Preface

The book "Advanced Holography – Metrology and Imaging" comprises four sections.

The first section has eight chapters on *Digital Holographic Interferometry*, a powerful tool in non-destructive testing and metrology. Among its applications are the study of fluid flow and mechanical strain (Chapters 1 and 2), surface contouring (Chapter 3) and characterization of optical waveguides (Chapter 4). The next three chapters cover spatial phase shifting for single shot quantitative holographic interferometry (Chapter 5), adaptive interferometry (Chapter 6) and incoherent interferometry (Chapter 7) which are likely to further extend the range of applications of digital holographic interferometry. Many of the problems associated with the use of photosensing arrays for hologram recording have been largely overcome and developments in bolometer arrays have enabled digital holography and holographic interferometry to enter the thermal infra-red region of the spectrum. This topic is discussed in Chapter 8.

The second section comprises Chapter 9 on *Digital Holographic Microscopy*.

In Section 3 devoted to *Imaging*, five chapters discuss synthetic holographic imaging (Chapter 10), the use of liquid crystal on silicon reflective spatial light modulators in digital holography (Chapter 11), holographic projection screens for image display (Chapter 12), computer-generated phase-only holograms for real-time image display (Chapter 13) and extreme ultraviolet holographic imaging (Chapter 14).

The final section is on *Seeing* and Chapter 15 is written from the perspective of the artist. It provides interesting insights into the nature of holographic images compared with other representations, and discusses some striking examples of holographic representation which exploit its unique characteristics. The second chapter (Chapter 16) in this section is on seeing and understanding through experimentation in optics in virtual and physical laboratory environments.

Many of the chapters describe the historical developments leading to the specific topic under discussion and will provide the reader with interesting and useful background information.

The following paragraphs give a brief summary of contents.

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*Real-Time Colour Holographic Interferometry (from Holographic Plate to Digital Hologram)* is applied to the study of unsteady flow downstream from a cylinder at Mach number less than one. Panchromatic silver halide plates are used as well as digital methods.

Preface XI

interferometry are explained. A number of unbalanced interferometers are described

*Infrared Holography for Wavefront Reconstruction and Interferometric Metrology* deals firstly with the basic principles of digital holography including methods of coping with reduced spatial resolution due to the pyroelectric sensor array used for IR detection and to the longer wavelength used in the IR. Numerical reconstruction of IR digital holograms is used to characterize vorticity of IR beams such as Laguerre-

*Alternative Reconstruction Method and Object Analysis in Digital Holographic Microscopy* describes a method of numerical reconstruction in digital holographic microscopy, which is similar to the double Fresnel-transform transform method in that it involves two steps with the intermediate plane coinciding with the Fourier transform plane for

*Synthetic Image Holograms* begins by analysing human vision and describes hologram synthesis at the hologram plane and at the eye-pupil plane. Color mixing, 3D properties, and kinetic behavior of the holograms are also discussed. The most

*Study of Liquid Crystal on Silicon Displays for their Application in Digital Holography* presents guidelines for optimizing the performance of these devices for the generation of digital holograms using Mueller-Stokes (M-S) formalism which allows effective depolarization in LCoS displays to be taken into account. Experimental evidence of temporal fluctuations in phase and their adverse effects on digital holograms are reviewed. A remedy based on the minimum Euclidean distance principle is tested by measuring the efficiency of optimized digital holograms written in a LCoS display.

*Holoimages on Diffraction Screens* begins by asking questions about the fundamental character of holograms and discusses how holographically recorded diffracting screens can produce stereoscopic images without requiring the audience to use special

*Computer-Generated Phase-Only Holograms for Real-Time Image Display* introduces a number of technical innovations that have enabled the realization of a real-time, phase-only holographic projection technology. By defining a new psychometrically determined optimization metric that is far more suited to human perception than the conventional mean-squared error (MSE) measure, a method for the generation of phase-only holograms which results in perceptually pleasing video-style images is

*Two and Three Dimensional Extreme Ultraviolet Holographic Imaging with a Nanometer Spatial Resolution* describes extreme ultraviolet (EUV) table-top holographic imaging using a compact EUV laser as the illumination source. This imaging method allows hologram recording without any previous object preparation, as required in electron

in which spatial coherence is not required.

the object. The advantages of this approach are explained.

common devices and recording materials are briefly described.

