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Key Terms

Abbreviations and Terminology

Prescription Abbreviations

An eyeglass prescription is a written order by an ophthalmologist or an optometrist to an optician for eyeglasses. It specifies the parameters to which the eyeglasses are to be made in order to correct blurred vision due to refractive errors, including myopia, hyperopia, astigmatism, and presbyopia.

Eyeglass prescriptions are written on paper pads that frequently contain a number of different abbreviations and terms:


D.V. is an abbreviation for "distance vision". This specifies the part of  the prescription designed primarily to improve far vision. In a bifocal lens, this generally indicates what is to be placed in the top segment.


N.V. is an abbreviation for "near vision". This may represent a single-vision lens prescription to improve near work, or the reading portion of a bifocal lens.


O.D. is an abbreviation for oculus dexter, Latin for "right eye


O.S. is an abbreviation for oculus sinister, Latin for "left eye".


SPHERE - The amount of long or short sightedness. This value specifies the optical power of a lens in diopters. It always has a plus (+) or a minus (-) sign in front or above it. There will always be a value for sphere, even if it is just zero (written as 0.00, Plano or P). If the vision in one eye is very poor, the word 'balance' or 'bal' may be written.


CYLINDER - This is the measure for astigmatism. Again, there is always a plus (+) or minus (-) sign present. There may not be a value of cylinder in your prescription.


This will only appear if there is a value for cylinder, as it relates to the cylindrical lens and denotes a reference point that indicates the direction of the power of the cylindrical lens. It will be a number between 1 and 180.

A Dioptre, or Diopter, is a unit of measurement of the optical power of a lens.

Eyeglass prescriptions will contain values here. The "spherical" and "cylindrical" columns contain lens strengths in diopters "axis" contains the direction of the cylinder axis in degrees.


Prism is a box on the Rx prescription form that is not normally filled in. Occasionally, when the two eyes are not properly aligned and looking directly at the same thing, PRISM can be ground into the lenses in order to re-align them. This can occur with strabismus or in situations where the eyes are properly aligned but are under a tremendous amount of strain in order to keep them aligned. The value under the 'prism' heading denotes the strength of the prism.


Rotation of the prism that is simply specified as 'base in',  'base out',  'base up' or 'base down'.  Only these four positions exist, however, orientations between these four positions can be specified by using combinations of horizontal and vertical prisms in the same lens.

Prism and Base are usually left empty; they refer to prescription features that are used to treat muscular imbalance or other conditions that cause errors in eye orientation, and are not seen in most prescriptions.


PUPILLARY DISTANCE - This measurement is used to accurately center the lenses in the frame that you have selected. PD is the distance from the center of the pupil (black circle) in one eye to the center of the pupil in the other eye. It is called Binocular PD. In cases where the pupils are not equidistant from the nose, a monocular PD is required. Monocular Pupillary Distance is measured from the center line of the nose to each pupil. In such a case you will have different values for the right eye (OD PD) and the left eye (OS PD). Another reason for more than one Pupillary Distance is the need to correct both distance and near vision. In this case the prescription will state Near PD and Dist PD. The Near PD measurement is shorter than the Distance PD by 2 to 5 mm because the eyes turn in at near distance.

P.D. measurement is a must for all RX orders for it insures that your prescription lenses in your glasses are aligned properly in front of the eyes. If you are unable have your Pupil Distance measured by optician, you can try to use a millimeter ruler.

Unless you specify an appropriate P.D. (pupillary distance), we will use an average P.D. to complete your order. The average P.D. has been provided to us by qualified opticians. By agreeing to these term and conditions you, the Customer, will be agreeing to let us use an average PD of 63mm for multifocal and single vision distance lenses, or a PD of 60mm for single vision reading lenses.

Reading Portion of the Eyewear Prescription


ADD POWER - Sometimes referred to as SEG POWER as well - It is additional plus power in the lower portion of a multifocal lens (progressive, bifocal, or trifocal) that is required for near vision.


SEGMENT HEIGHT - is the distance from the lowest portion of the eyewear to the top horizontal line of the Segment.

Please Note: If your doctor skipped any of the measured mentioned above (with the exception of PD and SEG HT, please leave it blank when submitting your oder.


Distance Between Lenses The distance between the lenses in a frame

Eye size

The size of the opening in an ophthalmic frame into which lenses are placed, measured horizontally at the widest point

Logo-style Rimless Frame (Rimless)

An ophthalmic frame that uses a monofiliment string to hold a grooved lens in a frame that does not entirely encircle the lens with metal or Zyl

Refractive index

The refractive index (or index of refraction) of a medium is a measure for how much the speed of light is reduced inside the medium.

Lens Materials

There are many different lens materials ranging from conventional plastic and glass; to state-of-the-art High Index materials that make lenses thinner and/or lighter than ever before possible. A lens' index of refraction is the measure of how steeply a material bends light. The higher the index of refraction, the more steeply it bends light; hence, the thinner the lens  can be made.


CR-39, or allyl diglycol carbonate, is a plastic polymer commonly used in the manufacture of eyeglass lenses. The CR stands for Columbia Resin.

