Contact lenses are medical devices placed directly on the surface of the eye to correct vision problems caused by refractive errors. These thin, curved lenses float on the tear film covering the cornea, acting as a new refracting surface for the eye’s optical system. They move in sync with the eye, offering a wider and more natural field of view compared to traditional spectacles.
The Optical Principle of Vision Correction
Vision depends on the eye’s ability to precisely bend, or refract, incoming light to focus it on the retina, the light-sensitive tissue at the back of the eye. Refractive errors occur when the eye’s shape or length causes light to focus incorrectly, resulting in blurred vision. Common issues include myopia (nearsightedness), where light focuses in front of the retina, and hyperopia (farsightedness), where light focuses behind it.
Contact lenses correct these errors by providing an additional, specific curvature that alters the path of light before it enters the eye’s natural lens and cornea. For myopia, the contact lens is designed as a diverging, or minus, lens, which gently pushes the focal point backward to land exactly on the retina. Conversely, for hyperopia, the lens acts as a converging, or plus, lens, bringing the focal point forward.
Astigmatism, where the cornea or lens has an irregular, non-spherical curvature, causes light to focus at multiple points, leading to distorted vision. This requires a toric contact lens, engineered with different powers across two perpendicular meridians to compensate for the eye’s uneven shape. Resting directly on the eye, the contact lens essentially replaces the irregular surface with a smooth, optically precise one, ensuring the image is perfectly focused on the retina.
Materials, Fit, and Corneal Oxygen Supply
The contact lens must be made from a material that is both optically clear and biologically compatible with the delicate tissues of the eye. Early soft lenses used hydrogels, plastic polymers that absorb water to become soft and flexible. Modern lenses primarily use silicone hydrogels, which incorporate silicone to dramatically increase the amount of oxygen that can pass through the lens.
The cornea, the clear front dome of the eye, is unique because it lacks blood vessels and must draw its oxygen directly from the air to remain healthy. When a contact lens covers the cornea, it creates a barrier that can reduce oxygen flow, potentially leading to complications like swelling or the growth of abnormal blood vessels. The oxygen transmissibility of a lens is quantified by the Dk/t value, which measures the material’s oxygen permeability (Dk) relative to the lens’s thickness (t).
A higher Dk/t value indicates better “breathability,” important for lenses worn for longer periods or overnight. For a lens to function safely, it must also fit correctly, centering properly on the cornea and moving slightly with each blink. This slight movement, facilitated by the tear film, helps to pump oxygenated tears under the lens and flush away debris, maintaining the eye’s natural physiological balance.
Categorization by Design and Wear Schedule
Contact lenses are broadly categorized by their material rigidity and their prescribed wear schedule. Soft lenses are the most common; they are made of flexible plastics that conform easily to the shape of the eye, offering immediate comfort. Soft lenses are available in a range of replacement schedules, including daily disposable, bi-weekly, and monthly, which are largely determined by hygiene and material durability.
Daily disposable lenses are discarded after a single use, offering the highest level of hygiene and eliminating the need for cleaning solutions. Lenses with a planned replacement schedule, such as two-week or monthly, require nightly removal, cleaning, and disinfection to prevent the buildup of protein deposits and microorganisms that naturally accumulate on the lens surface over time.
A second design category is Rigid Gas Permeable (RGP) lenses, often referred to as hard lenses. RGP lenses are made from durable plastics that hold their shape and provide exceptionally crisp vision, particularly for complex prescriptions like high astigmatism or keratoconus. While they may require a longer initial adjustment period than soft lenses, their rigid structure and high oxygen permeability make them a preferred option for certain visual and corneal requirements.