The Hologram E-Book

Martin J. Richardson

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Wiley - IEEE
• Sprache: Englisch

Seitenzahl: 622

Veröffentlichungsjahr: 2017

Beschreibung

The practical and comprehensive guide to the creation and application of holograms

Written by Martin Richardson (an acclaimed leader and pioneer in the field) and John Wiltshire, The Hologram: Principles and Techniques is an important book that explores the various types of hologram in their multiple forms and explains how to create and apply the technology. The authors offer an insightful overview of the currently available recording materials, chemical formulas, and laser technology that includes the history of phase imaging and laser science. Accessible and comprehensive, the text contains a step-by-step guide to the production of holograms. In addition, The Hologram outlines the most common problems encountered in producing satisfactory images in the laboratory, as well as dealing with the wide range of optical and chemical techniques used in commercial holography.

The Hologram is a well-designed instructive tool, involving three distinct disciplines: physics, chemistry, and graphic arts. This vital resource offers a guide to the development and understanding of the recording of materials, optics and processing chemistry in holography and:

•    Discusses the pros and cons of the currently available recording materials

•    Provides tutorials on the types of lasers required and optical systems, as well as diffraction theory and wave front reconstruction

•    Details the chemical formulations for processing techniques

Researchers and technicians working in academia and those employed in commercial laboratories on the production of holograms as well as students of the sciences will find The Hologram to be a comprehensive and effective resource.

Leseprobe

Table of Contents

Cover

Title Page

Foreword

Preface

Notes

Dedications and Acknowledgements

About the Companion Website

1 What is a Hologram?

1.1 Introduction

1.2 Gabor's Invention of Holography

1.3 The Work of Lippmann

1.4 Amplitude and Phase Holograms

1.5 Transmission Holograms

1.6 Reflection Holograms

1.7 Edge‐lit Holograms

1.8 “Fresnel” and “Fraunhofer” Holograms

1.9 Display Holograms

1.10 Security Holograms

1.11 What is Not a Hologram?

Notes

2 Important Optical Principles and their Occurrence in Nature

2.1 Background

2.2 The Wave/Particle Duality of Light

2.3 Wavelength

2.4 Representation of the Behaviour of Light

2.5 The Laws of Reflection

2.6 Refraction

2.7 Refractive Index

2.8 Huygens’ Principle

2.9 The Huygens–Fresnel Principle

2.10 Snell’s Law

2.11 Brewster’s Law

2.12 The Critical Angle

2.13 TIR in Optical Fibres

2.14 Dispersion

2.15 Diffraction and Interference

2.16 Diffraction Gratings

2.17 The Grating Equation

2.18 Bragg’s Law

2.19 The Bragg Equation for the Recording of a Volume Hologram

2.20 The Bragg Condition in Lippmann Holograms

2.21 The Practical Preparation of Holograms

Notes

3 Conventional Holography and Lasers

3.1 Historical Aspect

3.2 Choosing a Laser for Holography

3.3 Testing a Candidate Laser

3.4 The Race for the Laser

3.5 Light Amplification by Stimulated Emission of Radiation (LASER)

3.6 The Ruby Laser

3.7 Laser Beam Quality

3.8 Photopic and Scotopic Response of the Human Eye

3.9 Eye Safety I

3.10 The Helium–Neon Laser

3.11 The Inert Gas Ion Lasers

3.12 Helium–Cadmium Lasers

3.13 Diode‐pumped Solid‐state Lasers

3.14 Fibre Lasers – A Personal Lament!

3.15 Eye Safety II

3.16 The Efficiency Revolution in Laser Technology

3.17 Laser Coherence

Notes

4 Digital Image Holograms

4.1 Why is There Such Desire to Introduce Digital Imaging into Holography?

4.2 The Kinegram

4.3 E‐beam Lithographic Gratings

4.4 Grading Security Features

4.5 The Common “Dot‐matrix” Technique

4.6 Case History: Pepsi Cola

4.7 Other Direct Methods of Producing Digital Holograms

4.8 Simian – The Ken Haines Approach to Digital Holograms

4.9 Zebra Reflection Holograms

Notes

5 Recording Materials for Holography

5.1 Silver Halide Recording Materials

5.2 Preparation of Silver Bromide Crystals

5.3 The Miraculous Photographic Application of Gelatin

5.4 Why Has it Taken so Long to Arrive at Today’s Excellent Standard of Recording Materials for Holography?

5.5 Controlled Growth Emulsions

5.6 Unique Requirements of Holographic Emulsions

5.7 Which Parameters Control Emulsion Speed?

5.8 Sensitisation

5.9 Developer Restrictions

5.10 The Coated Layer

5.11 The Non‐typical Use of Silver Halides for Holography

5.12 Photopolymer

5.13 Photoresist

5.14 Dichromated Gelatin

5.15 Photo‐thermoplastics

6 Processing Techniques

6.1 Processing Chemistry for Silver Halide Materials

6.2 Pre‐treatment of Emulsion

6.3 “Pseudo‐colour” Holography

6.4 How Does Triethanolamine Treatment Work?

6.5 Wetting Emulsion Prior to Development

6.6 Development

6.7 Filamental and Globular Silver Grains

6.8 The H&D Curve

6.9 Chemical Development Mechanism

6.10 Pyro Developer Formulation

6.11 Ascorbic Acid Developers

6.12 “Stop” Bath

6.13 Fixing

6.14 Bleaching Solutions

6.15 Re‐halogenating Bleaches

6.16 Post‐process Conditioning Baths

6.17 Silver Halide Sensitised Gelatin (SHSG)

6.18 Surface‐relief Effects by Etching Bleaches

6.19 Photoresist Development Technique

Notes

7 Infrastructure of a Holography Studio and its Principal Components

7.1 Setting Up a Studio

7.2 Ground Vibration

7.3 Air Movement

7.4 Local Temperature Change

7.5 Safe Lighting

7.6 Organising Your Chemistry Laboratory

7.7 The Optical Table: Setting Up the Vital Components

7.8 Spatial Filtration

7.9 Filtering a “White” Laser Beam

7.10 Collimators

7.11 Organising Suitable Plate Holders for Holography

7.12 Hot Glue – The Holographer’s Disreputable Friend

7.13 Mirror Surfaces

7.14 Beam Splitters

7.15 Shutters

7.16 Fringe Lockers

7.17 Optics Stands

7.18 Safety – Reprise

Notes

8 Making Conventional Denisyuk, Transmission and Reflection Holograms in the Studio

8.1 Introduction

8.2 The Denisyuk Configuration

8.3 The Realism of Denisyuk Holograms

8.4 The Limitations of Denisyuk Holograms

8.5 The Denisyuk Set‐up

8.6 “Recording Efficiency”

