane framework in generation and processing of digital holograms

The wavefront recording plane (WRP), subsequently generalized to be known as the virtual diffraction plane (VDP), is a recent concept that has been successfully deployed in fast generation and processing of digital holograms. In brief, the WRP and its extension, the VDP, is a hypothetical plane that is located between the hologram and the object scene, and which is at close proximity to the latter. As such, the fringe patterns on the hypothetical plane are carrying the holistic information of the hologram, as well as the local optical properties of the object scene. This important property enables a hologram to be processed with classical image processing techniques that are normally unsuitable for handling holographic information. In this paper we shall review a number of works, that have been developed based on the framework of the WRP and the VDP.

Incoherent digital holographic adaptive optics

An adaptive optical system based on incoherent digital holography is described. Theoretical and experimental studies show that wavefront sensing and compensation can be achieved by numerical processing of digital holograms of incoherent objects and a guide star, thereby dispensing with the hardware components of conventional adaptive optics systems, such as lenslet arrays and deformable mirrors. The incoherent digital holographic adaptive optics (IDHAO) process is seen to be robust and effective under various ranges of parameters, such as aberration type and strength. Furthermore, low and noisy image signals can be extracted by IDHAO to yield high-quality images with good contrast and resolution, both for point-like and continuous extended objects, illuminated with common incoherent light. Potential applications in astronomical and other imaging systems appear plausible.

Adaptive optics by incoherent digital holography

Adaptive optics in astronomical and other imaging systems allows compensation of aberrations introduced by random variations of the refractive index in the imaging path. I propose what I believe is a new type of adaptive optics system that dispenses with the hardware lenslet arrays and deformable mirrors of conventional systems. Theoretical and experimental studies show that wavefront sensing and compensation can be achieved by numerical processing of digital holograms of the incoherent object and a guide star. The incoherent digital holographic adaptive optics is seen to be particularly robust and efficient, with envisioned applications in astronomical imaging, as well as fluorescence microscopy and remote sensing.

Microparticle characterization using digital holography

Digital holography is an effective 3D imaging technique, with the potential to be used for particle size measurements. A digital hologram can provide reconstructions of volume samples focused at different depths, overcoming the focusing problems encountered by other imaging based techniques. Several particle analysis methods discussed in the literature consider spherical particles only. With the object sphericity assumption in place, analysis of the holographic data can be significantly simplified. However, there are applications, such as particle analysis and crystallization monitoring, where non-spherical particles are often encountered. This paper discusses the processing of digital holograms for particle size and shape measurement for both spherical and arbitrarily shaped particles. An automated algorithm for identification of particles from recorded hologram and subsequent size and shape measurement is described. Experimental results using holograms of spherical and non-spherical particles demonstrate the performance of the proposed measuring algorithm.

High-speed generation of digital holographic interferograms and shearograms for non-destructive testing

Digital holographic interferometry is the state-of-the-art technique in holography having applications in the fields of non-destructive testing, flow analysis and vibration measurement covering a wide range from huge aerospace structures to micro objects such as MEMS [1,2,3] In holographic non-destructive testing (HNDT), transient thermal loading is an extensively used method of stressing the objects for defect detection [4] . Test objects are stressed thermally using infrared lamps, flat heater or hot-water tanks depending on the object configuration and geometry, defect type and nature and the safety aspects. Holograms at different stress states of the object are recorded continuously, either during heating or during cooling after the removal of the heating element, to obtain interferograms. However, in order to capture and analyse the fast transient response of the structures upon thermal stressing, a system that records holograms and generates holographic interferograms in high speed is essential. Added to this, if the system can simultaneously generate shearograms, which provide a displacement derivative fringe pattern, this would increase its defect detection capability during NDT. In this paper the development of a digital hologram processing system which can record a digital hologram in high speed and generate holographic interferograms and shearograms simultaneously is reported. The design of the high-speed image processing system and selection of digital camera is also explained. The paper also gives details of the software developed and its features. The need for the high-speed system to record and generate interferograms during NDT of highly porous materials is also demonstrated in this paper.

Fresnelets: new multiresolution wavelet bases for digital holography

We propose a construction of new wavelet-like bases that are well suited for the reconstruction and processing of optically generated Fresnel holograms recorded on CCD-arrays. The starting point is a wavelet basis of L2 to which we apply a unitary Fresnel transform. The transformed basis functions are shift-invariant on a level-by-level basis but their multiresolution properties are governed by the special form that the dilation operator takes in the Fresnel domain. We derive a Heisenberg-like uncertainty relation that relates the localization of Fresnelets with that of their associated wavelet basis. According to this criterion, the optimal functions for digital hologram processing turn out to be Gabor functions, bringing together two separate aspects of the holography inventor's work. We give the explicit expression of orthogonal and semi-orthogonal Fresnelet bases corresponding to polynomial spline wavelets. This special choice of Fresnelets is motivated by their near-optimal localization properties and their approximation characteristics. We then present an efficient multiresolution Fresnel transform algorithm, the Fresnelet transform. This algorithm allows for the reconstruction (backpropagation) of complex scalar waves at several user-defined, wavelength-independent resolutions. Furthermore, when reconstructing numerical holograms, the subband decomposition of the Fresnelet transform naturally separates the image to reconstruct from the unwanted zero-order and twin image terms. This greatly facilitates their suppression. We show results of experiments carried out on both synthetic (simulated) data sets as well as on digitally acquired holograms.

Electron Holography and Digital Imaging for Analysis of Nanostructured Materials

Abstract Many nanostructured materials are formed from powder precursors having ultra-fine particle sizes. Techniques of electron microscopy have proven invaluable for characterizing the structure of the precursor materials in order to better understand the fundamental processes that govern consolidation of the materials into the final nanophase structures. In recent years, the rapidly developing technique of electron holography has increasingly been applied for characterizing particle morphologies. The advent of the modern field emission microscope, which offers beam coherency sufficient to produce high contrast interference fringes for optimum hologram formation, and especially the availability of digital camera systems for hologram acquisition and rapid processing have both combined to bring electron holography to the forefront of techniques for characterization of nanostructured materials. Electron holograms typically yield phase images that can give quantitative information on crystal morphologies, but much additional information can result from digital processing of holograms.