2. The Optical System of the Human Eye

The eye's optical system consists of three main components: the cornea, the crystalline lens and in between them the iris ( Figure 1). The cornea, the outermost optical element, is responsible for about 2/3 of the optical power and aberrations of the eye. The iris controls the amount of light coming into the retina by regulating the pupil diameter. As in any optical system, the size of the pupil has important consequences for image formation: a smaller pupil increases the depth of focus and minimizes the effects of high-order aberrations. Conversely, the magnitude of aberrations increases with pupil dilation leading to a decrease in both visual performance and optical quality of the retinal image [1]. The crystalline lens accounts for about 1/3 of the optical power of the eye but it is capable of changing its focusing properties: controlled changes in the shape and thickness of the crystalline lens allow the eye to accommodate, the process by which the eye focuses on near objects. Even in the normal eye, departures from ideal focus (i.e., aberrations) exist and degrade the eye's optical performance [8–10]. LOA are the predominant optical aberrations (90% of the overall WA of the eye): defocus (positive or negative; i.e., hyperopia and myopia respectively) is the dominant aberration, followed by astigmatism (orthogonal or oblique). It is well known that the human eye suffers from HOA that cannot yet be accurately corrected and that they greatly diminish the overall optical quality of the eye, though their contribution to the overall WA of the eye is ≤10% [3, 8]. The presence of HOA, beyond defocus and astigmatism, has been known by researchers since the 19^th century, but only in the 1990s have wavefront sensors been developed to allow routine estimation of the eye's WA. The development of ocular wavefront sensors has allowed rapid, accurate and objective measurements of wave aberrations and made large population studies possible [2, 9–11]. Several authors [12–15] have measured the distribution and contribution of both LOA and HOA to the overall WA of the eye: between HOA, the magnitude of 3^rd order coma-like aberrations (vertical coma, horizontal coma, oblique trefoil and horizontal trefoil) and spherical aberration is higher than other higher aberration modes [1]. The eye's WA is not static but fluctuates over time: the eye's focus exhibits fluctuations about its mean value for steady-state accommodation with amplitudes ranging between 0.03 and 0.5 diopters (D). In addition, a general tendency for spherical aberration to change in a negative direction with increase in accommodation (–0.04 μm/D for accommodative levels of 1.0 to 6.0 D) has been measured, while the other HOA are not significantly influenced by accommodation [16, 17]. The largest source of temporal short-term instability (seconds and minutes) of HOA is then due to the micro-fluctuations in the accommodation of the lens: the anterior curvature increases centrally and flattens peripherally during accommodation, while at the same time, the lens thickness increases and the equatorial diameter decreases. These factors may contribute to the change in the measured aberrations. Another source of fluctuations is local changes in the tear film thickness over the cornea, due to evaporation and/or blinking [1, 18]. If considering a long period of time (over the course of the day and between successive days), the WA of the eye has been demonstrated to be sufficiently stable, with no significant changes in the magnitude and contributions of HOA [1, 17]. An AO ophthalmic device can measure and correct for the fluctuations of the eye's WA, thus improving the resolution of images taken from the retina of patients.

Figure 1