Gaussian beams.

filters.

demonstrated.

*Three-dimensional Displacement and Strain Measurements by Windowed Phase-Shifting Digital Holographic Interferometry* exploits the idea that a hologram can be regarded as made up of a number of spatially contiguous windows in a single plane. Phase differences obtained from different windows, using the same sensitivity vector, are the same, but speckle causes values to be calculated from different values of light intensity. More reliable results are obtained by weighting each phase difference value according to the intensities of light used in its calculation.

*Multiple-Wavelength Holographic Interferometry with Tunable Laser Diodes* uses very small wavelength changes between holographic recordings to obtain large synthetic wavelengths for surface contour measurement. Phase jumps are avoided by simply using larger synthetic wavelengths. The precision with which laser wavelengths must be known when phase unwrapping is required, is obtained and an algorithm for adjustment of pixel size in Fresnel reconstruction at different wavelengths is presented.

*Digital Holographic Interferometric Characterization of Optical Waveguides* utilizes phase shifting for the reconstruction of the optical phase differences along graded index waveguides. A simple algorithm avoids the problem of tilted GRIN optical waveguides inside the optical field. Refractive index profiles of waveguides are obtained, as well as effective indices and the mode field distribution across symmetric and asymmetric waveguides.

*Single-Shot Phase-Shifting Digital Holography Based on the Spatial Carrier Interferometry and its Tolerance Analysis* describes how angularly multiplexed phase shifting in a single digital hologram enables precise quantitative information about a wavefront to be obtained. The authors discuss the tolerances on the angles of incidence of the reference beams of each wavelength employed and consider procedures to be adopted when the tolerances are not met.

*Multi-Channel Adaptive Interferometers Based on Dynamic Holograms Multiplexing*  discusses the basic principles of wave mixing in a photorefractive crystal and introduces a detection limit for adaptive interferometry, a technique offering considerable immunity to slow temporal variations in phase difference. The authors describe practical multichannel adaptive interferometers using dynamic hologram multiplexing in photorefractives.

*Incoherent Holographic Interferometry* introduces an incoherent interference mechanism which seems to contradict our existing knowledge of the requirements for interference. The reasons why temporal and spatial coherence are needed for holographic interferometry are explained. A number of unbalanced interferometers are described in which spatial coherence is not required.

X Preface

presented.

and asymmetric waveguides.

when the tolerances are not met.

multiplexing in photorefractives.

*Real-Time Colour Holographic Interferometry (from Holographic Plate to Digital Hologram)* is applied to the study of unsteady flow downstream from a cylinder at Mach number less than one. Panchromatic silver halide plates are used as well as digital methods.

*Three-dimensional Displacement and Strain Measurements by Windowed Phase-Shifting Digital Holographic Interferometry* exploits the idea that a hologram can be regarded as made up of a number of spatially contiguous windows in a single plane. Phase differences obtained from different windows, using the same sensitivity vector, are the same, but speckle causes values to be calculated from different values of light intensity. More reliable results are obtained by weighting each phase difference value

*Multiple-Wavelength Holographic Interferometry with Tunable Laser Diodes* uses very small wavelength changes between holographic recordings to obtain large synthetic wavelengths for surface contour measurement. Phase jumps are avoided by simply using larger synthetic wavelengths. The precision with which laser wavelengths must be known when phase unwrapping is required, is obtained and an algorithm for adjustment of pixel size in Fresnel reconstruction at different wavelengths is

*Digital Holographic Interferometric Characterization of Optical Waveguides* utilizes phase shifting for the reconstruction of the optical phase differences along graded index waveguides. A simple algorithm avoids the problem of tilted GRIN optical waveguides inside the optical field. Refractive index profiles of waveguides are obtained, as well as effective indices and the mode field distribution across symmetric