8.7 Diffraction Efficiency

8.8 Spectrum of the Viewing Illumination

8.9 Other Factors Influencing Apparent Hologram Brightness

8.10 Problems Faced in the Production of High‐quality Holograms

8.11 Selecting a Reference Angle

8.12 Index‐matching Safety

8.13 Vacuum Chuck Method to Hold Film During Exposure

8.14 Setting the Plane of Polarisation

8.15 Full‐colour “Denisyuk” Holograms

8.16 Perfect Alignment of Multiple Laser Beams

8.17 “Burn Out”

8.18 Hybrid (Boosted) Denisyuks

8.19 Contact Copying

8.20 The Rainbow Hologram Invention

8.21 A Laser Transmission Master Hologram

8.22 Laser Coherence Length

8.23 The Second Generation H2 Transmission Rainbow (Benton) Hologram

8.24 Developments of the Rainbow Hologram Technique

8.25 Using the α‐Angle Theory to Produce Better Colour Rainbow Images

8.26 Aligning the Master Hologram with the α‐Angle

8.27 Producing an α‐Angle H2 Transfer

8.28 Utilising the Full Gamut of Rainbow Colours

8.29 Reflection Hologram Transfers

8.30 “Pseudo‐colour” Holograms

8.31 Real‐colour Holograms

Notes

9 Sources of Holographic Imagery

9.1 The Methods for Incorporation of 3D Artwork into Holograms

9.2 Making Holograms of Models and Real Objects

9.3 Models Designed for Multi‐colour Rainbow Holograms

9.4 Supporting the Model

9.5 Pulse Laser Origination

9.6 The “2D/3D” Technique

9.7 The Rationale Behind Holographic Stereograms

9.8 Various Configurations for Holographic Stereograms

9.9 The Embossed Holographic Stereogram

9.10 Stereographic Film Recording Configuration

9.11 Shear Camera Recording

9.12 The Number of Image Channels for a Holographic Stereogram

9.13 Process Colours and Holography – An Uncomfortable Partnership

9.14 Assimilating CMYK Artwork with Holography

9.15 Interpretation of CMYK Separations in the RGB Format

Notes

10 A Personal View of the History of Holography

Notes

Epilogue: An Overview of the Impact of Holography in the World of Imaging

Index

Supplementary Material

End User License Agreement

List of Tables

Chapter 02

Table 2.1 Refractive index of common materials.

Chapter 03

Table 3.1 Inert gas ion laser strong spectral lines.

Chapter 05

Table 5.1 Atomic weight vs. ionic radius chart.

Chapter 06

Table 6.1 Optical density (OD) key values.

List of Illustrations

Chapter 01

Figure 1.1 (a) The amplitude fringe recording; (b) the image it produces in laser light.

Figure 1.2 Making a transmission hologram with a single laser beam.

Figure 1.3 Young’s slits visualisation [1].

Figure 1.4 Twin‐slit diffraction of green and red lasers.

Figure 1.5 The dispersion of white light by a transmission hologram.

Figure 1.6 The planar index modulations (fringes) reflect light of a single wavelength only.

Figure 1.7 Light introduced into the edge of the substrate layer is diffracted so as to emerge from the surface of the assembly.

Figure 1.8 Sharp edges of the shadow of the model are contaminated by the effects of edge diffraction.

Figure 1.9 Circular wave front emanating from a point source.

Figure 1.10 Second generation transfer hologram.

Figure 1.11 (a) First generation (H1) recording of a real object; (b) second generation (reflection) “image‐planed” recording (H2); (c) the “contact copy” configuration for mass production.

Figure 1.12 Reconstruction of a dot‐matrix hologram.

Figure 1.13 Schematic of the original theatre manifestation of “Pepper’s Ghost”.

Figure 1.14 Stereo.

Figure 1.15 Barney – the one‐eared Border Collie.

Figure 1.16 Mode of operation of lenticular devices.

Figure 1.17 Animated lenticular portrait of David Bowie.

Figure 1.18 (a) Vinyl record grooves; (b) CD pit tracks.

Chapter 02

Figure 2.1 Blue Morpho butterfly.

Figure 2.2 Magnetic and electric fields in transverse oscillation.

Figure 2.3 Electromagnetic wave period.

Figure 2.4 Electromagnetic spectrum.

Figure 2.5 Reflection at a flat surface.

Figure 2.6 Reflection in a plane mirror.

Figure 2.7 Behaviour of the wave front at a refractive index boundary.

Figure 2.8 Practical example of the effect of travelling in different media.

Figure 2.9 Snell’s Law.

Figure 2.10 The Brewster condition.

Figure 2.11 Brewster angle incidence at a glass plate.

Figure 2.12 Underwater view from a swimming pool [4].

Figure 2.13 Ray diagrams explaining underwater view in a swimming pool.

Figure 2.14 Dispersion of white light by (a) a prism and (b) a raindrop.

Figure 2.15 Diminution of constructive interference with phase change.

Figure 2.16 Diffraction at a small aperture.

Figure 2.17 Calculating the path difference between adjacent grating centres.

Figure 2.18 Orders of diffraction for a transmission grating.

Figure 2.19 The Braggs’ x‐ray crystallography analysis.

Figure 2.20 Fringes bisect the angle between incident beams.

Figure 2.21 (a) Fringe spacing as calculated in air; (b) fringe spacing as calculated in glass.

Figure 2.22 Bragg spacing between fringe planes.

Figure 2.23 Principle of the “white light reflection” (Lippmann) hologram.

Figure 2.24 Summation of in‐phase reflection.

Figure 2.25 Off‐axis (Denisyuk) phase preservation geometry.

Chapter 03

Figure 3.1 Mercury spectrum.

Figure 3.2 Twin spectral lines of sodium.

Figure 3.3 Pioneer brothers‐in‐law.

Figure 3.4 Ruby laser schematic.

Figure 3.5 “Burn‐paper” sequence of ruby laser tuning.

Figure 3.6 “The Mathematical Chef” – self‐portrait of Martin Richardson.

Figure 3.7 Ruby beam without filtration.

Figure 3.8 The HeNe equivalent.

Figure 3.9 Pulse laser wavelengths as part of the spectrum of human vision.

Figure 3.10 Schematic of the helium–neon laser principle.

Figure 3.11 Schematic operation of a 532 nm d.p.s.s. laser.

