The “add power” measurement is a requirement for bifocal and multifocal contacts in order describe the amount of additional power of the lens required to assist with near vision. The “add power” measurement is always indicated by a plus sign (+). Some lenses will list the “add power” as low, medium or high rather than with a numerical figure; the numerical equivalent can vary by brand.
Aspheric lenses have a complex curvature design that helps to reduce some distortion and aberrations in vision. Aspheric lenses can also be made thinner and flatter than a simple lens design.
Astigmatism is a condition caused by an irregular curvature of the cornea, the covering of the front of the eye. This causes light waves to refract incorrectly. This refractive error causes blurry or distorted vision at all ranges. Astigmatism can be corrected using a toric lens.
The axis specifies the orientation of the irregular corneal curvature in a patient with astigmatism. The axis of the lens is generally indicated by a number ranging between one and one hundred and eighty degrees.
This measurement refers to the shape of the back surface of a contact lens, the part that fits over the cornea. Base curves are typically measured in millimeters of curvature and generally range between eight millimeters and ten millimeters. Some brands of contact lenses use the less-specific references such as flat, median and steep base curve.
Bifocal and multi-focal lenses are used to correct both distance and near vision. Unlike a traditional lens that features one consistent prescription throughout, bifocal or multi-focal lenses have two or more different prescriptions.
The brand of contact lenses refers to the trade name of a particular kind of lens. This usually includes the company and make of a particular lens, such as Acuvue Oasys or Focus Dailies. Only the particular brand prescribed by a doctor should be worn, regardless of personal preference. Unlike with some prescription medications, there are no generic brands of contact lenses.
While many people wear clear contacts purely to correct their vision, both corrective and cosmetic lenses are also sold with color added. There are three variations:
Enhancing: colored lenses that are intended to deepen the color of pale eyes, enhancing the natural shade. They are ineffective on dark eyes.
Opaque: colored lenses intended to mask and replace the natural eye color. They are most effective on dark eyes.
Visibility tint: lenses tinted lightly in a way that will not affect eye color. They are intended to assist a lens wearer in seeing a pair of lenses in a case or if dropped or lost.
There are many options in contact lens solutions, and not all solutions are the same. Some solutions may not be appropriate to use for patients with certain sensitivities and not all solutions are compatible with all contact lens materials. It is best to only use the solution recommended by your doctor.
Multipurpose solution: a solution used to remove dirt and debris from the lens, and also to disinfect the lenses to kill bacteria. Most multipurpose solutions can also be used to rinse the contact lenses. Multipurpose solutions are the most common solutions sold today.
Cleaning solution: a solution intended to remove dirt, mucus or other debris that builds up during lens wear. A cleaning solution typically needs to be rinsed from the lens with a rinsing solution prior to disinfecting or inserting the lens.
Rinsing solution: a solution that removes other solutions and dirt from lenses prior to insertion or before disinfecting.
Disinfecting solution: a solution used to prevent bacteria from contaminating lenses.
Rewetting solution or drops: a solution used to lubricate lenses throughout the day for increased comfort.
Conventional or traditional lenses are usually used for a period of six to twelve months and can be soft lenses or hard lenses while disposable lenses are worn for a day to a month and then replaced. Conventional lenses are currently prescribed much less than disposable lenses. The first contact lenses released onto the market were conventional lenses.
The cylinder is a lens measurement that refers to the amount of power needed to correct astigmatism. A cylinder is generally abbreviated CYL and is usually indicated with a minus sign (-) in the middle portion of a contact lens prescription.
DK/t is an abbreviation used to describe the amount of oxygen that can pass through a lens. The more oxygen a lens lets in, the safer it is in general for the wearer. “Dk” represents the permeability of a particular substance while “t” represents the thickness of the lens. Early contact lenses were hard to wear due to a low level of oxygen permeability. Modern soft lenses and rigid gas permeable lenses have higher DK/t values and are much safer to wear and far more comfortable over long periods of time during the day.
As opposed to extended wear lenses, daily wear lenses are worn during the day and removed at night. For health reasons, daily wear contacts are more popular and more commonly prescribed. Daily wear contacts can be disposable or conventional.
The diameter of a lens refers to the width of a lens, generally measured in millimeters and abbreviated as DIA. Most brands come in one or two sizes, although some lenses are made to an individual’s particular specifications. The diameter of a soft contact lens is usually between thirteen and fifteen millimeters across, with fourteen as the average.
Disposable contacts, rather than conventional lenses, are worn for a shorter period of time. These contact lenses are replaced on a schedule, usually daily, weekly or monthly. Different materials and lens types are designed for specific replacement schedules. It is always important to follow the replacement schedule recommended by your doctor even if the lenses don’t feel like they need to be replaced.
