For hundreds of years, glasses have been used to correct for the eye's basic refractive errors. The earliest spectacles simply provided a defocus (or spherical) correction. These types of glasses slowly evolved to include a correction of astigmatism (or cylinder and axis) and are similar to the types of glasses we wear today. The following two figures illustrate how glasses are used to correct for the defocus, or the amount of nearsightedness (myopia) or farsightedness (hyperopia), in an individual's eye. The left side of the figure shows the case for a nearsighted eye, in which the overall length of the eye is too long. Therefore, all of the rays of light coming from a distant object, like a star, are focused in front of the retina (red spot), as shown on the left. This focus problem can be corrected, as illustrated on the right, by inserting a diverging, or negative, lens of the appropriate power in front of the eye. This causes the rays to slightly diverge before entering the eye, and allows the light to become focused on the retina (blue spot). In order to have a perfectly crisp view of the world, all of the rays of light from an object must be precisely focused on the retina.

The following figure represents the case of an eye that is farsighted. As shown on the left, a farsighted eye tends to focus distant objects behind the retina (red spot). By placing a positive, or converging, lens in front of the eye, light from an object can be brought to a nice focus on the retina (shown on the right).

Contact lenses, on the other hand, have not been around for nearly as long as glasses. Even though Leonardo da Vinci is often credited with the first concept and drawings of contact lenses in the late 15th century, the first contacts were not actually fabricated until the turn of the 20th century. These initial contact lenses were made of hand blown glass and were primarily used to treat certain corneal pathologies. It was not until the early 1950s that glass contact lenses were replaced with Plexiglas and PMMA lenses. The soft contact lenses we enjoy today did not get their start until the late 1960s and early 1970s. Contact lenses work on the same principles as above to correct nearsightedness and farsightedness, and can also correct astigmatism.

Another popular technique used to improve vision is laser refractive surgery, namely laser in-situ keratomileusis (LASIK). In this procedure, a microkeratome (shown on the left) is placed on the eye and is used as a precise knife to cut a thin corneal flap. The flap is then retracted, as shown in the rightmost picture below, and an excimer laser is used to ablate the underlying corneal tissue in order to achieve the desired correction. After the tissue is removed by the laser, the thin flap is laid back on top of the cornea and it quickly settles back into place.

For a nearsighted (or myopic) eye, the overall length of the eye is too long, or alternatively, one can think of the cornea as being too steep and possessing too much power. In this case, more corneal tissue needs to be removed from the center of the eye than in the periphery. This is illustrated in the top three figures, which shows how individual laser pulses are applied to the eye's pupil (large black circle) over time. In the beginning of the procedure, laser pulses are primarily applied to remove tissue at the center of the pupil (top leftmost figure). As the procedure continues (top central figure), more pulses are centrally applied while some additional ablation begins to occur closer to the edge of the pupil. In these figures, the darker the shade of purple for the laser pulse, the more tissue has been removed in the given spot location. At the end of the procedure, the laser has removed some tissue from the entire pupil, with more tissue being removed from the pupil center (darkest purple spots) than from the pupil margins (lightest shade of purple spots). How does this nearsighted correction actually affect how images are focused on the retina? This is illustrated in the two bottom components of this figure. In the lower leftmost figure, we again see that a nearsighted eye is too long and light from a distant object is focused in front of the retina (red spot), creating a blur on the retina. In the lowermost figure, we see an extremely exaggerated result of a typical LASIK ablation for a nearsighted eye in which more tissue has been removed from the central cornea, increasing its flatness and reducing its overall power. As a result, the image focus is pushed back onto the retina (blue spot), allowing distant objects to be sharply focused on the retina.

The top three components of the following figure show a similar time sequence for a farsighted (or hyperopic) laser ablation. At the beginning of a farsighted correction, a ring of pulses are laid down in the mid-periphery of the pupil, and no laser ablation occurs in the central cornea (top leftmost figure). As the procedure wears on, more pulses are applied in this mid-peripheral range, still sparing the central cornea and pupil (top central figure). Finally, at the end of the ablation (top rightmost figure), one can see that some additional tissue removal has occurred at the edge of the pupil, but a majority of the ablation has occurred in a small annulus surrounding the center of the eye (darkest purple spots). No ablation has actually occurred over the center of the pupil due to the fact that we are trying to steepen the cornea in a farsighted correction (shown in the two bottom component figures). A normal farsighted eye is too short, or alternatively has too little power, to focus an object on the retina (red spot, lower leftmost figure). However, the process of removing more tissue peripherally than centrally in a laser ablation serves to steepen the cornea and provide more power to pull the object back into focus on the retina (blue spot, lower rightmost figure). This effect has again been exaggerated for illustrative purposes in the figure.

Even though laser refractive surgery (LASIK) can accurately correct the eye's defocus and astigmatic errors, it does induce some higher order aberrations, namely spherical aberration.

Close Window