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Graefes Arch Clin Exp Ophthalmol (2008) 246:167168 DOI 10.1007/s00417-007-0624-3
BOOK REVIEW
Jason Porter, Hope Queener, Julianna Lin, Karen Thorn, Abdul Awwal (eds): Adaptive optics for vision science
Wiley-Interscience, New York, 2006, hardcover, $88.50, 591pp, hardcover, ISBN 0-471-67941-0
Barry R. Masters
Received: 31 May 2007 / Accepted: 1 June 2007 / Published online: 6 July 2007 # Springer-Verlag 2007
Sooner or later most ophthalmologists, optometrists, and vision scientists will use instruments that are based on adaptive optics. At the start, I define adaptive optics for the reader. Adaptive optics is an optical system that adapts in order to compensate for optical aberrations caused by the medium between the object and the image. While adaptive optics were first developed for military and astronomical applications, there are two applications in vision science. First, clinical vision applications involving vision correction methods that compensate for higher order aberrations instead of the usual corrections of defocus and astigmatism. Second, vision researchers using adaptive optics to image the retina, and in particular the photoreceptors, with optical resolution that was not possible prior to the introduction of a new generation of retinal imaging systems.
Previously, I reviewed two books that describe the clinical applications of wavefront sensing (the measurement of wavefront aberration in the human eye): Aberration-Free Refractive Surgery, New Frontiers in Vision Second Edition, (J. F. Bille, C. R. H. Harner, F. H. Loesel, (Eds, 2004) and Corneal Topography in the Wavefront Era: A Guide for Clinical Application (Wang, 2006). Both books discussed corneal keratorefractive surgery and topographic measurements. While these books stress the clinical applications of wavefront technology, they only provided a cursory introduction to the fundamentals of the technology.
Hence, there was a need for an in depth book that explains, describes, and compares the various adaptive optic technolo-
gies, their advantages and limitations in both research and clinical settings. This gap is filled with a new book. Adaptive Optics for Vision Science is a clearly written, carefully illustrated, and comprehensive account of the fundamentals and applications of adaptive optics in vision science. Four sections make up the book: wavefront measurement and correction, retinal imaging applications, vision correction applications, and design examples that provide details of the instruments used in clinical adaptive optic systems.
This book will be of great benefit to researchers and clinicians in ophthalmology and optometry, and vision science, as well as students who work with instruments that incorporate adaptive optic systems. Also, this book would be invaluable to engineers in the biomedical field who are developing new medical devices that are based on adaptive optics.
How did this revolution in adaptive optics come about? Astronomers faced the problem of imaging stars through the turbulent atmosphere. A solution was provided by Horace Babcock, who first proposed that an adaptive optical element could be used to correct for the time-varying aberrations caused by atmospheric turbulence. Adaptive Optics in Astronomy, (Ed, F. Roddier, 2004) is a high level text that covers the engineering aspects of designing, constructing, and calibrating an adaptive optics system. In spite of the focus on applications in astronomy I found this book to be an excellent resource for the advanced reader.
The introductory chapter of Adaptive Optics for Vision Science correctly credits Junzhong Liang, a graduate student in the laboratory of Josef Bille at the University of Heidelberg, for his work with a Shack-Hartmann wavefront sensor applied to measure the wavefront aberrations from the human eye. Their pioneering work led the way to the closed-loop adaptive systems. Another graduate student, Andreas Dreher, also working in Billes laboratory, demonstrated how
B. R. Masters (*)
Department of Biological Engineering, MIT, Cambridge, MA, USAe-mail: [email protected]
168 Graefes Arch Clin Exp Ophthalmol (2008) 246:167168
a deformable mirror could be used to improve retinal images obtained with a scanning laser ophthalmoscope. Later, Liang, Don Miller, and David Williams at the University of Rochester constructed a closed-loop adaptive optics system, consisting of a wavefront sensor, a control computer, and a wavefront corrector, with the capacity to correct higher order aberrations in the human eye.
In strong support of my high recommendation of this book is the fact that it is clearly written and explains the concepts, theories, and instrumentation at a level that actually teaches the reader. For example, in the section on the location of monochromatic aberrations within the eye the author states that optical aberrations differ from individual to individual, with the size of the pupil, with the age of the subject, with the state of accommodation, with retinal eccentricity, with refractive state, and according to other parameters. The author then explains how to compute the optical aberrations that are associated with the anterior corneal surface. Next, the reader is shown how to compute the aberrations from the different ocular surfaces to produce retinal image quality.
Another example of the comprehensive nature of this book is the discussion on wavefront sensors. In contrast to other books, Adaptive Optics for Vision Science presents a detailed discussion of several types of wavefront sensors that are used in vision science: the spatially resolved refractometer, the laser ray tracing technique, and the Shack-Hartmann (lenslet array) wavefront sensor. The key principle is an indirect measurement of local wavefront slopes (the first derivative of the wavefront) together with the reconstruction of the complete wavefront that is obtained by the integration of these slopes or gradients. The authors provide significant details on the Shack-Hartmann wavefront sensor, including a discussion of the design compromise between dynamic range and measurement sensitivity.
This book provides a detailed discussion and description of another component of the adaptive optics system: wavefront correctors. In vision research there are four types of wavefront correctors: discrete actuator deformable mirrors that consist of a continuous reflective surface and an array of actuators situated under the surface; segmented correctors, which are segmented mirrors that consist of an array of planar mirrors
whose axial motion is independently controlled; membrane mirrors consisting of a flexible membrane between a transparent top electrode and an array of patterned electrodes below; and bimorph mirrors in which a piezoelectric layer lies between a continuous top electrode and a patterned electrode. A mirror layer is situated on top of the continuous electrode. Adaptive optics is an emerging field of technology, i.e., micromechanical system (MEMS) mirrors have the potential to provide low-cost and very compact devices.
I found the section on the application of adaptive optics to retinal imaging well organized. It contains a review of the fundamental properties of the retina, and a chapter on high-resolution retinal imaging. Vision researchers will continue to make new advances in our understanding of vision and its disorders with the help of the new retinal imaging devices that are based on adaptive optics.
Ophthalmologists and optometrists will find the last section on vision correction applications a useful textbook and guide to the current technology. Readers will be interested in new technologies that permit post-surgical alteration of the power of an implanted IOL. These technologies could reduce the higher order aberrations of the IOL. Another important application is the use of wavefront measurement to provide patients with customized corneal ablation. This is an example of the great progress that has been made in laser photo-refractive keratectomy since 1985 when Seiler performed the first procedure on blind eyes. An advantage of wavefront technology coupled with laser refractive surgery (LASIK, LASEK, or PRK) is the possibility to correct for higher order aberrations in addition to the lower order aberrations sphere and cylinder.
Other useful features of Adaptive Optics for Vision Science include an appendix that contains the Optical Society of Americas Standards for Reporting Optical Aberrations, a glossary of terms, a table of symbols, and a detailed index. The glossary is a useful source of definitions as well as links to the relevant book chapters. Each chapter ends with a list of key references that often point to web sites. I highly recommend Adaptive Optics for Vision Science to the readers of Graefes Archive for Clinical and Experimental Ophthalmology.
Springer-Verlag 2007