Figure 3.12 Schematic for the visible fibre laser.

Figure 3.13 Image‐gang hologram exposed at 413 nm in resist.

Chapter 04

Figure 4.1 Photomicrograph of “Kinebar” hologram.

Figure 4.2 Compact dot matrix.

Figure 4.3 Spaced dot matrix.

Figure 4.4 Dot‐matrix system.

Figure 4.5 Filling the image space in DM.

Figure 4.6 Microtext.

Figure 4.7 Cross‐talk between dots.

Figure 4.8 “One‐step” digital origination system.

Figure 4.9 “Digital Head” by Zebra.

Chapter 05

Figure 5.1 Titration electrode potential.

Figure 5.2 Silver halide grain growth plot.

Figure 5.3 Schematic of the electrode‐controlled deposition mechanism.

Figure 5.4 Conventional emulsion research laboratory.

Figure 5.5 “Wet test”.

Figure 5.6 Migrant ion creates a Frenkel defect in a doped AgBr crystal.

Figure 5.7 Wine glass hologram.

Figure 5.8 Rayleigh scatter as a function of grain size and wavelength. Figure courtesy of Alex Cabral.

Figure 5.9 Typical photopolymer layer assembly.

Figure 5.10 Schematic of photo‐thermoplastic assembly.

Figure 5.11 Mechanism of photo‐thermoplastic exposure.

Chapter 06

Figure 6.1 Density profile vs. crystal population.

Figure 6.2 Silver halide SEM image.

Figure 6.3 Pseudo‐colour image of Grommet.

Figure 6.4 Formation of latent image.

Figure 6.5 The H&D curve (characteristic curve).

Figure 6.6 Contrast effects. (a) Photographic; (b) satisfactory contrast; (c) reduced contrast.

Figure 6.7 The action of developer.

Figure 6.8 “Shreddies” hologram card.

Figure 6.9 Solvent bleaching action.

Figure 6.10 The “diffusion transfer” bleach mechanism.

Figure 6.11 EDTA molecule.

Figure 6.12 Phenosafranine molecule.

Chapter 07

Figure 7.1 Pneumatic suspension of honeycomb table.

Figure 7.2 The spatial filter principle.

Figure 7.3 (a) Unfiltered spread; (b) after filtration.

Figure 7.4 (a) Magnetic‐type spatial filter; (b) ring‐type spatial filter.

Figure 7.5 Alignment of a spatial filter lens.

Figure 7.6 (a) Positioning the pinhole beyond the focal point to locate the focus; (b) photographic example of locating the focal plane.

Figure 7.7 Vertical misalignment of pinhole.

Figure 7.8 Focussed filter.

Figure 7.9 Beyond the focal point.

Figure 7.10 Misalignment of component laser beams.

Figure 7.11 The choice between lens and mirror collimators.

Figure 7.12 Comparison of reflections from spherical and parabolic mirrors.

Figure 7.13 The off‐axis parabolic mirror and its origin.

Figure 7.14 (a) Direct illumination of a spherical collimator; (b) folding the illumination beam for economy of space.

Figure 7.15 Advantage of correct orientation of a plano‐convex lens.

Figure 7.16 Finding the approximate focal length of a lens.

Figure 7.17 Lens tissue cleaning.

Figure 7.18 Rotary beam.

Figure 7.19 Spurious back reflections in a rotary beam splitter.

Chapter 08

Figure 8.1 Hologram reference.

Figure 8.2 Denisyuk hologram realism.

Figure 8.3 Denisyuk configuration.

Figure 8.4 Incident and reflected wave front approach.

Figure 8.5 The shadow of the recording film.

Figure 8.6 Diffuse reflection.

Figure 8.7 Incandescent bulb spectrum.

Figure 8.8 Fluorescent lamp spectrum.

Figure 8.9 “White” LED spectrum.

Figure 8.10 (a) The transmission rainbow hologram where the “master hologram” is a narrow slit window; (b) the reflection transfer H2 hologram where the real image of the master hologram is generally larger than the final hologram; (c) the Denisyuk configuration where there is little restriction upon the range of positions from which the image can be seen by the viewer, other than the critical angle effect which was described in Chapter 2.

Figure 8.11 “Newton’s rings.”

Figure 8.12 Viewing conditions.

Figure 8.13 The effect of Snell’s Law at the recording plate.

Figure 8.14 Reflection of 50 mW p‐polarised laser from glass plate.

Figure 8.15 Vacuum chuck.

Figure 8.16 Horizontal Denisyuk camera.

Figure 8.17 Vertical Denisyuk camera with horizontal object.

Figure 8.18 Vertical Denisyuk camera with upright object.

Figure 8.19 Dental cast.

Figure 8.20 Full‐colour Denisyuk teapot.

Figure 8.21 Dielectric laser combiner.

Figure 8.22 The localised “burn out” in a Denisyuk image.

Figure 8.23 Denisyuk shadow reduction.

Figure 8.24 Denisyuk boosted by backlit diffuser screen and “top light.”

Figure 8.25 Denisyuk principle in the contact copy process.

Figure 8.26 Contact copying in transmission mode.

Figure 8.27 (a) Rainbow transmission; (b) reflection volume.

Figure 8.28 H1 transmission master recording.

Figure 8.29 Michelson–Morley interferometer.

Figure 8.30 (a) Projected real image schematic; (b) photograph of the actual real image projected from a laser transmission H1.

Figure 8.31 Colour‐smeared image from collimated white light illumination of H1.

Figure 8.32 H1 image produced by three‐component “white” laser” beam.

Figure 8.33 Dispersed white‐light reconstruction.

Figure 8.34 Rainbow transfer.

Figure 8.35 “Laser‐foil” method.

Figure 8.36 Combined effect of prism and lens.

Figure 8.37 Diffractive optics combination.

Figure 8.38 The α‐angle phenomenon.

Figure 8.39 Tilting a “real‐colour” embossed hologram from its central viewing position.

Figure 8.40 α‐angle master hologram.

Figure 8.41 α‐angle transfer to H2.

Figure 8.42 Alpha‐angle reconstruction.

Figure 8.43 The H1 mask as a template for colour design.

Figure 8.44 Reflection hologram H2 transfer camera.

Figure 8.45 Pseudo‐colour hologram.

Figure 8.46 Separate channels of a full‐colour reflection security hologram.

Chapter 09

Figure 9.1 3D colour hologram of model.

Figure 9.2 Fabergé egg clock.

Figure 9.3 Fruit bat in flight.

Figure 9.4 Principle of the “2D/3D” technique.

