Vision correction

Summary: Modern optometry depends on precise measurements to construct corrective lenses.

Human vision is subject to a variety of ailments and disorders. Some are congenital; others are age-related. Faulty vision results in blurriness, coupled with headaches and ocular tiredness. However, for many years, humans have been perfecting the art of using external implements to aid vision. Technologies exist in the twenty-first century that can restore perfect vision to people suffering from common vision-related problems, such as myopia or astigmatism. The methods used to diagnose vision issues and to construct corrective lenses rely on precise mathematical measurements and understanding of the geometric principles behind light refraction. Vision may also be modeled in various ways, including using a concept called “orthonormal polynomials,” such as the Fourier series and optic wavefronts. This has many applications, including laser vision correction. In stereoscopic vision, two-dimensional projections of the world onto the retina of each eye are combined and compared to form a three-dimensional image. It was once thought of as virtually impossible to cure stereoblindness, but in the early twenty-first century, vision therapists use a variety of techniques to help patients perceive stereoscopic depth in three spatial dimensions.

Lens Power

The optical power of a lens, also known as “dioptic power,” “refractive power,” or “focusing power,” is a measure of the curvature of the lens and the degree to which a lens converges or diverges light. It is equal to the reciprocal of the focal length of the lens in meters. Its unit is “diopter.” Prescriptions for eyeglasses specify the optical power of the lenses. The human eye has a refractive power of 60 diopters. Stacking lenses helps to combine their optical power.

Eyeglasses and Bifocals

A simple pair of eyeglasses contains nothing more than two pieces of glass shaped in such a way that they act like a pair of lenses. Lenses exploit the physical property of light called “refraction.” Refraction occurs when light travels between mediums of different densities, such as air and glass. The change in the medium causes light to bend in a certain calculable way. This property of lenses is suitable to refocus the image back onto the retina in people suffering from long-sightedness and short-sightedness.

The focal length of a lens in air can be calculated using the lensmaker’s equation, given by

where f is the focal length of the lens, n is the refractive index of the material, R₁ is the radius of curvature of the lens surface closest to the light source, R₂ is the radius of curvature of the lens surface farthest from the light source, and d is the thickness of the lens.

To address people suffering from vision problems such as myopia, hyperopia, and astigmatism, bifocal lenses were invented. These lenses have a section of magnification at the lower portion of the frames to allow the wearer to read small print. Benjamin Franklin is generally associated with the invention of the first pair of bifocals.

Contact Lenses

Contact lenses are corrective or cosmetic lenses placed on the cornea of the eye. Their performance is similar to that of eyeglasses but they can be shaped somewhat differently. Spherical lenses are the typical shape of contact lenses on both the inside and the outside surfaces, whereas toric contact lenses, often used for people with astigmatism, are created with curvatures at different angles and cannot move on the eye. Contact lenses are extremely lightweight and are virtually invisible when compared to eyeglasses. However, they are also not held in place by a rigid framework like glasses. Mathematical models are useful for understanding the various movements of lenses within the eye, especially hard contact lenses.

In the twenty-first century, technology has advanced to a level where it is possible to imprint electronics onto the contact lenses themselves, resulting in the ability to project a virtual display onto the eye directly. While this technology by itself does not directly correct any vision problems, it could be used to assist people in their everyday activities, such as locating objects, or reading street signs by magnifying letters.

LASIK

Laser-assisted in situ keratomileusis (LASIK) is becoming an increasingly popular alternative to contact lenses and eyeglasses. LASIK is a type of refractive surgery performed using a laser. A “laser” (Light Amplification by Stimulated Emission of Radiation) is a highly concentrated beam of light capable of focusing high energy in a small area.

The technology was invented by a Colombia-based Spanish ophthalmologist Jose Barraquer. His technique involved cutting thin flaps in the cornea and altering its shape. After the laser was invented, Dr. Bhaumik, in 1973, announced the breakthrough in using lasers to treat vision problems.

LASIK involves creating a flap of corneal tissue, remodeling the cornea underneath the flap with the help of a laser, and then repositioning the flap. Mathematical computations are used to determine the depth of the cuts used in the surgery, and these are often a function of the average cornea thickness of 550 micrometers. One alternative is to leave some fixed tissue depth.

Bibliography

Barry, Susan. Fixing My Gaze: A Scientist’s Journey Into Seeing in Three Dimensions. New York: Basic Books, 2009.

Dai, Guang-ming. “Wavefront Optics for Vision Correction.” SPIE Press Monograph PM 179 (2008).

Hecht, Eugene. Optics. 4th ed. Addison Wesley, 2002.