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{{short description|Order written by an eyewear prescriber}}
This article describes the optics of an ordinary eyeglass prescription, which is used to correct small refractive errors in the optical system of the eye. The effect of these errors is to create a blurred image. By correcting these errors, the eyeglass lens removes the blur.
{{Refimprove|date=August 2014}}


[[Image:GlassPerscriptionRig.jpg|thumb|right|Using a [[phoropter]] to determine a prescription for eyeglasses]]
==Abbreviations==


An '''eyeglass prescription''' is an order written by an eyewear prescriber, such as an [[optometrist]], that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient. If an [[eye examination]] indicates that corrective lenses are appropriate, the prescriber generally provides the patient with an eyewear prescription at the conclusion of the exam.
<div style="float:right;width: 400px;margin-left:5px;text-align:center">
[[image:Specrx-prescription2.jpg]]
</div>
*''R<sub>x</sub>'' means ''prescription'' and is derived from the abbreviation "rx" from the [[Latin]] word ''recipire,'' "to take." Literally, ''Rx'' is an instruction to the patient "to take" what is specified in the prescription. See [[Medical prescription]].


The parameters specified on spectacle prescriptions vary, but typically include the patient's name, power of the lenses, any prism to be included, the [[pupillary distance]], expiration date, and the prescriber's signature. The prescription is typically determined during a [[Eye examination#Refraction|refraction]], using a [[phoropter]] and asking the patient which of two lenses is better, or by an [[Autorefractor|automated refractor]], or through the technique of [[retinoscopy]]. A [[dispensing optician]] will take a prescription written by an [[optometrist]] and order and/or assemble the frames and lenses to then be dispensed to the patient.
*''O.D.'' is an abbreviation for ''oculus dexter,'' Latin for "right eye." (Some eyeglass prescriptions simply say "left" and "right" instead of "O.S." and "O.D.") ''Oculus'' means "eye." An eye doctor is sometimes called an "oculist" (although eye doctors themselves usually prefer to use either the term ''ophthalmogist'' or ''optometrist''). The Latin word ''dexter'' means "right" (as opposed to "left"). The words "dexterity" and "dextrous" derive from this root, because the right hand is usually more skillful than the left.
{{TOC limit|3}}


==Components of a sphero-cylindrical correction==
*''O.S.'' is an abbreviation for ''oculus sinister,'' Latin for "left eye." The Latin word "sinister" means "left," which was once associated with the idea of evil. The "bar sinister" in medieval heraldry meant that the bearer was of illegitimate descent.


===Sphere component===
::''Note:'' The word [[ophthalmologist]] has an "h" after the "p". The first syllable is pronounced "aahf," like the "oph" in "sophomore." It comes from the Greek root "ophthalmos-" meaning "eye;" ophthalmology literally means "the science of eyes." The word [[optometrist]] comes from a different Greek root, "opto-," meaning "vision." An optometrist literally means "one who measures vision." In the United States, these are specific terms referring to specific state licensing and courses of professional study. There is considerable overlap in the scope of the two specialties. Optometrists undergo four years of training and have received a doctor of optometry (OD) degree. They are doctors, though not ''medical'' doctors, and are licensed to detect, treat, manage, and refer cases of eye disease. Ophthalmologists are medical doctors (MD) who have typically performed an additional three to five years of residency in ophthalmology, and received board certification. The American Academy of Ophthalmology emphasizes the distinction by promoting the phrase "Eye MD" to refer to ophthalmologists. The American Optometric Association characterizes optometrists as "primary eye care providers." Very broadly, ophthalmogists tend to be oriented toward the treatment of eye disease, and optometrists toward the maintenance of optimum vision in healthy eyes.
[[File:Basecurve power.png|thumb|right|upright=1.4|Positive [[meniscus lens]]es provide positive diopters, while negative meniscus lenses provide negative diopters.]]
Every corrective lens prescription includes a spherical correction in [[diopter]]s. Convergent powers are positive (e.g., +4.00 D) and condense light to correct for farsightedness/long-sightedness ([[hyperopia]]) or allow the patient to read more comfortably (see [[presbyopia]] and [[binocular vision#Disorders|binocular vision disorders]]). Divergent powers are negative (e.g., −3.75 D) and spread out light to correct for nearsightedness/short-sightedness ([[myopia]]). If neither convergence nor divergence is required in the prescription, "plano" is used to denote a refractive power of zero.