Figure 9.5 The stereogram principle.

Figure 9.6 The “Holodisk” technique.

Figure 9.7 Two views of the Shakespeare embossed stereogram.

Figure 9.8 The process train for stereogram production.

Figure 9.9 Shear camera mechanism.

Figure 9.10 Working on the elevated Mitchell camera in Bruges.

Figure 9.11 Centre of rotation for a portrait.

Figure 9.12 Separating a colour scene through a red filter.

Figure 9.13 Creating the Cyan printing plate.

Figure 9.14 Separating a colour scene through a green filter.

Figure 9.15 Creating the Magenta printing plate.

Figure 9.16 Separating a colour scene through a blue filter.

Figure 9.17 Creating the Yellow printing plate.

Figure 9.18 (a) Subtractive and (b) additive colour models.

Figure 9.19 1902 colour print.

Chapter 10

Figure 10.1 Benton rainbow hologram.

Figure 10.2 RPS inaugural meeting.

Figure 10.3 Applied Holographics placing.

Figure 10.4 JDW with “The HoloCopier”.

Figure 10.5 “Mirror Man” achromat.*

Figure 10.6 “Mirror Man” laser lit.*

Figure 10.7 Die‐cut sheet of Hasbro Visionaries holograms. Visionaries © 2016 Hasbro.

Figure 10.8 Laser portrait of Simon Brown.*

Figure 10.9 Animated watch.*

Figure 10.10 Photopolymer coin hologram.*

Figure 10.11 Craig and Daniel approach the Vatican.

Figure 10.12 Pope John Paul II in 2D/3D.*

Figure 10.13 Braxted’s first dot‐matrix hologram.*

Figure 10.14 The journey to Rome.

Figure 10.15 Andrew with Bill McGowan.

Figure 10.16 Andrew unloading cameras at the Vatican.

Figure 10.17 Michelangelo’s La Pieta.

Figure 10.18 Wembley Stadium hologram.

Figure 10.19 Christmas hologram.

Figure 10.20 Biker stereogram.

Figure 10.21 The end of the road – that sinking feeling!

Figure 10.22 The end of the road – that drinking feeling!

Figure 10.23 The set awaits David Bowie.

Figure 10.24 Picture neg. report.

Figure 10.25 Biometrigram.

Figure 10.26 Ver‐tec’s Rubik’s Cube.

Figure 10.27 Holographers in pensive mood.

Figure 10.28 Jonathan Betts prepares Harrison’s “H4”.

Figure 10.29 Mike with the successful “H4” hologram.

Figure 10.30 Colour hologram by Iñaki Beguiristain.

Epilogue: An Overview of the Impact of Holography in the World of Imaging

Figure E.1 My hologram of Martin Scorsese.

Figure E.2 The author’s hologram of Sir Peter Blake.

Figure E.3 Photograph of the author’s hologram “Primal Scream”.

Figure E.4 Opto‐Clone egg hologram.

Guide

Cover

Table of Contents

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The Hologram

Principles and Techniques

Martin J. Richardson

De Montfort University, Leicester, UK

John D. Wiltshire

Independent Consultant, UK

This edition first published 2018© 2018 John Wiley and Sons Ltd

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.

The right of Martin J. Richardson and John D. Wiltshire to be identified as the authors of this work has been asserted in accordance with law.

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ISBN: 9781119088905

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Dedications and Acknowledgements

Martin J. Richardson

Dedicated to my daughters Elizabeth and Florence, who are inspirational, and my partner Nicky, for persevering more than thirty years of holographic mayhem, thank you!

John D. Wiltshire

For Carol, Jonathan and Darren.

Many thanks to Martin for the invitation to join him in the creation of this book. After 45 years working in the production, transport and recording of light, I hope my experience in practical issues will be useful to readers.

My uncle, Harold Swannell, an electrician at the Royal Small Arms Factory, Enfield, inspired and nurtured my lifelong interest in electricity, light and chemistry over sixty years ago.

Later, Joyce and Stanley Wiltshire, my late mother and father, coped patiently with explosions, fires and evil odours in our home throughout my trying youth. Thank you.

My Mum had ten siblings – her legacy: “The Turner Heritage.”

Thanks to everyone who lived the “Holography Dream” together at Applied Holographics from 1983, during my 14 years at Braxted Park, and especially to Paul Dunn and Andrew Rowe for their recent help as I sought to remember and record those halcyon days.

My earliest inspiration for holography came from the late Graham Saxby, Nick Phillips and Steve Benton.

For the inspiring technical discussions I’ve had over the years with Peter Howard, Howard Buttery, Dave Oliff, Simon Brown, Jeff Blyth, Craig Newswanger, Peter Miller, David Winterbottom, Nigel Abraham, Mike Medora, Brian Holmes, Gideon Raeburn, Hans Bjelkhagen, Patrick Flynn, Satyamoorthy “Kabi” Kabilan, Jonathan Wiltshire and many other great scientists and engineers.

For my friends who didn’t make it this far: Rob Rattray, Hamish Shearer, Micky Finlay and my soulmate Barney, I’m carrying the baton.

Thanks for irreplaceable contributions to this book and 20 years of friendship with Alexandre Cabral.

My sincere gratitude to project editor Samanaa Srinivas at John Wiley & Sons for invaluable help and advice in the realisation of a working manuscript.

To my friends throughout Europe – still love you – back soon!

Thank you.

Foreword

It was a great honour to receive the invitation to write this foreword. Holography has been part of my life both at the research level, applied to security of documents and products, and at the academic level, as a powerful tool to teach many of the complex aspects of optics.

The idea behind this book clearly detaches it from existing holography books that focus on the physics of the technique, requiring some considerable background in optics and mathematics, on the exquisite chemistry around the recording of the hologram, or on the artistic concept that surrounds this fabulous creative tool. In this book it is possible to navigate the interfaces between various types of knowledge involved and, when essential, the required physical/optical/chemical concepts are explained in a simple and pragmatic way, allowing the content to be easily explored by someone not entirely familiar with the subject, or to be appreciated by a specialist due to the simplified and abridged approach.

Reading this book reminded me of an anecdote (that I adapted to holography) about a complex holography camera that was having problems of consistency for several months. After using all the expertise available from scientists of all possible areas, the institution decided to call an old holographer who had worked in holography all his life. After a detailed analysis, the holographer fastened one screw with the proper torque and the holographic camera immediately started to give wonderful results. The institution was profoundly thankful to the holographer but considered the cost to be unexpectedly high for the activity performed. As a reply, the invoice from the holographer detailed: 1% of the cost – fastening the screw, 99% of the cost – knowing which screw to fasten!

