Holography Holography is the synthesis of interference and diffraction. In recording a hologram, two waves interfere to form an interference pattern on the recording medium. When reconstructing the hologram, the reconstructing wave is diffracted by the hologram. When looking at the reconstruction of a 3-D object, it is like looking at the real object. It is therefore said that: ‘A photograph tells more than a thousand words and a hologram tells more than a thousand photographs’. Although holography requires coherent light, it was invented by Gabor back in 1948, more than a decade before the invention of the laser | Optical Metrology. Kjell J. Gasvik Copyright 2002 John Wiley Sons Ltd. ISBN 0-470-84300-4 6 Holography INTRODUCTION Holography is the synthesis of interference and diffraction. In recording a hologram two waves interfere to form an interference pattern on the recording medium. When reconstructing the hologram the reconstructing wave is diffracted by the hologram. When looking at the reconstruction of a 3-D object it is like looking at the real object. It is therefore said that A photograph tells more than a thousand words and a hologram tells more than a thousand photographs . Although holography requires coherent light it was invented by Gabor back in 1948 more than a decade before the invention of the laser. By means of holography an original wave field can be reconstructed at a later time at a different location. This technique therefore has many potential applications. In this book we concentrate on the technique of holographic interferometry. Because of the above-mentioned properties we shall see that holographic interferometry has many advantages compared to standard interferometry. THE HOLOGRAPHIC PROCESS Figure a shows a typical holography set-up. Here the light beam from a laser is split in two by means of a beamsplitter. One of the partial waves is directed onto the object by a mirror and spread to illuminate the whole object by means of a microscope objective. The object scatters the light in all directions and some of it impinges onto the hologram plate. This wave is called the object wave. The other partial wave is reflected directly onto the hologram plate. This wave is called the reference wave. In the figure this wave is collimated by means of a microscope objective and a lens. This is not essential but it is important that the reference wave constitutes a uniform illumination of the hologram plate. The hologram plate must be a light-sensitive medium . a silver halide film plate with high resolution. We now consider the mathematical .
