Preface

A good preface of a book about holography is to put the systems and methods described herein in their historical context. The early holograms and some of the holograms described in this book are recorded as the result of interference between two light waves. The phenomenon of optical two-wave interference has been well known since the first decade of the 19th century when Thomas Young published his famous double-slit experiment. In Young's experiment, the interference happens between two waves where none of them carries any image. Therefore, Young's experiment produces an interference pattern between two light waves but not a hologram. The revolutionary transition from a simple interference pattern to a hologram happened in 1948 in Dennis Gabor's pioneering work presenting for the first time what is known today as the Gabor hologram. The methods described in Chapters 1–4 in this book are conceptually close in the sense that the holograms are recorded by two-wave interference between a wave carrying the object information and another wave called the reference wave, which does not contain any object information.

The holograms described in Chapters 5 and 6 are additional technological steps in the evolution of holography beyond the holograms recorded with a non-image reference beam. These holograms of incoherently illuminated objects are recorded using the principle of self-interference. Self-interference holograms are a new stage in the evolutionary chain in which both interfering waves carry the object's image. However, the image information is never the same in both interferometer channels. In holograms described in Chapters 5 and 6, the images are in-focus at different distances from the aperture. The hologram described in Chapter 7, COACH, is an additional evolutionary stage in which one of the two images passes through a coded scattering mask. The other image is in focus at an infinite distance from the aperture.

From the stage of COACH, the technology could be evolved to a system in which both interfering waves pass through scattering masks and/or to a system without two-wave interference at all. At the time of this writing, the second option has been chosen, mainly because the interference-less COACH is simpler and more efficient than the COACH with two-wave interference. The acoustic holograms of Chapter 8 also belong to the type of holograms recorded without two-wave interference because the entire phase and amplitude information of the scene can be detected directly from a single acoustic wave without the need to record interference with any reference wave. What are the future systems in this evolutionary chain? The answer is that only those systems providing any advantage over other existing systems at a reasonable cost will probably justify their existence.

The holography of today is in transition from using traditional photo materials to the use of digital cameras and computers in what are called digital holograms. However, not all applications of holography can be implemented by digital holograms. Chapters 9 and 10 describe methods of recording holograms on non-traditional photo materials.

The unique applications of holographic display in Chapter 9 and of the holographic lens in Chapter 10 dictate the photo material in these holograms. Regarding other holography applications, various chapters in the second section of the book discuss diverse applications, such as holographic microscopy in Chapters 11–13, heat transfer measurements in Chapter 14, quantum computing in Chapter 15, augmented reality in Chapter 16, beam-shaping in Chapter 17, and holographic encryption in Chapter 18.

Holography is a broad field developed in the meeting between optics, and material sciences, and contains more or different information on the observed scene than a regular image of the same scene. To extract the required information from raw holograms, a digital algorithm is applied according to the specific application of the system. The development of the field has been accelerated lately due to the improvement of digital cameras, computers, and spatial light modulators.

As a multidisciplinary area, holography connects experts in electro-optical engineering, image processing, computer algorithms, and sometimes in material engineering.

More experts are needed when holography is utilized in various applications such as microscopy, industrial inspection, biomedicine, entertainment, and others. This book provides an overview of the world of holography from the aspect of concepts, system architectures, and applications. The various chapters deal with ongoing research of many current developments in the field of holography. Thus, the present book is only an interim summary of a research field that aims to encourage better technology for current and new applications. Many thanks and deep appreciation to all the authors for contributing excellent chapters to this book. I sincerely thank Author Service Managers Zrinka Tomicic and Romina Rovan and the staff of IntechOpen for their valuable support. We hope that the scientific community dealing with these and related topics will find this book interesting, helpful, and inspiring.

Finally, I would like to mention here my two dear colleagues and teachers, Gabriel Popescu (1971–2022) and John T. Sheridan (1964–2022), both of whom prematurely passed away while preparing this book. These two scientists contributed many original ideas to the science of holography and have greatly influenced numerous researchers in the field. May they rest in peace.

> **Joseph Rosen** School of Electrical and Computer Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel

Section 1

Theoretical Concepts

and Methods

Section 1