This simple joke applies perfectly to the challenging complexity behind holography and makes us aware of something that is (apparently more than in other fields) fundamental and, above all the academic/scientific knowledge, required to make good holograms: Experience. This book also conveys to the reader know‐how gathered from several decades of experience, and this is undoubtedly a fundamental instrument if someone wants to take holography to the next level.

In conclusion, whether a researcher in a science institute, a teacher or a student in an optics class, or an artist in a holography studio, this book is a highly valuable tool for those starting to take the first steps on the difficult journey that is holography, and an excellent complement to the physics and chemistry books for those more advanced in the subject. Joyce Carol Oates, an American writer, once wrote that, “Beauty is a question of optics. All sight is illusion.” If holography is an optical illusion, it is undoubtedly the most beautiful one.

Dr Alexandre Cabral

Preface

Hologram - The Thinking Picture

With the legacy of the inventions of Lippmann, Gabor et al., we are lucky enough to have been the generation who have experienced the dawn of the Age of Holography first hand. Still, when we think of holograms, we think of the future, a place full of wondrous inventions. A place where driverless cars defy gravity, civilisation is established on Mars or cities are built under Earth’s oceans. And then, of course, holographic images that materialise in thin air and communicate with each other using artificial intelligence, indistinguishable from human consciousness.

Such is the legacy of science fiction. In reality, this vision of the future may fall far beyond the laws of physics, but nevertheless there are some truly astounding developments taking place right now in the field of holography. I know, because as a research professor at De Montfort University, I’ve had the privilege of experiencing some of the world’s most incredible three‐dimensional holograms, and it’s a glimpse into a future I want to share with you.

It could be that the laws of physics, outlined in Chapter 2 of this book, will prevent holography from fulfilling the science fiction vision of Star Wars. Instead, the holographic medium serves another function, a function that underlines the very nature of technological advance, as science strives to catch up with science fiction. The spin‐offs are often more interesting than the original research intention! The fantasy of holography is a conceptual lubricant that facilitates the birth of other great ideas. Could it be, then, that the intrinsic, but unvoiced, value of holographic illusion points the way for next‐generation immersive augmented reality, promising to evolve into something other, something unpredictable?

When Microsoft recently announced its technical breakthrough toward interactivity with holograms, it was a jaw‐dropping moment for those within the holographic research community. The claim that Windows was about to enter our physical world through holographic technology owes much to the dream of science fiction and certainly adds another chapter to the history of three‐dimensional imaging.

It took me several days of thought regarding the implications this would have on the research community and, after going through my initial feeling of elation, thoughts slowly slipped into its darker meaning. Was Microsoft misleading the public into thinking they had found the Holy Grail of 3D? The thought of holograms populating our everyday lives also felt somehow unsettling. It simply didn’t align itself with current experience and, therefore, something seemed intrinsically wrong. Was the world on the brink of really merging with the digital Matrix? Was it a mirror rather than a window – a mirror reflecting another’s identity, thoughts, desires and, therefore, needs? The idea of the mirror seen through another’s eyes – someone else’s view of reality – seemed beyond our perception, because Microsoft’s HoloLens™ system threatens to invade the small amount of unencumbered reality we currently have, a space rapidly diminishing because of the ubiquitous screens on our walls, on our desks and in our pockets. Real space is an endangered, diminishing asset.

However, it quickly became clear that this interaction with our physical environment was through a head‐set that superimposes the Microsoft operating system on the actual world. The Microsoft HoloLens™ system may not be holographic in the purest sense (“What is Not a Hologram?” Section 1.11 of this book), but the fact that Gabor’s word “hologram”’ continues to inspire innovation in the twenty‐first century means our trip is far from over!

Perhaps The HoloLens™ could be said to be the modern‐day equivalent to “Pepper’s Ghost”, a historical device used to create spectacular visual illusions by the use of projection systems, explained in Chapter 1. By utilising an angled, partially reflective surface, of which the viewer is not aware, between the audience and the main subject of a display, it is possible to produce a ghostly or ethereal image which appears to the audience to be superimposed in the same space as the principal (“real”) display. Documentation of such a principle is recorded as far back as the sixteenth century in the writings of Giambattista della Porta. Later, such inventors as Rock [1] have attempted to improve the technique by suggesting improvements such as a method to hold a light, foil screen in position with a minimum of wrinkles in the film, which would normally be detrimental to the reflected image quality and thus to the illusion; ways to improve the brightness and contrast ratio of the projected images, and ways to eliminate extraneous light reflecting in the mirror screen, which tend to reduce the effectiveness of the illusion. Maas [2] also describes ways to improve the presentation of the basic principle. O’Connell [3] has shown ways of using this technique for video tele‐presence methods.

In 1987, Stephen Benton, one of the world’s great holographers and Professor at MIT, suggested that a stereographic three‐dimensional hologram display should be confined within a limited viewing space (the Benton Alcove Hologram [4]) so as to restrict the viewer from coming into contact with the angular viewing limitations of the holographic image, which we all agree tend to be the “Achilles Heel” of display holography.

Recent technological developments by Zebra Inc. and XYZ have improved horizontal viewing angles and also provided vertical parallax by digital ray‐tracing techniques, described in Chapter 8. The restriction of the ability to view the image from oblique angles is a severe disadvantage in comparison with Denisyuk holograms recently produced by Yves Gentet, Colour Holographic and Hans Bjelkhagen. But Denisyuk holograms of this type, whilst providing exceptionally realistic images (approaching facsimiles of reality), do not have the ability to represent computer‐generated animated images, such as may be realised by digital techniques.

In the 1980s, holographer Peter Miller produced a two‐colour reflection hologram with integrated sound system, featuring an image of a Barracuda Car Radio. Proximity switches behind the glass plate caused an audio effect to “change channel” when the viewer placed a finger in the real image of the channel selector button. This was a brilliant innovation which pre‐dated the modern iterations of interactivity with a hologram!

In common with so many modern optical systems, a key component of the Microsoft HoloLens™ technique is a holographic optical element (HOE). These optical devices have a similar effect to a conventional glass optic but take the form of a thin film that is optically clear and has unique abilities in the manipulation of light. Previous to the HOE, non‐holographic optical elements, made with a mechanical ruling device, were used in spectrophotometers, for example, as a dispersive grating to divide the spectrum from a white source into its separate colours, on an angular basis. An HOE is a convenient and relatively low‐cost component with a highly efficient grating made by laser imaging of a photosensitive material. It is possible, using holographic methods, however, to produce more complex optics such as diffusers which control the exact direction of scattered light, or volume holograms such as head‐up displays (HUDs) which direct reflected light of a narrow band of wavelengths in a specific (off‐axis) direction; for a holographic mirror, the angle of incidence is not necessarily equal to the angle of reflection! HOEs to assist the collection of solar power will inevitably follow. So, positive commercial statistics predicting the future of holography seem relatively clear‐cut.

