Optical Aberrations

Wavefront technology allows us to better understand the complex optical aberrations that degrade visual function in the human eye. This technology may usher in a new era of refractive surgery, by not only attaining supernormal visual acuities, but by improving other aspects of visual function as well.

There are many different aberrations in the normal eye, but the one that has significant deleterious effects on vision is called the spherical aberration. Reduced scotopic (low light) contrast sensitivity and the presence of halos are two manifestations of decreased visual function that directly relate to this higher-order aberration.

Spherical Aberration

The natural human cornea is steeper in the center and flatter towards the periphery. This configuration, known as a prolate surface, provides the ideal refractive configuration for stigmatic optics: parallel rays focusing at one point on the retina (Figure 1). What would happen if the cornea did not have a prolate shape but was a perfect sphere with the same steepness at the visual axis and towards the periphery? The optics of this system are no longer stigmatic.

Fig. 1
An ideal cornea is steeper in the center and flatter towards the periphery. This prolate configuration allows light rays entering along the visual axis and in the corneal periphery to focus at a single point on the retina.

While paraxial rays still come to a point focus on the retina, light rays entering the peripheral cornea would be bent greatly and would focus in front of the fovea (central point on the retina). These light rays degrade contrast sensitivity and visual acuity when the pupil is enlarged, such as in low-light conditions.

Fig. 2
Positive spherical aberration: Peripheral rays are bent greatly; focus in front of the fovea and degrade image quality.

If we continue to flatten the central cornea, we move into an oblate configuration; the cornea is flat in the center and steep towards the periphery (Figure 3). This configuration also suffers from positive spherical aberration. When we perform conventional LASIK or PRK myopic refractive surgery, we are inducing positive spherical aberration by flattening the central optical zone while leaving the peripheral cornea untouched. We are converting the cornea from the more optically efficient prolate configuration to a less efficient oblate surface. When the positive spherical aberration is particularly pronounced, the patient may experience a myopic shift, as the best focus of the image shifts anteriorly.

Fig. 3
An 'Oblate' cornea is flat centrally and steep towards the far periphery. Peripheral rays are bent greatly, focus in front of the fovea and degrade image quality.

As mentioned earlier, the ideal cornea (Figure 1) has a prolate configuration, namely steep in the center and flat towards the periphery. However, If we take this to an extreme and continue to steepen the central cornea and flatten the periphery, we will produce optics that contain significant negative spherical aberration (Figure 4). In this extreme prolate configuration, peripheral rays are not bent enough and focus at a virtual point behind the fovea, again degrading contrast sensitivity and visual function.

Fig. 4
Negative spherical aberration: In this extreme prolate cornea, peripheral rays are not bent enough and focus at a virtual point behind the fovea, degrading image quality.

Let's examine two major areas of visual function degraded by spherical aberration: glare/halos and decreased contrast sensitivity. Spherical Aberration causes glare and halos. In looking at Figure 3, one can imagine that the peripheral rays from a point light source might degrade image quality in the form of a myopic shift, manifest most commonly during scotopic conditions.

CSF: Spatial frequency is shown increasing to the right while image contrast decreases along the vertical axis. There is a range of frequencies in the middle that are easier to appreciate at low contrast than those on either end. The red line highlights this and represents the contrast sensitivity function of the youthful human eye. The presence of spherical aberration shifts this curve downward to the blue curve, denying the eye from detecting subtle contrast changes and image detail.

Fig. 5
To understand contrast sensitivity, let's examine Figure 5, which shows a contrast sensitivity grading. The spatial frequency of alternating light and dark stripes increases from left to right. The contrast decreases from bottom to top.

The grading appears to have a hump in the middle at the spatial frequencies for which the human eye is most sensitive.

The red curve in Figure 5 demonstrating this sensitivity, represents the contrast sensitivity function (CSF) for the youthful human eye. Spherical aberration shifts this CSF curve downward, to the blue curve, for example, degrading contrast sensitivity at all spatial frequencies. Another measure of visual function, the modulation transfer function (MTF), relates spatial frequency and contrast acuity. Spherical aberration also negatively affects the MTF.

The complementary nature of the cornea and lens breaks down with age. As we move past age 40, the lens shape changes such that it begins to contribute positive spherical aberration, which adds to that of the cornea, reducing contrast sensitivity and visual acuity in dark conditions.