The term "sphere" comes from the geometry of lenses. Lenses derive their power from curved surfaces. A spherical lens has the same curvature in every direction perpendicular to the [[optical axis]]. Spherical lenses are adequate correction when a person has no [[astigmatism]]. To correct for astigmatism, the "cylinder" and "axis" components specify how a particular lens is different from a lens composed of purely spherical surfaces.
*''D.V.'' and ''N.V.'' stand for "distant vision" and "near vision" and refer to the different corrections in the upper and lower portions of a bifocal lens. The "N.V." portion of a "single vision" lens prescription will be blank.


===Cylinder component===
*''Spherical,'' ''Cylindrical,'' and ''Axis'' will be explained further below. Most eyeglass prescriptions will contain values here. The "spherical" and "cylindrical" columns contain lens strengths in ''diopters'' (see below); "axis" contains the direction of the cylinder axis in degrees.
Patients with [[astigmatism]] need a [[cylindrical lens]], or more generally a [[toric lens]] to see clearly. The geometry of a toric lens focuses light differently in different meridians. A meridian, in this case, is a plane that is incident with the optical axis. For example, a toric lens, when rotated correctly, could focus an object to the image of a horizontal line at one focal distance while focusing a vertical line to a separate focal distance.


The power of a toric lens can be specified by describing how the cylinder (the meridian that is most different from the spherical power) differs from the spherical power. Power evenly transitions between the two powers as you move from the meridian with the most convergence to the meridian with the least convergence. For regular toric lenses, these powers are perpendicular to each other and their location relative to vertical and horizontal are specified by the axis component.
*''Prism'' and ''Base'' are usually blank; they are refer to prescription features that are used to treat muscular imbalance or other conditions that cause errors in eye orientation.


There are two different conventions for indicating the amount of cylinder: "plus cylinder notation" and "minus cylinder notation". In the former, the cylinder power is a number of diopters more convergent than the sphere power. That means the spherical power describes the most divergent meridian and the cylindrical component describes the most convergent. In the minus cylinder notation, the cylinder power is a number of diopters more divergent than the sphere component. In this convention, the sphere power describes the most convergent meridian and the cylinder component describes the most divergent. Europe typically follows the plus cylinder convention while in the United States the minus cylinder notation is used by optometrists and the plus cylinder notation is used by ophthalmologists. Minus cylinder notation is also more common in Asia, although either style may be encountered there. There is no difference in these forms of notation and it is easy to convert between them:<ref>{{Cite web |title=NHS Optical Benefits in the UK - Wikibooks, open books for an open world |url=https://en.wikibooks.org/wiki/NHS_Optical_Benefits_in_the_UK#Transposing_the_Format |access-date=2017-11-01 |website=en.wikibooks.org |language=en}}[https://www.mouqy.com/guides/how-to-buy-prescription-glasses-online/]</ref>
==Blur==
* Add the sphere and cylinder numbers together to produce the converted sphere
* Invert the sign of cylinder value
* Add 90° to axis value, and if the new axis value exceeds 180°, subtract 180° from the result
For example, a lens with a vertical power of −3.75 and a horizontal power of −2.25 could be specified as either −2.25 −1.50 × 180 or −3.75 +1.50 × 090.


===Axis component===
Here are some examples of the kind of blurred images that can result from refractive errors; we will be discussing them in more detail below. For now, note that not all kinds of blur are the same.
[[File:Geraet beim Optiker.jpg|thumb|right|This [[phoropter]] is set to an axis of 180 for each eye. This can be seen by noticing the tiny white arrows that are horizontal as they point to the tiny numbers that line the opening the patient looks through. Click the image to see a full resolution version where the individual axis markings become more visible.]]The axis defines the location of the sphere and cylinder powers. The name "axis" comes from the concept of generating a cylinder by rotating a line around an axis. The curve of that cylinder is 90° from that axis of rotation. When dealing with toric lenses, the axis defines the orientation of the steepest and flattest curvatures relative to horizontal and vertical. The "3 o'clock" position is defined as zero, and the 90th meridian is a vertical line. A horizontal line passes through both zero and the 180th meridians. By convention, a horizontal axis is recorded as 180.