ECP is an abbreviation that stands for Eye Care Provider, a generic term for the health care professionals who diagnose, prescribe and treat refractive errors and fit corrective lenses. Eye care providers can be optometrists or ophthalmologists. Both optometrists and ophthalmologists attend an additional four years of schooling after college, ensuring high levels of training and professionalism.
Extended range lenses offer additional power options beyond what is normally available in a contact lens. This power is required for those who have refractive errors that cannot be corrected with a more typical contact lens.
In contrast to daily wear lenses, extended wear lenses are worn continuously for any period of time from several days to a month. While daily wear lenses must be removed after sixteen to eighteen hours, extended wear contacts do not have to be taken out each day. If suggested by a health care professional, individuals can sleep in their lenses every night. There are specific lenses designed to transmit enough oxygen to the eye to be safe enough to sleep in. Not all contact lenses can be worn on an extended wear basis.
Farsightedness, clinically known as hyperopia, is a condition in which individuals can see distant objects more easily than objects that are close by. Hyperopia is a condition that can be treated with both glasses and contacts, as well as corrective surgery.
In contact lenses, “flat” refers to a base curve measurement. Rather than using a numerical value, some brands of lenses use terms like flat, median, and steep to describe a base curve. A flat base curve has less curvature than a steep base curve. While the term is uniform, the specific measurements will vary by brand.
Flex wear is another term for extended wear. Flex wear lenses can be worn continuously for a period of days or one month without being removed, depending on a doctor’s recommendation. Many individuals sleep in flex wear lenses, although some take them out for health reasons. Like many contacts, flex wear contacts come in a variety of brands, styles and prescriptions.
The original contact lenses approved in 1945 were what are now known as hard contacts. Originally made of glass, modern hard lenses are currently made from a material called polymethyl methacrylate (PMMA) better known as Plexiglas. Hard lenses do not allow much oxygen to permeate the lenses, making them less comfortable for long periods of time. For this reason, hard lenses are rarely used; rigid gas permeable lenses and soft lenses are more common. Although still an option for treating some specific corneal problems, few people wear this type of contact lens.
In contact lenses, “high” refers to a stronger add power measurement for bifocal or multifocal lenses.
In contact lenses, “low” refers to a weaker add power measurement for bifocal or multifocal lenses.
Median is a base curve measurement. Rather than using a numerical value, some brands of lenses use terms like flat, median, and steep to describe a base curve. While the term is uniform, the specific measurements will vary by brand.
Monovision lenses are an alternative to bifocal or multifocal lenses in which one eye is corrected to clearly see close objects and the other is corrected to see far objects. This means that one eye is always out of focus when looking at both near and far objects. In time, the brain acclimates to the prescriptions and learns to pay attention to only one eye at a time rather than both as is normal. Monovision correction works very well for many patients and may be the best option for patients who have to wear lenses that correct astigmatism.
Multifocal lenses are designed to correct vision at a distance, at an intermediate range, such as at a computer, and also for close up. Unlike a tradition lens that features one consistent prescription throughout, multi-focal lenses have two or more different prescriptions.
Nearsightedness, clinically known as myopia, is a condition in which individuals can see near objects more clearly than distant objects. Myopia is a condition that can be treated with corrective lenses, both glasses and contacts, as well as corrective surgery.
“OD” is an abbreviation that indicates lens specifications for the right eye on a corrective lens prescription. OD stands for “oculus dexter,” the Latin phrase for “right eye.”
“OS” is an abbreviation that indicates lens specifications for the left eye on a corrective lens prescription. OS stands for “oculus sinister,” the Latin phrase for “left eye.”
“OU” is an abbreviation that indicates lens specifications for both eyes on a corrective lens prescription. OU is only used when both eyes require the same levels of refractory correction. OU stands for “oculus uterque,” the Latin phrase for “both eyes.”
Ophthalmologists are medical doctors who attended medical school, experienced clinical rotations and underwent at least four years of specialized residency, occasionally preceded by an internship year. They are licensed MDs (Doctor of Medicine) or DOs (Doctor of Osteopathic Medicine) and are trained to diagnose and treat vision problems and perform eye surgery when necessary. Some ophthalmologists receive additional training in more specialized areas of eye care.
Opticians, unlike ophthalmologists and optometrists, are not doctors. Most opticians have a high school diploma and a two-year technical degree. They are not permitted to diagnose, prescribe or treat eye disorders. Instead, opticians specialize in fitting glasses, choosing frames and ensuring prescribed products fit and function properly. While some opticians are licensed, there is no national board regulating the education of opticians and the care they can provide.
