The human eye, one of the most exquisitely constructed organs in all of nature, transmits information from the surrounding world to the brain by sensing the intensity and color of the light. As an optical system, however, the human eye exhibits its imperfection by the existence of ocular aberrations. Therefore, images formed on the retina are blurred, resulting in a degradation in visual performance, with the most prominent clinical parameters being the visual acuity and contrast sensitivity. The ocular aberrations consist of the low-order and high-order aberrations, or the spherocylindrical error and irregular error.
Traditionally, the spherocylindrical error has been corrected by spectacles. More recently, contact lenses and intraocular lenses have also been used to correct low-order ocular aberrations. In 1983, Dr. Steven Trokel and his co-workers proposed tissue ablation on human cornea using excimer lasers at an ultraviolet frequency of 193 nm. In 1985, Dr. Theo Seiler performed the first procedure of phototherapeutic keratectomy in Germany. Subsequently in 1988, Dr. Marguerite McDonald performed the first photorefractive keratectomy (PRK) on a normally sighted human eye in the United States. In 1989, Dr. Ioannis Pallikaris performed the first laser assisted in situ keratomileusis (LASIK) on human eyes in Greece. Today, LASIK is the most popular procedure for refractive surgery. Newer techniques, such as the laser assisted sub-epithelial keratomileusis (LASEK) and Epi-LASIK, have also been developed. As such, laser vision correction has become a popular and effective means for vision correction. It has been estimated that a few millions of laser vision correction procedures are performed in the world each year at the time of this writing.
1.1 Wavefront Optics and Vision Correction
As laser vision correction becomes more and more mature, the improvement of the visual outcome after the procedure becomes somewhat saturated. Higher expectations from consumers drove the refractive laser manufacturers to greater technological innovation. At the same time, research on correcting high-order ocular aberrations in human eyes has been very active. In the early 1990s, Dr. Junzhong Liang, then a PhD student of Prof. Josef F. Bille's at the University of Heidelberg, Germany, developed the first Hartmann-Shack wavefront sensing device for ocular aberration measurements.