In a regular toric lens, the flattest and steepest curvatures are separated by 90°. As a result, the axis of the cylinder is also the meridian with the same power as the recorded sphere power. The cylinder power, as defined above is the power that is most different from the sphere power. Because they are defined relative to each other, it is important to know if the lens is being described in minus cylinder notation, where the sphere power is the most convergent / least divergent power. When using plus cylinder notation, the opposite is true.
[[Image:Specrx-letterscamblur.png]]<br>
[[Image:Specrx-lettersastigblur.png]]<br>
[[Image:Specrx-lettersastigblur2.png]]<br>
[[Image:Specrx-letterseyeblur.png]]<br>


If the lens is spherical (there is no cylinder component) then there is no need for an axis. A prescription like this is written with D.S. (diopters sphere) after the sphere power (e.g., −3.00 D.S.). This verifies that the prescription is truly spherical rather than the cylinder power being omitted in error.


==Lenses==
====Summary====
* correction power is measured in diopters
* by convention, an axis of 90° is vertical, 0° or 180° are horizontal
* if the cylinder power is positive, the lens is most convergent 90° from the axis
* if the cylinder power is negative, the lens is most divergent 90° from the axis
* if the cylinder power is zero, the lens is spherical and has the same power in every meridian


=====Sample prescription=====
Depending on the optical setup, lenses can act as magnifiers, lenses can introduce blur, and lenses can correct blur. Many people first encounter lenses in the form of magnifying glasses, and think of lenses as magnifiers. Eyeglasses may, in fact, have a small magnifying or reducing effect, but that is an unintentional (and undesirable) side effect. Eyeglasses do not improve vision by magnifying images; they improve vision by reducing blur.
A prescription of ''−1.00 +0.25 × 180'' describes a lens that has a horizontal power of −1.00 D and a vertical power of −0.75 D. In the US, only ophthalmologists write prescriptions in + cylinder. An optometrist would write a prescription in - (minus) cylinder. All spectacle and contact lenses would be made in minus cylinder. Therefore, the above prescription would be written as −0.75 −0.25 × 90.


==Abbreviations and terms==
==Lens strength==
[[Image:Specrx-prescription2.jpg|thumb|right|An eyeglass prescription]]


Similar to [[medical prescription]]s, eyeglass prescriptions are written on paper pads or included in a patient's [[electronic health record]], and contain a number of different abbreviations and terms:
The values given in the "sphere" and "cylinder" columns of an eyeglass prescription are lens strengths in [[diopter]]s, abbreviated D. The higher the number of diopters, the stronger the lens.
*''DV'' 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.
*''NV'' is an abbreviation for ''near vision''. This may represent a [[Corrective lens#Single vision|single-vision]] lens prescription to improve near work, or the reading portion of a bifocal lens.
*''OD'' is an abbreviation for ''oculus dexter'', [[Latin]] for ''right eye'' from the patient's point of view. ''Oculus'' means ''eye''.
*''OS'' is an abbreviation for ''oculus sinister'', Latin for ''left eye'' from the patient's point of view.
*''OU'' is an abbreviation for ''oculi uterque'', Latin for ''both eyes''.
*:N.B.: In some countries, such as the United Kingdom, ''RE'' (right eye), ''LE'' (left eye), and ''BE'' (both eyes) are used. Sometimes, just ''right'' and ''left'' are used.
*''SPH'', ''CYL'', and ''AXIS'' are values for describing the power of the lens using plus cylinder or minus cylinder notation.
*''ADD'' is an abbreviation for ''Near Addition''. This is the additional refractive power to be combined, or added, to the distance power to achieve the ideal near power.
*''Prism'' and ''Base'' Prism refers to a displacement of the image through the lens, often used to treat [[strabismus]] and other binocular vision disorders. The [[Prism correction|prism]] value is measured in [[prism diopter]]s, and ''Base'' refers to the direction of displacement.
* PL is an abbreviation for plano or plain, meaning no prescription
*''PD'' or ''IPD'' Pupillary Distance or Interpupillary Distance, respectively. It is the distance between pupil centers.
*''BVD'' Back [[vertex distance]] is the distance between the back of the spectacle lens and the front of the [[cornea]] (the front surface of the eye). This is significant in higher prescriptions (usually beyond ±4.00D) as slight changes in the vertex distance for in this range can cause a power to be delivered to the eye other than what was prescribed.