In fact, modern holography offers many alternatives to light‐shaping devices in industry and may be compared with the role electronic circuits and microprocessors played at the beginning of the 1960s as an alternative to the valve. As mass‐produced holographic optical elements start replacing micro‐lens arrays, and holographic phase memory is poised ready to replace today’s standard magnetic hard drives, each has commercial potential previously thought impossible.

It remains to be seen if our ever‐increasing dependence on technology will impair our physical or mental faculties and our adaptability to nature, but we do know that the advantage modern holography gains over existing technology will be long term and, in some cases, life‐changing.

Creative computing will play a major role in the development of computer synthetic holograms within numerous applications including the arts, entertainment, games, mobile applications, multimedia, web design and other pervasive interactive systems. Due to the nature of these applications, computing technology needs to be developed specifically to tackle the conceptual complexity that does not exist in other applications. The challenges faced by creative computing come from the need to originate applications that involve knowledge in the disciplines of the Humanities and Arts more traditionally used to describe activities at human behaviour level. The main feature of these applications is that a creative system will directly serve people’s needs to improve quality of life. It is the rapid development of computing technology that will enable new creative industry and it is also this rapid development that requires serious academic discussion.

Creative computing supports the vision that computing technology will become an integral part of the design industry, where computing offers new design tools for artists and designers to extend the traditional products, and computing technology itself will be developed and enriched by reference to knowledge from the Humanities and the Arts.

Today, holograms are standard security issue on bank cards and banknotes, event tickets, postage stamps and passports – all aimed specifically at halting counterfeiting. They are a typical component in the validation of safety‐critical items – such as medicines and machine parts – and therefore save lives.

The list of applications of holography will increase in length as a growing number of five‐star research labs in universities and technical companies, including Microsoft, find new applications for these amazing devices. We are developing new types of holograms with the long‐term aim of progressing the medium beyond its ability simply to capture and replay three‐dimensional images, pursuing their general ability to diffract and manipulate light. Extensive technical documentation concerning holography has established it as an exciting, emerging medium. However, its potential still remains relatively untapped. So, how did we arrive at this juncture of technology? Why does holography have the potential power to change the way we see our world and, as we start our journey into the Age of Photonics, where did the holographic journey begin?

The word “hologram” means many things to many people. The word was used by Professor Stephen Hawking as a metaphor to describe concepts in quantum mechanics. Hawking related the holographic principle with that of the need to explain the anomalous behaviour at a black hole, comparing the way it flattens time and space with the way a two‐dimensional surface of a holographic recording carries a three‐dimensional image. Other theoretical scientists suggest that the universe has qualities resembling a hologram, in that information about the whole exists in every constituent part. I’m reminded of William Blake’s poem Auguries of Innocence:

To see a world in a grain of sand

And a Heaven in a wild flower

Hold infinity in the palm of your hand

And eternity in an hour

The word may also be used to describe the complete works of Shakespeare or a description of time, for example an organic life span from conception to death. Others tailor the word to promote idiosyncratic philosophy. The authors revert to the Greek roots of Gabor’s term “holo” (entire) and “graph” (message).

In the following chapters, the intention is to offer a stepping stone for those who have an interest in this fascinating area of holographic imaging. We hope to provide an entry point into the philosophical and practical aspects of hologram‐making; to understand how and why some of the holograms with which we are familiar today were made, and what the future holds for a relatively young technology as the related science develops.

Martin J. Richardson

Notes

1

James Rock. Patent application US20070201004A1: Projection apparatus and method for Pepper’s Ghost illusion.

2

Uwe Maas – Musion Eyeliner 3D Projection

www.eyeliner3d.com

3

O’Connell, I. (2009) “Video Conferencing Technique”.

New Scientist

, 26 November.

4

Benton, S.A. (1987) “‘Alcove’ Holograms for Computer‐Aided Design,”

Proceedings of SPIE

, 0761, True Three‐Dimensional Imaging Techniques and Display Technologies, 53.

About the Companion Website

Don’t forget to visit the companion website for this book:

www.wiley.com/go/richardson/holograms

There you will find valuable material designed to enhance your learning, including:

Video

Figure PPTs

1What is a Hologram?

1.1 Introduction

First, we should define in technical terms what precisely is meant by the term “hologram” and discuss some of the important milestones in the development of the technique.

Some years ago, holography was tauntingly dubbed “the solution looking for a problem” and it has taken some time to establish the true nature of the technology and disperse some of the urban myths which it seemed to attract so readily in earlier years.

We shall mention some of the important types of hologram recording which are possible, and end this chapter with a view of the public perception of holography; explaining a number of 3D systems which are frequently labelled “holograms” by the public, but which do not realistically meet the criteria for inclusion in this category.

Before embarking on a career in holography, remember that the holographer has to be prepared for the frequent request to re‐create R2‐D2’s projection of Princess Leia, and also patiently to stand fast through the knowing wink accompanying the statement “Oh yes that’s the method where the broken hologram still contains all of the image!”

1.2 Gabor's Invention of Holography

The word “hologram”, coined by Denis Gabor from Greek roots, seems to be difficult to define precisely, so the authors prefer to assume the most appropriate English meaning to be “the entire message”.

Of course, in an era where the Hellenic Institute of Holography plays a prominent role in development of new techniques for ultra‐real representation of museum artefacts, the word has now ironically become “anglicised” to the extent that it now translates directly back into modern Greek as “ολόγραμμα”!

Gabor’s intention was to refer, in the name, to the unique ability of this technique to record an incoming wave of light from an object in terms of both its phase and amplitude. Gabor was working in electron microscopy when he observed that interference recording was a way to achieve recordings of ultra‐high resolution without the difficulties introduced in optical systems by the limitations of recording materials, lenses and conventional optics.

The hologram differs greatly from the simple photographic recording of an image based solely on the amplitude of light arriving from the field of view of the optical system, because when we look at a black and white film exposed in an everyday camera, after applying developer, we see a perfectly recognisable “negative” image of the subject beyond the lens. In comparison, in its simplest form, the hologram recording itself, in the appropriate high‐resolution silver halide film or plate, tends to be something that, at first sight, is a meaningless jumble of lines or zones in varying tones.

