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The optical performance of the human eye has been intensively investigated over the past 200 years. In the early years, however, measurements were strictly limited to subjective investigations due to the technology available at that time. Most studies of higher-order imaging errors - such as spherical aberration and coma - never gained clinical attention, because eye surgeons were unable to correct such "irregular" optical structures.

This has changed due to modern excimer-laser systems for refractive correction and customized corneal laser surgery. The latter technique combines objective wavefront sensing with reliable scanning-spot excimer-laser platforms to correct for more than the well-known optical errors of the eye, such as long-sight, short-sight and astigmatism. Customized surgery also enables the correction of higher-order imaging errors of the eye, and today it appears to be on the verge of making a big commercial impact.

These imaging errors, called aberrations, affect the image quality at the retina of the eye and, consequently, they reduce visual performance. Aberrations can be shape or positioning errors, as well as inhomogeneities of refractive-index values of the eye's optical elements. A point light source, such as a star, should form a sharp single point at the retina. But optical aberrations, which exist in all human eyes, increase the blur of such a light source and decrease the contrast of the light-spot edges.

This means that the fine detail of an object observed by an eye with significant higher-order aberrations appears blurred. Clinicians and researchers realized that some visual symptoms reported by patients correlate with their degree of higher-order optical aberrations. Patients treated using standard laser vision correction, such as LASIK, often complain of vision problems in poor light conditions (i.e. at night). In particular, patients have reported effects such as halos, glare and double vision after LASIK treatment.

As a result, the primary aim of customized corneal laser surgery is to avoid this increase in optical aberrations after laser vision correction. However, clinical studies showed that the procedure also reduced pre-existing optical aberrations. This means that surgeons can actually improve the optical quality of the patient's eye. Patients with serious higher-order aberrations before corneal laser surgery have reported a significant vision improvement after customized treatment.

The customized technique is based on an objective measurement of an eye's optical aberrations. In the past three years, two technologies, the Tscherning aberrometer and the Hartmann-Shack sensor, have been applied to the human eye and have gained clinical acceptance for wavefront-based treatments.

In the aberrometer a red laser diode (wavelength 670nm) is optically coupled to a monomode fibre (diameter 25mm) and collimated at the end of the fibre tip. The resulting beam is split into a group of parallel rays by a mask with a regular matrix of fine holes (diameter 0.3mm). This light pattern is projected onto the retina (see figure 1). After exiting an eye with perfect optical performance, this recognizable pattern of spots would have the same regular and undistorted structure as the rays before they enter the eye.

In a real eye, the pattern is distorted according to the individual's ocular optical aberrations. To determine the optical aberrations objectively, this spot pattern is imaged onto the sensor array of a video camera. The co-ordinates of the geometric centres of all imaged retinal spots are calculated by image-processing software. For each spot the displacement from its ideal position is measured. It is possible to find the deviation of the wavefront from its ideal spherical shape with the known relationship between the positions of the rays at the cornea and their locations along the retinal plane (length of the eye).

Hartmann-Shack sensors work differently (figure 2). Generally used in astronomical and industrial applications, they consist of an infrared laser (beam diameter <1mm) that is focused on the retina to form a single light spot. The wavefront from that light source is roughly spherical, but it is deformed while travelling through the eye. In this approach, the exit pupil of the eye is imaged onto the Hartmann-Shack sensor by a telescope.

The sensor has a microlens array and a CCD camera. The array forms a set of light spots over the pupil plane. Optical aberrations are determined by the deviation of spot positions from their reference positions. The angle between the focal length and the location of each spot is a measure of the wavefront slope at this position within the pupil plane. As the eye has a poor optical quality compared with technical optics, all wavefront sensors used in clinical wavefront sensing should have a high dynamic range and good lateral resolution of several hundred spots with the measured pupil diameter (typically 7mm).

US firm Alcon, which uses a Hartmann-Shack system, received approval from the FDA in the US for customized corneal laser surgery in October 2002. In clinical trials, more than 500 eyes were treated for short-sight, with 98% achieving 0.8 vision or better, 88% realizing 1.0 (i.e. 20:20) vision or better, and 63% achieving 1.25 vision or better.

In these trials, the procedure was proved to address higher-order aberrations, provide an increase in contrast sensitivity and demonstrate an improvement in vision quality over conventional laser vision correction. Clinical trials of various laser systems from different manufacturers to determine its effectiveness for long- and short-sight correction with and without astigmatism are ongoing, as well as the treatment of other special visual disorders, such as pre-existing night-vision problems and post-LASIK complications.