== References ==
A +10 diopter lens would make a good magnifying glass. Eyeglass lenses are usually much weaker, because eyeglasses do not work by magnifying; they work by correcting focus.
{{reflist}}


== External links ==
Stacking lenses combines their strength. A +1 diopter lens combined with a +2 diopter lens forms a +3 diopter system.
* [[Wikibooks:NHS Optical Benefits in the UK|UK optical vouchers explained]]


{{Use dmy dates|date=March 2017}}
[[Image:Specrx-1d2d.png]]


{{DEFAULTSORT:Eyeglass Prescription}}
Lenses come in positive (plus) and negative (minus) strengths. You can usually tell whether a lens is positive or negative by looking through it. Positive lenses tend to enlarge things when you look through them; negative lenses tend to diminish the size of things when you look through them. Because eyeglass lenses are usually weak, they don't enlarge or diminish very much.
[[Category:Ophthalmology]]

[[Category:Corrective lenses]]
Positive eyeglass lenses can concentrate sunlight, like a burning glass. Usually, however, they are much too weak to set fire to anything.
[[Category:Optometry]]
[[Category:Glasses]]
[[Image:Specrx-rglass1.jpg]]
[[Image:Specrx-rglass2.jpg]]
[[Image:Specrx-rglass3.jpg]]
[[Image:Specrx-rglass4.jpg]]<br>

This series of pictures show the shadow cast by a pair of 1 diopter drugstore "reading glasses" outdoors in sunlight as we hold it farther and farther away from a wall. As the distance from the wall increases, the shadow of the frame seems to thicken and the bright area in the center gets smaller and brighter. It slowly changes from being "eyeglass-shaped" to circular.

Negative lenses spread sunlight instead of concentrating it.

A negative lens combined with a positive lens removes some of its strength. A -2 lens combined with a +5 lens forms a +3 diopter system.

[[Image:Specrx-5dm2d.png]]

A -3 lens stacked on top of a +3 lens looks almost like clear glass, because the combined strength is 0.

[[Image:Specrx-3dm3d.png]]

In science textbooks, positive lenses are usually diagrammed as convex on both sides; negative lenses are usually diagrammed as concave on both sizes. In a real optical system, you usually get the best optical quality when most rays of light are roughly normal to the lens surface. In the case of an eyeglass lens, this means that the lens should be roughly shaped like a cup with the hollow side toward the eye. So most eyeglass lenses are meniscus in shape.

==Spherical lenses and spherical correction==

Usually:
*the "spherical" component is the main correction
*the "cylindrical" component is "fine tuning."

Depending on the optical setup is, lenses can act as magnifiers, lenses can introduce blur, and lenses can correct blur.

Whatever the setup, spherical lenses act equally in all directions; it magnifies, blur or corrects blur the same amount in every direction.

An ordinary magnifying glass is a kind of spherical lens. When a spherical lens acts as a magnifier, it magnifies equally in all directions. Here, note that the magnified letters are magnified both in height and in width.

[[Image:Specrx-sphermag1.png]]

Similarly, when a spherical lens puts an optical system out of focus and introduces blur, it blurs equally in all directions:

[[Image:Specrx-StarBlur.png]]

Here is how this kind of blur looks when viewing an eye chart. This kind of blur involves no astigmatism at all; it is equally blurred in all directions.

[[Image:Specrx-letterscamblur.png]]

==Cylindrical lenses and cylindrical correction==

Some kinds of magnifying glasses, made specifically for reading wide columns of print, are cylindrical lenses. When a cylindrical lens acts as a magnifier, it magnifies only in one direction. For example, the magnifier shown magnifies letters only in height, not in width.