So, what is the “hologram”? In the etymological sense, in the early years there was a move to call the recording itself, the “holograph” and the reconstructed image, the “hologram”. This seemed a logical and useful division, as it is often confusing during discussion in the workplace, as to whether we are referring to the glass plate in the laboratory or to the image which springs forth from it when the laser is switched on.

However, this terminology appears to have fallen by the wayside, and today’s dictionary does not generally acknowledge the word “holograph” except in a separate sense with reference to the special legal value of handwritten documents.

When we look though a microscope at a silver halide holographic recording plate which has been exposed to a suitable “standing wave” of interference, and processed in a suitable developer and either “fixed” or “stopped”, we see black and white lines in what appears to be a random pattern, or at least a pattern which does not appear to relate directly to the subject of the recording.

Its fringes might well, in some cases, be predominantly linear – leading to the concept of “surface‐relief holograms”. If the subject matter is relatively close to the plate, we may begin to recognise the shape of the object, but in a true “redundant” Fraunhofer hologram, it is quite impossible to relate the pattern to the subject matter (see Figure 1.1, which shows a magnified image of part of the recorded pattern in a holographic plate and, beside it, a black and white photograph of the recorded 3D image seen when the hologram was lit with helium–neon laser light).

Figure 1.1 (a) The amplitude fringe recording; (b) the image it produces in laser light.

So what is happening here?

The conditions for recording a hologram involve the use of a coherent light source. Lasers were not available in the era when Gabor invented holography, but nowadays we have a wide choice of laser types that can produce holograms, which will be detailed later.

Using a single laser, in the simplest format, we can arrange for one part of its emitted light to be incident upon a three‐dimensional object. The remainder of the beam travels towards a high‐resolution recording plate, and the light direct from the laser (“reference beam”) coincides near the plate with light reflected from the 3D object (“object beam”). The light reflected from the object contains information about the shape and tonality of the object and is “coherent” with the light arriving at the recording plate direct from the laser (Figure 1.2). In the vicinity of the plate these two beams “interfere” to produce a “standing wave of interference” which can be recorded in the photosensitive emulsion on the plate provided certain conditions of stability exist – by definition, this “standing wave” must not move or change during the recording process.

Figure 1.2 Making a transmission hologram with a single laser beam.

So what is the nature of this “standing wave”?

We are familiar with the classical experimental demonstration by Thomas Young early in the nineteenth century. By using sunlight issuing from a tiny hole in a window blind (which was a simplistic way to provide a beam of partially coherent light), he was thus able to demonstrate the wave nature of light. This was achieved by passing light from this single source through two adjacent narrow slits in such a way that the two waves issuing from slightly displaced sources continued towards a screen. His famous sketch in Figure 1.3 was made by the visualisation of waves on water.

Figure 1.3 Young’s slits visualisation [1].

Now that we are routinely able to utilise laser light, we can easily demonstrate the analogous wave effect in electromagnetic radiation. If the screen CDEF is stationary, the extended row of spots which results demonstrates the interference of light from the two separate sources A and B.

As shown in Figure 1.3, the “wave fronts” from the two sources provide an orderly sequence of high and low intensity in accordance with the distance between the slits, the wavelength of the light and the distance of the screen from the slits (i.e. the angle between the beams).

Of course, if we were to place a sheet of photosensitive film in the position of the line marked CDEF by Young, in an optical set‐up, we could record an interference pattern provided the “standing wave” was stationary.

Nowadays, we can very easily use a laser as a fully coherent light source and Figure 1.4 shows the effect of inter‐changing the laser wavelength between recordings with the same slit apertures, in this case a pair of thin (0.1 mm) lines spaced by 1.5 mm etched into a black‐developed photographic plate.

Figure 1.4 Twin‐slit diffraction of green and red lasers.

It is clear that increasing the distance between the slits in Figure 1.4 or reducing the distance to the screen will radically change the frequency of the fringes in accordance with “the grating equation” (see Chapter 2).

This fact will also lead us to see the advantage of the “off‐axis” methods of recording holograms, later invented by Upatnieks and Leith in the USA simultaneously with the work of Denisyuk in the USSR; but it will also show the necessity for a spectacular improvement of the resolution characteristics of the films used for this more advanced form of holography. Recording materials for holography are discussed in detail in Chapter 5.

1.3 The Work of Lippmann

Gabriel Lippmann, working at the turn of the twentieth century, effectively anticipated the resolution and silver halide grain‐size requirements of holographic recording materials. Using natural light, Lippmann’s technique involved the production of colour photographs by a process which bears a close relationship to today’s reflection holography. He was awarded the 1908 Nobel Prize for his work.

Essentially, he recognised the need for fine‐grain silver halide emulsions and a method to process them which produced a transparent layer, rather than using the simple black metallic silver grains, which normally produce image contrast with the white background, that we associate with traditional “black and white” photography.

Lippmann produced photo emulsions with grain size of the order of 50 nm. This is difficult to do, even nowadays, for reasons explained later in Chapter 5.

By sandwiching this emulsion in intimate contact with a mirror surface in the form of liquid mercury, Lippmann was able to set up a recording process whereby a standing wave was produced by light reflected from the mirror, as it met light entering from the lens of his camera. If the camera was entirely still, the stationary wave formed by each individual wavelength of light would be recorded in the appropriate position in the photograph.

But, in common with today’s “full‐colour holography”, achieved by coherent laser light, the ability of the emulsion to record a range of wavelengths simultaneously in a single zone of the recording material was limited. Furthermore, the use of incoherent light to produce a planar grating in the depth of the emulsion effectively meant that Lippmann was limited to a very thin layer of his scattering emulsion.

The natural “speed” of a photographic material tends to be proportional to its grain size, so Lippmann’s exposure times were long – requiring great stability of the equipment and the subject itself; this still presents a problem today in modern holography. Processing fine‐grain emulsion is also complicated, as explained in Chapter 6; these minute crystals of silver halide may well be soluble in chemicals designed for use with more typical, everyday photographic materials.

We will return to the connections between Lippmann’s work and that of Denisyuk over 60 years later, and to the complex issues of photographic emulsion‐making in subsequent chapters.

1.4 Amplitude and Phase Holograms

The first holograms were, quite naturally, recorded in silver halide material. Given its natural speed, this has obviously been the material of choice for the photographic industry for many years, only recently challenged by digital electronic technology. Only the problems outlined above, as addressed by Lippmann, regarding the resolution of the recording materials for holographic use have prevented silver from remaining the material of choice to this day, when new materials have emerged. We discuss the alternative recording materials for holography in Chapter 5.