*Single-Shot Phase-Shifting Digital Holography Based on the Spatial Carrier Interferometry and its Tolerance Analysis* describes how angularly multiplexed phase shifting in a single digital hologram enables precise quantitative information about a wavefront to be obtained. The authors discuss the tolerances on the angles of incidence of the reference beams of each wavelength employed and consider procedures to be adopted

*Multi-Channel Adaptive Interferometers Based on Dynamic Holograms Multiplexing* discusses the basic principles of wave mixing in a photorefractive crystal and introduces a detection limit for adaptive interferometry, a technique offering considerable immunity to slow temporal variations in phase difference. The authors describe practical multichannel adaptive interferometers using dynamic hologram

*Incoherent Holographic Interferometry* introduces an incoherent interference mechanism which seems to contradict our existing knowledge of the requirements for interference. The reasons why temporal and spatial coherence are needed for holographic

according to the intensities of light used in its calculation.

*Infrared Holography for Wavefront Reconstruction and Interferometric Metrology* deals firstly with the basic principles of digital holography including methods of coping with reduced spatial resolution due to the pyroelectric sensor array used for IR detection and to the longer wavelength used in the IR. Numerical reconstruction of IR digital holograms is used to characterize vorticity of IR beams such as Laguerre-Gaussian beams.

*Alternative Reconstruction Method and Object Analysis in Digital Holographic Microscopy*  describes a method of numerical reconstruction in digital holographic microscopy, which is similar to the double Fresnel-transform transform method in that it involves two steps with the intermediate plane coinciding with the Fourier transform plane for the object. The advantages of this approach are explained.

*Synthetic Image Holograms* begins by analysing human vision and describes hologram synthesis at the hologram plane and at the eye-pupil plane. Color mixing, 3D properties, and kinetic behavior of the holograms are also discussed. The most common devices and recording materials are briefly described.

*Study of Liquid Crystal on Silicon Displays for their Application in Digital Holography*  presents guidelines for optimizing the performance of these devices for the generation of digital holograms using Mueller-Stokes (M-S) formalism which allows effective depolarization in LCoS displays to be taken into account. Experimental evidence of temporal fluctuations in phase and their adverse effects on digital holograms are reviewed. A remedy based on the minimum Euclidean distance principle is tested by measuring the efficiency of optimized digital holograms written in a LCoS display.

*Holoimages on Diffraction Screens* begins by asking questions about the fundamental character of holograms and discusses how holographically recorded diffracting screens can produce stereoscopic images without requiring the audience to use special filters.

*Computer-Generated Phase-Only Holograms for Real-Time Image Display* introduces a number of technical innovations that have enabled the realization of a real-time, phase-only holographic projection technology. By defining a new psychometrically determined optimization metric that is far more suited to human perception than the conventional mean-squared error (MSE) measure, a method for the generation of phase-only holograms which results in perceptually pleasing video-style images is demonstrated.

*Two and Three Dimensional Extreme Ultraviolet Holographic Imaging with a Nanometer Spatial Resolution* describes extreme ultraviolet (EUV) table-top holographic imaging using a compact EUV laser as the illumination source. This imaging method allows hologram recording without any previous object preparation, as required in electron

### XIV Preface

microscopy, and free of any interaction with a probe as in scanning microscopes. A detailed discussion of the processing of the reconstructed holographic images, performed by changing object-hologram distance in the reconstruction code is presented.

*The Visual Language of Holograms* considers the properties of the representational systems of different hologram types and how pictorial qualities are expressed within those systems. A number of intriguing examples of holographic representation are discussed. Comparison is also made with more traditional expressions of pictorial qualities. The author also considers how aspects of the particular experience of the holographic image may influence reception and interpretation of the visual language that is being used.

*A Contribution to Virtual Experimentation in Optics* provides a set of Matlab based software tools allowing virtual and physical laboratory exploration of different optical phenomena, including computer generated holography.

The editor of this book would like to express her gratitude to Prof. Vincent Toal for his useful advice in the process of preparation of the book "Advanced Holography – Metrology and Imaging".

> **Dr. Izabela Naydenova** Dublin Institute of Technology Ireland