[[Image:Specrx-cylmag1.png]]

Similarly, when a cylindrical lens puts an optical system out of focus and introduces blur, it blurs only in one direction.

[[Image:Specrx-StarBlurA.png]]

This is the kind of blur that results from uncorrected ''astigmatism.'' The letters are smeared out directionally, as if an artist had rubbed his or her thumb across a charcoal drawing.A cylindrical lens of the right strength can correct this kind of blur. When viewing an eye chart, this is how this kind of blur might appear:

[[Image:Specrx-lettersastigblur.png]]

Compare it to the kind of blur that is equally blurred in all directions.

[[Image:Specrx-letterscamblur.png]]

When an eye doctor measures your eye&mdash;a procedure known as ''refraction''&mdash;usually he or she begins by finding the best spherical correction. If there is astigmatism, the next step is to remove it by adding the right amount of cylindrical correction.

==Axis==

Spherical lenses just have a strength, such as +1.0D, or -2.5D.

Astigmatism, however, causes a ''directional'' blur. Here are two examples of the kind of blur you get from astigmatism. The letters are smeared out directionally, as if an artist had rubbed his or her thumb across a charcoal drawing.

A cylindrical lens of the right strength can correct this kind of blur. The second example is a little bit more blurred, and needs a stronger cylindrical lens.

But notice that in addition to being smeared more, the second example is smeared out in a different ''direction.''

[[Image:Specrx-lettersastigblur2.png]]

[[Image:Specrx-lettersastigblur.png]]

A spherical lens is the same in all directions; you can turn it around, and it doesn't change the way it magnifies, or the way it blurs:

[[Image:Specrx-sphermag2.png]]<br>

[[Image:Specrx-sphermag1.png]]

A cylindrical lens has both a strength and an ''axis.'' Turning it around so that the axis points in different directions changes the way it magnifies, and the way it blurs.

[[Image:Specrx-cylmag2.png]]

[[Image:Specrx-cylmag1.png]]

The ''axis'' specification gives the orientation of the axis of the cylindrical correction.

== Distant vision and near vision ==


The DV portion of the prescription describes the corrections for '''d'''istant '''v'''ision. For most people under forty years of age, this is the only part of the prescription that is filled in. The NV or near-vision portion of the prescription is blank, because a separate correction for near vision is not needed.

The NV portion is used in prescriptions for bifocals.

In younger people, the lens of the eye is still flexible enough to accommodate over a wide range of distances. With age, the lens hardens and becomes less and less able to accommodate.

This is called "presbyopia;" the "presby-" root means "old" or "elder." (It is the same root as in the words "priest" and "presbyterian.")

The hardening of the lens is a continuous process, not something that suddenly happens in middle age. It is occurring all along. All that happens around middle age is is that the process progresses to the point where it starts to interfere with reading.

When nursery school children want to examine something carefully, they just hold it very close to their eyes. They don't need magnifying glasses because they have such good near vision.

This chart (which is ''approximate'') shows that a schoolchild has over ten diopters of accommodation, while a fifty-year-old has only two. This means that a schoolchild is able to focus on an object about 10 cm. (4") from the eye, a task for which an adult needs a magnifying glass with a rated power of about 3.5X.

[[Image:Specrx-accom.png]]

== Variations in prescription writing ==

There is a surprising amount of variation in the way prescriptions are written; the layout and terminology used is not uniform.

When no correction is needed, the spherical power will sometimes be written as "0.00" and sometimes as "Plano" or "Pl" (because the lens, although not flat, is optically equivalent to a flat piece of glass).

When cylindrical correction is needed, the mathematics and optics of the way lenses combine mean that there are two different ways to write the same correction. One is called the plus-cylinder form and the other the minus-cylinder form. These two prescriptions are equivalent:

{| border=1
|Spherical
|Cylindrical
|Axis
|-
|2.00
|1.00
|90
|}

{| border=1
|Spherical
|Cylindrical
|Axis
|-
|3.00
| -1.00
|180
|}

Both of them specify a power of 2.00 diopters at the 180 degree axis and 3.00 diopters at the 90 degree axis.