Silver halide, as explained in detail in Chapter 5, produces, after initial development, grains of black silver metal (which may appear tinted dark red or green in transmitted light, as we will explain later) that produce an “amplitude” record of the zones/fringes which are associated with areas of high intensity (additive or constructive) interference of the light in the standing wave recorded, and which, in turn, represent the interaction of the reference beam with the object wave.

This record is called an amplitude hologram: a three‐dimensional matrix of zones of clear gelatin interrupted by grains of opaque silver metal which prevent the direct passage of light rays through the layer. This works relatively well for transmission holograms, where, as we will show later, the fringe microstructure is principally perpendicular to the layer; a predominantly superficial recording.

But if we then continue the emulsion processing with a bleaching solution, we can change the amplitude hologram into a phase‐modulated microstructure. The black grains of silver which prevent the passage of light rays through the layer can be changed advantageously into translucent crystals of silver bromide. This is a very simple chemical step:

The loss of an electron is a simple oxidation process which can be brought about by any number of common oxidising agents (for example, a solution of ferric [iron III] sulphate). Loss of the electron results in the creation of a silver ion, Ag+, which then combines in solution with a negative bromide ion in the bleaching solution to produce translucent (pale yellow) silver bromide.

Now that we have effectively removed attenuating black material from the layer, we have a condition where light rays entering the layer are not significantly absorbed, but are now subject to diffraction at interfaces within the layer which are manifested by local “image‐wise” modulation of the refractive index of the constituents of the layer.

Now, whereas silver halide is historically the most important recording material, we must consider other, later materials which directly provide phase modulation. These include dichromated gelatin (DCG), which found an important commercial outlet in hologram pendants for jewellery, and, at the opposite end of the market, the use of holograms as optical elements such as the head‐up displays which were introduced in fighter aircraft in the 1970s.

The DCG medium became unfashionable later due to technical difficulties, general chemical undesirability and archival problems, and has been replaced predominantly by modern photopolymer materials in the role of direct phase‐modulated materials.

1.5 Transmission Holograms

Simplistically, a transmission hologram is a recording where, at the reconstruction or viewing stage, the illumination source directs its light through the recording film or plate so that the viewer will interrogate the image from the opposite side to the light source, as seen in Figure 1.5.

Figure 1.5 The dispersion of white light by a transmission hologram.

Figure courtesy of Alex Cabral.

This phenomenon is a function of the recording arrangement. Of course, the irony is immediately apparent that, as we discuss in detail in Chapter 8, the format for recording a transmission hologram, which is viewed from the side opposite to the illumination source, to enable the appropriate fringe structure to form, is a configuration where both object and reference beams arrive from the same side of the plate (the opposite is true for reflection holograms!).

The fact is that, despite a certain level of popular disbelief, a transmission hologram may be either a thin hologram or a thick (volume) hologram. This is entirely dependent upon the recording medium and we will detail in Chapter 5 why certain materials are more appropriate than others in the various hologram recording configurations, due to their ability to record either surface or volume microstructures.

But, as shown in Figure 1.5, the diffractive properties of a transmission hologram work in such a way as to disperse incident white light into its component colours. The hologram acts, in its effect, in a similar way to a prism, although the colour distribution appears reversed. This fundamental property of the transmission hologram made it impossible in the early years to view the image in white light, until the incredible invention by Dr Stephen Benton in 1967 of the rainbow hologram, which we describe later in detail and which has facilitated the major commercial outlet for the technology in security applications.

So, the embossed holograms with which we are so familiar are, in fact, transmission holograms. By mounting the transmissive layer upon a metallic foil substrate, we are able to view the image from the same side as the illumination source. Similar means have been used to display large‐format transmission rainbow holograms; that is, by mounting a glass or film hologram layer upon a mirror before framing the whole assembly.

1.6 Reflection Holograms

In the simplest terms, reflection holograms are those which are illuminated in the reconstruction or replay step from the same side of the plate or film as the viewer sees the image. These holograms are often called volume reflection or Lippmann holograms.

Note: The use of the term “volume hologram” works very well in the technical environment, but the authors have frequently been seriously frustrated in commercial discussions by the interpretation of this term as relating either to the quantity of holograms to be produced or to the fact that the image itself has depth or volume!

As with transmission hologram technology, the configuration for recording such a microstructure is the opposite of the viewing configuration, so that to create the required microstructure here, we must arrange for the recording object and reference beams to be incident from opposite sides of the recording medium. The result is that the fringes are typically planar within the layer and, like Lippmann’s photographs, they tend to act somewhat like a mirror, rather than showing the prism‐like dispersive effect of the transmission hologram. This is why the concept has been summarised as being “like a mirror with a memory”.

This “mirror” has the quality of being monochromatic, so it acts rather like a filter to incident white light, in that a single wavelength is selectively reflected, whereas rays of all other wavelengths pass directly through the layer. This is because rays of light, shown in orange in Figure 1.6, whose wavelengths coincide with the particular selected frequency of the planar grating, constructively interfere, in accordance with Bragg’s Law, at the successive parallel index interfaces, in order to create a reflected summation of their energy, whereas other wavelengths fail to meet the conditions for constructive interference. It is important to recognise that Bragg diffraction follows a mechanism of constructive interference, whilst thin holograms actually function quite differently.

Figure 1.6 The planar index modulations (fringes) reflect light of a single wavelength only.

Figure courtesy of Alex Cabral.

Because the reflection hologram is thus able to reflect selected light of a certain wavelength, and of course this may be in the form either of a (simple) plane wave or a complex object wave, as previously discussed, the device itself may take the form of a clear, almost colourless layer, which bears a three‐dimensional or animated image, so that such an extremely simple, compact, ethereal film layer is a very attractive proposition in security applications, where it may be used to overlay conventional printed graphics, allowing an unobstructed view of printed information on a document, in the “off‐Bragg” conditions of illumination.

In this configuration, we see the vital importance of the phase (bleached) hologram set‐up. Interestingly, if we make an exposure in the reflection mode to a suitable silver halide film, we can develop the film to a low density and if, at that time, we dry the film and view the hologram with strong illumination, we will see a very dim monochromatic reflection image in the black silver layer. If we imagine the fringes of this “Lippmann–Bragg” hologram to be of similar planar structure to the pages of a book, albeit tilted perhaps at a small angle to the surface of the film, as shown in Figure 1.6