CR-39 is transparent in visible spectrum and almost completely opaque in the ultraviolet range, it has high abrasion resistance, in fact the highest abrasion/scratch resistance of any uncoated optical plastic. CR 39 is about half the weight of glass . A wide range of colors can be achieved by dyeing of the surface or the bulk of the material. CR-39 is also resistant to most of solvents and other chemicals, to gamma radiation, to aging, and to material fatigue. It can withstand the small hot sparks from eg. welding.

What is high index?

The index of refraction refers to the ability of a material to bend light. The higher the index the more ability to bend light. High index lenses are thinner and lighter than their standard counterparts making them more cosmetically appealing and more comfortable to wear since they are lighter.

What is Aspheric?

An aspheric lens uses a parabolic curvature versus a spherical curvature. It has two benefits, one is that it reduces the thickness of a lens making it lighter and more cosmetically appealing, secondly it reduces magnification (or minification) making the wearer's eyes appear more realistic in size.


Polycarbonate is characterized as a very durable material. Although polycarbonate has high impact-resistance, it has low scratch-resistance and so a hard coating is applied to polycarbonate eye-wear lenses.  This polymer is highly transparent to visible light and has better light transmission characteristics than many kinds of glass.

Polycarbonate Generic name for the brand name plastic Lexan used to make highly shatter resistant ophthalmic lenses and a wide variety of other products which require optical clarity and shatter resistance (Space Shuttle windows, jet fighter canopies, bullet proof "glass")

The ultimate comfort! Extraordinarily light and thin. Exceptionally strong and durable. Farsighted prescriptions look much better in Ultralight Plus due to a flatter lens design that reduces the magnification of your eyes. The latest in polycarbonate technology, these lenses are up to 28% thinner, 36% lighter and 47% flatter than regular plastic lenses.

Crown Glass (optics)

Crown glass is type of optical glass used in lenses and other optical components.

Photochromic (Transition)

Transition lenses are lenses that darken on exposure to UV radiation. Once the UV is removed (for example by walking indoors), the lenses will return to their clear state. Photochromic lenses may be made of either glass or plastic. The reason these lenses darken in sunlight but not indoors under artificial light, is that room light does not contain the UV (short wavelength light) found in sunlight. Automobile windows also block UV so these lenses will not work very well in a car. Because photochromic compounds fade back to their clear state by a thermal process, the higher the temperature, the less dark photochromic lenses will be. This thermal effect is called "temperature dependency" and prevents these devices from achieving true sunglass darkness in very hot weather. Conversely, photochromic lenses will get very dark in cold weather conditions.

Glasses frames are commonly made from metal, horn or plastic. Lenses were originally made from glass, but many are now made from various types of plastic, including CR-39 or polycarbonate. These materials reduce the danger of breakage and weigh less than glass lenses. Some plastics also have more advantageous optical properties than glass, such as better transmission of visible light and greater absorption of ultraviolet light. Some plastics have a greater index of refraction than most types of glass; this is useful in the making of corrective lenses shaped to correct vision abnormalities such as myopia, allowing thinner lenses for a given prescription. Scratch-resistant coatings can be applied to most plastic lenses giving them similar scratch resistance to glass. Hydrophobic coatings designed to ease cleaning are also available, as are anti-reflective coatings intended to improve night vision and make the wearer's eyes more visible.

Ametropia Correction

Ametropia is a general term for an eye experiencing a refractive error, meaning that the image formed on the retina is not in sharp focus. Common refractive errors include:


This is a condition in which the image is focused in front of the retina, resulting in blurred images. Myopia (commonly referred to as nearsightedness) is a refractive error in which the eye exhibits too much power and is generally neutralized with a minus powered lens. Myopia can be either refractive (power of the eye is too strong) or axial, meaning that the eye is too long.


This is a condition in which the image is focused behind the retina, resulting in blurred images. Hyperopia (com­monly referred to as farsightedness) is a refractive error in which the eye seems to lack power and is generally neutralized with a plus or magnifying lens. Hyperopia can either be refrac­tive, meaning that the eye lacks refractive power, or axial, mean­ing the eye would be too short.


This is a condition in which the crystalline lens or more commonly the cornea is not spherical. The radii of cur­vature would differ 90 degrees apart from one another. The general shape would look more like the side view of an egg or a football. Astigmatism can be classified as Compound Hyperopic Astigmatism, a form of astigmatism in which both meridians focus light behind the retina. This form of astigmatism is cor­rected by a lens that has differing degrees of convergence in each meridian (both meridians magnify). Simple Hyperopic Astigmatism is a form of astigmatism in which one meridian focuses light on the retina and the other focuses light behind the retina. This refractive error would be corrected by a lens that would be pIano (having no power) in one meridian while the other would have converging (magnifying) power.

Compound Myopic Astigmatism

This is a condition in which both meridians focus light in front of the retina. This would be cor­rected by a lens that had differing degrees of diverging (minify­ing) power in each meridian. Simple Myopic Astigmatism is a condition where one meridian focuses light on the retina while the other focuses light in front. This form of astigmatism is cor­rected with a lens that would be pIano (having no power) in one meridian while the other would be diverging (minifying).