The first one specifies a 2.00 spherical component, which by itself would give a power of 2.00 diopters along both the 180 and 90 degree axis, and adds a 1.00 cylindrical component at 90 degrees. The result is 2.00 diopters at 180 degrees and 2.00 + 1.00 = 3.00 diopters at 90 degrees.

The second specifies a 3.00 spherical component, which by itself would give a power of 3.00 diopters along both the 180 and 90 degree axis, and subtracts a 1.00 cylindrical component at 180 degrees. The result is 3.00 - 1.00 = 2.00 diopters at 180 degrees and 3.00 diopters at 90 degrees.

== See also ==

*[[Eye]]
*[[Lens]]

Latest revision as of 02:42, 7 March 2025

Using a phoropter to determine a prescription for eyeglasses

An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient. If an eye examination indicates that corrective lenses are appropriate, the prescriber generally provides the patient with an eyewear prescription at the conclusion of the exam.

The parameters specified on spectacle prescriptions vary, but typically include the patient's name, power of the lenses, any prism to be included, the pupillary distance, expiration date, and the prescriber's signature. The prescription is typically determined during a refraction, using a phoropter and asking the patient which of two lenses is better, or by an automated refractor, or through the technique of retinoscopy. A dispensing optician will take a prescription written by an optometrist and order and/or assemble the frames and lenses to then be dispensed to the patient.

Components of a sphero-cylindrical correction

[edit]

Sphere component

[edit]
Positive meniscus lenses provide positive diopters, while negative meniscus lenses provide negative diopters.

Every corrective lens prescription includes a spherical correction in diopters. Convergent powers are positive (e.g., +4.00 D) and condense light to correct for farsightedness/long-sightedness (hyperopia) or allow the patient to read more comfortably (see presbyopia and binocular vision disorders). Divergent powers are negative (e.g., −3.75 D) and spread out light to correct for nearsightedness/short-sightedness (myopia). If neither convergence nor divergence is required in the prescription, "plano" is used to denote a refractive power of zero.

The term "sphere" comes from the geometry of lenses. Lenses derive their power from curved surfaces. A spherical lens has the same curvature in every direction perpendicular to the optical axis. Spherical lenses are adequate correction when a person has no astigmatism. To correct for astigmatism, the "cylinder" and "axis" components specify how a particular lens is different from a lens composed of purely spherical surfaces.

Cylinder component

[edit]

Patients with astigmatism need a cylindrical lens, or more generally a toric lens to see clearly. The geometry of a toric lens focuses light differently in different meridians. A meridian, in this case, is a plane that is incident with the optical axis. For example, a toric lens, when rotated correctly, could focus an object to the image of a horizontal line at one focal distance while focusing a vertical line to a separate focal distance.

The power of a toric lens can be specified by describing how the cylinder (the meridian that is most different from the spherical power) differs from the spherical power. Power evenly transitions between the two powers as you move from the meridian with the most convergence to the meridian with the least convergence. For regular toric lenses, these powers are perpendicular to each other and their location relative to vertical and horizontal are specified by the axis component.

There are two different conventions for indicating the amount of cylinder: "plus cylinder notation" and "minus cylinder notation". In the former, the cylinder power is a number of diopters more convergent than the sphere power. That means the spherical power describes the most divergent meridian and the cylindrical component describes the most convergent. In the minus cylinder notation, the cylinder power is a number of diopters more divergent than the sphere component. In this convention, the sphere power describes the most convergent meridian and the cylinder component describes the most divergent. Europe typically follows the plus cylinder convention while in the United States the minus cylinder notation is used by optometrists and the plus cylinder notation is used by ophthalmologists. Minus cylinder notation is also more common in Asia, although either style may be encountered there. There is no difference in these forms of notation and it is easy to convert between them:[1]

  • Add the sphere and cylinder numbers together to produce the converted sphere
  • Invert the sign of cylinder value
  • Add 90° to axis value, and if the new axis value exceeds 180°, subtract 180° from the result

For example, a lens with a vertical power of −3.75 and a horizontal power of −2.25 could be specified as either −2.25 −1.50 × 180 or −3.75 +1.50 × 090.

