polycarbonate lenses are eyeglass lenses made from a durable plastic polymer called polycarbonate. They are designed to correct vision like other spectacle lenses, while emphasizing impact resistance and lighter weight. They are commonly used in children’s glasses, safety eyewear, sports eyewear, and some rimless or semi-rimless frames. In clinical practice, they are one of several lens material options selected based on prescription needs and lifestyle risks.

anti-reflective coating is a thin, transparent layer applied to optical surfaces to reduce glare and reflections. It is most commonly used on eyeglass lenses, but it is also used on instrument optics in eye clinics. By reducing surface reflections, it can improve visual clarity and cosmetic appearance. Its performance and durability vary by material and manufacturer.

photochromic lenses are eyeglass (and some contact or implant) lenses that darken in response to ultraviolet (UV) light and lighten again indoors. They are commonly used in everyday prescription glasses and non-prescription eyewear to reduce brightness outdoors. They aim to provide convenient, automatic light adaptation without switching between clear glasses and sunglasses. Performance and features vary by material and manufacturer.

prism glasses are eyeglasses that include prism power in one or both lenses. They shift where images fall so the eyes can aim more comfortably together. They are commonly used for double vision and certain binocular vision problems. They are prescribed by optometrists and ophthalmologists after eye alignment testing.

progressive addition lenses (PAL) are eyeglass lenses that change power gradually from top to bottom. They are commonly used to help people see clearly at distance, intermediate (computer range), and near. They are most often prescribed for presbyopia, the age-related loss of near focusing ability. They are worn in everyday glasses for work, reading, and driving.

trifocals are vision-correcting lenses designed with three distinct focal zones. They are commonly used in eyeglasses to help people see far away, at intermediate distance, and up close. They are most often prescribed for presbyopia, the age-related reduction in near focusing ability. In some settings, “trifocal” can also describe certain intraocular lenses used during cataract surgery.

bifocals are eyeglass lenses with two different optical powers in a single lens. They are commonly used to correct distance vision and near vision at the same time. They are most often prescribed for age-related near focusing difficulty (presbyopia). They are used in everyday settings like reading, computer tasks, and driving.

single vision lenses are lenses with one optical power across the entire lens. They are used to focus light for one viewing distance, such as far away or up close. They are commonly prescribed in eyeglasses and contact lenses. They are a standard option for correcting everyday refractive errors.

glasses are wearable optical devices that place prescription lenses in front of the eyes. They are commonly used to improve focus for clearer vision at distance, near, or both. They are also used for eye protection and comfort in certain environments. glasses are prescribed and fitted in routine optometry and ophthalmology care.

spectacles are wearable optical devices that place lenses in front of the eyes to change how light focuses. They are most commonly used to correct vision blur from refractive errors such as myopia or astigmatism. spectacles are also used for eye protection, comfort, and selected clinical needs. They are prescribed and fitted in optometry and ophthalmology settings and then worn in daily life.

chromatic aberration is an optical effect where different colors of light focus at different points. It can make edges look fringed with color, especially in high-contrast scenes. It occurs in cameras, microscopes, and also in the human eye and corrective lenses. In eye care, it matters for image quality, lens design, and interpreting diagnostic images.

trefoil is a type of optical distortion (aberration) that can reduce visual quality even when a glasses prescription seems correct. It is named for its three-lobed, clover-like pattern in optical measurements. In eye care, trefoil is most commonly discussed as a higher-order aberration measured by wavefront aberrometry or inferred from corneal imaging. Clinicians use it to help explain symptoms like glare or “smearing” and to plan certain customized vision procedures.

coma is a type of optical blur caused by an “off-center” focusing error in the eye. It is classified as a higher-order aberration, meaning it is different from simple nearsightedness, farsightedness, and astigmatism. coma is commonly discussed in refractive surgery planning, corneal disease evaluation, and advanced contact lens fitting. People may describe its visual effect as streaking, smearing, or “comet-like” tails around lights.

spherical aberration is an optical focusing error that can make images look less sharp. It happens when light rays passing through the edge of a lens focus differently than rays passing through the center. In eye care, it is discussed in relation to the cornea, the natural crystalline lens, and artificial intraocular lenses. It is commonly measured with wavefront testing and considered in refractive surgery planning and lens design.

higher-order aberrations are subtle optical imperfections that can reduce the *quality* of vision even when glasses or standard contact lenses provide good clarity. They are different from common focusing errors like nearsightedness and astigmatism. They are often discussed in wavefront testing, refractive surgery planning, and specialty contact lens care. They can help explain symptoms like glare, halos, or “ghosting,” especially at night.

aberrations are optical imperfections that make light focus less precisely on the retina. They help explain why vision can feel “not crisp” even when a glasses prescription seems accurate. In eye care, aberrations are commonly discussed in wavefront testing, refractive surgery planning, and lens selection. They are also used to describe visual symptoms such as glare, halos, ghosting, and reduced contrast.

post-LASIK ectasia is a condition where the cornea becomes progressively weaker and changes shape after LASIK surgery. It can lead to increasing myopia (nearsightedness) and irregular astigmatism that distort vision. It is most commonly discussed in refractive surgery follow-up and corneal specialty clinics. It is evaluated using corneal imaging and clinical exam findings.

corneal ectasia is a condition where the cornea becomes thinner and bulges outward. It can change the eye’s focusing power and cause irregular vision. It is commonly discussed in corneal disease clinics and refractive surgery follow-up. It is also a key concept in contact lens fitting and corneal imaging.

keratoglobus is a rare corneal ectasia, meaning the cornea becomes abnormally thin and bulges outward. It typically causes a more “globular” (ball-like) corneal shape rather than a cone shape. The term is used in eye clinics to describe a specific pattern of corneal thinning and protrusion. It is most often discussed in cornea specialty care, contact lens fitting, and surgical planning.

pellucid marginal degeneration is a corneal ectasia, meaning the cornea becomes thin and mechanically unstable. It typically causes progressive, irregular astigmatism and blurred or distorted vision. The thinning most often occurs in the lower (inferior) peripheral cornea while the cornea stays relatively clear. The term is commonly used in cornea clinics, optometry practices, and refractive surgery screening to describe a specific thinning pattern.