Optometrists are primary eye care doctors who must hold a Doctor of Optometry (OD) degree. Their training focuses entirely on the eye. Optometrists must attend an undergraduate institution as well as a four-year professional program to earn the OD designation. Optometrists are also licensed by a board and are held to high standards of care. Optometrists diagnose and treat disorders and diseases of the eyes and vision often by prescribing glasses, contact lenses or prescription medications. Optometrists are licensed to perform some surgical procedures, but will refer a patient to an appropriate ophthalmologist if an invasive eye surgery is required.
Planned replacement contact lenses are a form of contact lens that requires a set replacement schedule, such as daily, weekly, monthly or quarterly. Planned replacement contact lenses are also called disposable lenses.
Plano is a term that describes a lens prescription that has no spherical power. This measurement is often used in regards to cosmetic contacts, like color contacts, that are not worn for any corrective purposes. Occasionally, a toric lens will also be have a plano power if an individual only needs to correct an astigmatism not in conjunction with another condition like hyperopia or myopia.
Power, often abbreviated “PWR,” is a measurement used to indicate the amount of correction needed in a prescription and is also known as sphere or strength. Power is calculated in a unit of measurement equal to the reciprocal of the focal length of a lens called a diopter. The power of a lens is indicated with a plus sign (+) for hyperopia and a minus sign (-) for myopia. A prescription that is -2.00 will allow light waves to converge at half a meter for someone with myopia while a +2.00 diopter lens will do the same for a person who has hyperopia.
Also known as aging eye, presbyopia is a condition in which the crystalline lens, the part of the eye that allows us to focus, becomes more rigid and less flexible. When this happens, individuals lose the ability to focus on small, near details like words and images, such as in books or on menus. Most people first notice presbyopia somewhere around age 40. It occurs in all people, regardless of other vision problems. Presbyopia is unavoidable and most people treat it by wearing bifocals or reading glasses.
A progressive bifocal has a gradual change in lens power included in a single lens. Progressive lenses allow patients to focus at a range of distances from far away, to intermediate to close up.
All contacts have a certain period of time for which they are usable. All contact lenses should be replaced according to brand and recommendation of your eye doctor.
Rigid Gas Permeable Contacts, also known as RGPs, are an alternative to both hard and soft contact lenses. They are made of a stiff plastic that is oxygen permeable, allowing more oxygen to reach they eye than both soft and hard contacts do. RGP’s are also known as “gas perms.”
“Rx” is a standard abbreviation for a prescription that is used in all fields of medicine, not just eye care. It is derived from the Latin verb “recipe,” which means “to take” or “take thus,” as prescriptions are prescribed to be taken in a particular manner.
The sphere of a lens, often abbreviated “SPH,” is another way of indicating the power of a lens. The sphere is a measurement used to indicate the amount of correction needed in a prescription. The sphere of a lens is calculated in a unit of measurement equal to the reciprocal of the focal length of a lens known as a diopter. The sphere of a lens is indicated with a plus sign (+) for hyperopia and a minus sign (-) for myopia.
A spherical lens is one that does not include any correction for astigmatism. This is the most common type of contact lens prescribed and is very effective in treating nearsighted (myopic) and farsighted (hyperopic) individuals.
Steep is a base curve measurement. Rather than using a numerical value, some brands of lenses use terms like flat, median, and steep to describe a base curve. A steep base curve has more curvature on the back surface. While the term is uniform, the specific measurements will vary by brand.
The strength of a lens, often abbreviated “STR,” is another way of indicating the spherical power of a lens. The strength is a measurement used to indicate the amount of correction needed in a prescription. Strength is calculated in a unit of measurement equal to the reciprocal of the focal length of a lens called a diopter. The strength of a lens is indicated with a plus sign (+) for hyperopia and a minus sign (-) for myopia.
Toric lenses are cylindrical lenses that rely on gravity and eyelid interaction to rotate to the correct orientation in order to correct astigmatism. Toric lens prescriptions generally have two extra measurements, cylinder and axis, that indicate the amount and orientation of the astigmatism.
A vial is an alternate term sometimes used to describe a conventional lens. The term “vial” refers to the small vials in which most conventional lenses are packaged.
All contacts have a certain period of time for which they are usable and should be replaced according to brand and professional suggestion. The wear time, or replacement schedule, indicates how long a particular lens is intended to last. Brand manufacturers produce lenses with a particular wear time recommended, but your eye care provider will let you know what wear time is appropriate for you. Wear times include:
Conventional (six months to a year), daily replacement, one-to-two week replacement, and monthly replacement.
XW is an alternate abbreviation for extended wear lenses. While daily wear lenses must be removed after sixteen to eighteen hours, extended wear contacts do not have to be taken out each day. If recommended by a health care professional, individuals can sleep in their lenses. Due to health concerns, however, a doctor might suggest a patient remove his or her lenses at night, either regularly or occasionally. Extended wear lenses are often seen abbreviated either as EW or XW. They can also be called flex wear lenses.