Axis component

[edit]
This phoropter is set to an axis of 180 for each eye. This can be seen by noticing the tiny white arrows that are horizontal as they point to the tiny numbers that line the opening the patient looks through. Click the image to see a full resolution version where the individual axis markings become more visible.

The axis defines the location of the sphere and cylinder powers. The name "axis" comes from the concept of generating a cylinder by rotating a line around an axis. The curve of that cylinder is 90° from that axis of rotation. When dealing with toric lenses, the axis defines the orientation of the steepest and flattest curvatures relative to horizontal and vertical. The "3 o'clock" position is defined as zero, and the 90th meridian is a vertical line. A horizontal line passes through both zero and the 180th meridians. By convention, a horizontal axis is recorded as 180.

In a regular toric lens, the flattest and steepest curvatures are separated by 90°. As a result, the axis of the cylinder is also the meridian with the same power as the recorded sphere power. The cylinder power, as defined above is the power that is most different from the sphere power. Because they are defined relative to each other, it is important to know if the lens is being described in minus cylinder notation, where the sphere power is the most convergent / least divergent power. When using plus cylinder notation, the opposite is true.

If the lens is spherical (there is no cylinder component) then there is no need for an axis. A prescription like this is written with D.S. (diopters sphere) after the sphere power (e.g., −3.00 D.S.). This verifies that the prescription is truly spherical rather than the cylinder power being omitted in error.

Summary

[edit]
  • correction power is measured in diopters
  • by convention, an axis of 90° is vertical, 0° or 180° are horizontal
  • if the cylinder power is positive, the lens is most convergent 90° from the axis
  • if the cylinder power is negative, the lens is most divergent 90° from the axis
  • if the cylinder power is zero, the lens is spherical and has the same power in every meridian
Sample prescription
[edit]

A prescription of −1.00 +0.25 × 180 describes a lens that has a horizontal power of −1.00 D and a vertical power of −0.75 D. In the US, only ophthalmologists write prescriptions in + cylinder. An optometrist would write a prescription in - (minus) cylinder. All spectacle and contact lenses would be made in minus cylinder. Therefore, the above prescription would be written as −0.75 −0.25 × 90.

Abbreviations and terms

[edit]
An eyeglass prescription

Similar to medical prescriptions, eyeglass prescriptions are written on paper pads or included in a patient's electronic health record, and contain a number of different abbreviations and terms:

  • DV 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.
  • NV 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.
  • OD is an abbreviation for oculus dexter, Latin for right eye from the patient's point of view. Oculus means eye.
  • OS is an abbreviation for oculus sinister, Latin for left eye from the patient's point of view.
  • OU is an abbreviation for oculi uterque, Latin for both eyes.
    N.B.: In some countries, such as the United Kingdom, RE (right eye), LE (left eye), and BE (both eyes) are used. Sometimes, just right and left are used.
  • SPH, CYL, and AXIS are values for describing the power of the lens using plus cylinder or minus cylinder notation.
  • ADD is an abbreviation for Near Addition. This is the additional refractive power to be combined, or added, to the distance power to achieve the ideal near power.
  • Prism and Base Prism refers to a displacement of the image through the lens, often used to treat strabismus and other binocular vision disorders. The prism value is measured in prism diopters, and Base refers to the direction of displacement.
  • PL is an abbreviation for plano or plain, meaning no prescription
  • PD or IPD Pupillary Distance or Interpupillary Distance, respectively. It is the distance between pupil centers.
  • BVD Back vertex distance is the distance between the back of the spectacle lens and the front of the cornea (the front surface of the eye). This is significant in higher prescriptions (usually beyond ±4.00D) as slight changes in the vertex distance for in this range can cause a power to be delivered to the eye other than what was prescribed.

References

[edit]
  1. ^ "NHS Optical Benefits in the UK - Wikibooks, open books for an open world". en.wikibooks.org. Retrieved 1 November 2017.[1]
[edit]

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