As we get older the lens behind our cornea, which is elastic and is capable of changing its power, begins to lose its elasticity. Near viewing becomes increasingly difficult as time progresses. For a presbyope to read or do tasks that require near viewing, they need a reading prescription. This reading pre­scription may be in the form of an ADD power. This is power that is added to the Rx to allow for near viewing. Presbyopes can be either myopic, hyperopic, or both. A condition in which one eye requires a plus power and the other a minus power is called antimetropia. Any of the above mentioned types of astigmatism may be present as well.

Presbyopia is not a disease as such, but a condition that affects everyone at a certain age. The first symptoms are usually noticed between the ages of 40-50, though in fact the ability to focus declines throughout life. For those with good distance vision, it may start with difficulty reading fine print, particularly if the lighting is poor, or with eyestrain when reading for long periods. Many advanced presbyopes complain that their arms have become "too short" to hold reading material at a comfortable distance.

Presbyopia, like other focus defects, becomes much less noticeable in bright sunlight. This is not the result of any mysterious 'healing effect' but just the consequence of the iris closing to a pinhole, so that depth of focus, regardless of actual ability to focus, is greatly enhanced, as in a pinhole camera which produces images without any lens at all.


During cataract surgery the often clouded crystalline lens is removed. The result is a significant lack of power. When corrected with spectacles the lenses appear bulbous. Today nearly all cataract patients are implanted with an 10L (Intraocular Lens). This lens can help replace the power lost by the removal of the crystalline lens.

Keep in mind that not all people who wear glasses have a refrac­tive error (though most do). In some cases the powers of the eyes are normal but they are not able to work as a team. Each eye sends its own images and if these two don't fuse up the result is a condition called diplopia (double vision). Misalignment of the eyes, called strabismus, is commonly the culprit. This can often be the result certain of muscle imbal­ances. To correct for this, prism can often be prescribed. The lens may have no refractive power but can still displace. the image. By displacing the image fusion can be obtained making vision more comfortable.

Protective eyewear is another industry staple. Not just industri­al safety glasses but lenses that protect us from harmful forms of radiation such as UV and IR. Anyone who regularly engages in sports, work or other activities in which harm can come to the eye as a result of projectiles or radiation (UV & IR) should seriously consider investing in a pair of protective eyewear.

Industrial Safety

According to Prevent Blindness America (Feb 2004) "Each business day, more than 2,000 U.S. workers experience job-related eye injuries, with 10 to 20 percent of them disabling because of temporary or permanent vision loss. Ninety percent of the injuries could have been prevented with protective eyewear." Today's industrial safety glasses use thick and very tough lens materials, polycarbonate or Trivex, combined with a sturdy frame. Combining these can effectively protect the eye from physical trauma. Safety lenses and frames must be ANSI approved and appropriately marked.

Sports Safety

Many sports can cause severe eye trauma, either as a result of impact from a ball or damage from accidental contact from another player. There is protective eye gear available for every sport. Just like industrial safety glasses, these incorporate tough and extremely impact resistant lens materi­als such as polycarbonate or Trivex. Often these frames will include in their design a strap which keeps the frame secure during active movement.


People may have certain lifestyles that require them to spend considerable time outdoors. In this case they may be exposed to high amounts of UV and wind. Lenses that absorb Ultraviolet Radiation can help reduce the chance of cataracts. Lenses can also be seen as enhancing the performance of the wearer. Lens colors such as browns and yellows can give the wearer higher contrast vision. This can be useful for golf, target shooting and driving.

Thinking in terms of what we need lenses to do and finding ways to incorporate as many of their uses into a single pair of glasses for the patient is essential. Even before the exam, enquir­ing about a patient's hobbies and work environment can be useful to deduce what they will be best suited for. The last thing we want is for the patient to use their glasses for something they were not intended for.

AR Coating

Patients want their eyeglass lenses to give them the best possible vision. But lenses without AR create reflections that interfere with vision, causing halos around light sources, ghost images and a loss of visual acuity. By virtually eliminating these reflections, AR coatings deliver peak performance.

For Driving: - AR coatings improve safety by allowing the wearer to quickly recognize potential hazards on the road, especially at night. Lens reflections from street lamps and headlights are virtually eliminated. For The Office: - Artificial light in an office environment quickly leads to eye fatigue and working on a computer puts a strain on the eyes. AR coatings provide the sharpest, most comfortable vision for a long work day. For Best Appearance & Communication: - AR coatings improve transparency and allow the wearer's eyes to be seen. Wearers enjoy direct eye contact, unhindered by distracting reflections.

Single Vision

A lens that has one focal power, usually a correction for nearsightedness, farsightedness, and/or astigmatism.


In addition to a correction for nearsightedness, farsightedness, and/or astigmatism, a bifocal contains an addition for near vision. There are two focal powers.

Progressive Lens

A progressive lens make a progressive change from distance vision to near vision providing an infinite range of powers in between. An additional advantage of progressive lenses is the lack of a bifocal line.