Optical prescription – Wikipedia

Use of a refraction unit to select optical correction in glasses glasses

A Optical prescription is a prescription written by a vision professional, such as an ophthalmologist (in France) or by an optometrist (in Anglo-Saxon countries or in Belgium), which indicates the value of all the parameters that the prescriber deems necessary for the realization of equipment in corrective glasses for a patient.

If the view examination indicates that corrective glasses are justified and necessary, the prescriber generally writes a prescription with prescription of optical equipment.
The parameters indicated on the prescription include the dioptric power of each glass to be made to correct the visual vagueness due to a refractive disorder such as myopia, hyperopia, astigmatism, and/or presbyopia. The power of the corrective glass is determined by the use of a refractor (or refractive unit) or manually, associated with a questionnaire with the patient (“With this glass, it’s better, it’s worse?”) , and an automatic refractor.
An optician or a French orthoptist is not currently authorized to prescribe optical equipment.
An optometrist, which holds a qualification diploma greater than that of an optician, can prescribe an optical correction in France. But unlike a prescription made by an ophthalmologist, this act cannot be reimbursed by Social Security.
Following the prescription of the glasses, the ophthalmologist can request an orthoptic assessment, in order to verify the oculo-motor balance (if the eyes work well at the same time) of the patient with the new prescribed correction. Indeed, some patients who have an imbalance may have headaches, dizziness, the eyes that burn or who blush quickly, …
An orthoptist holds a diploma in ability in orthoptics and has been trained by ophthalmologists (notably in refraction), neurologists, … He will verify that the prescribed correction does not cause imbalance in the patient and if so The case he can advise the prescribing doctor a small readjustment. But also, he checks the patient’s imbalances without correction, to bring better comfort of the latter when the glasses have been prescribed (if imbalance there). These acts are reimbursed by social security.
The optician receives the prescription, advises and commands the glasses and the frame after having taken morphological measures on the carrier, then size and sets up the glasses, assembles the optical equipment, and adjusts the frame, delivers, sells the optical equipment And provide maintenance advice.

In France, with regard to optical prescription, we are talking about “3 os” (ophthalmologists, orthoptists and opticians “). Optometry is not yet recognized in France.

Like a medical prescription, optical prescriptions are written with many abbreviations and different terms:

• VL is an abbreviation for V far . This means the necessary correction to improve vision in the distance.
• VP is an abbreviation for V nearby ision . This means the necessary correction to improve vision closely, to read for example.
• ADD , who means to ddition , indicates the refractive power to add to the spherical power of each eye for the creation of progressive glasses.
• FROM is an abbreviation for oculus dexter, Latin, that is to say, he right .
• YOU is an abbreviation for oculus sinister, Latin, that is to say, he left .
• AND is an abbreviation for œ he left , which is much more used.
• OR is an abbreviation for Eyes each , Latin, that is to say both eyes .
• Dg is a much more used abbreviation on a daily basis than OR .
• A power spherical Corrects a refractive (or Ametropy) disorder of the eye with a convergent (for hyperopia) or divergent (myopia) refractive power in all meridians.
• A power cylindrical Corrects the astigmatism of the eye by adding or subtracting a cylindrical power in a meridian described by the prescribed axis.
• Axe Indicates the angle, in degrees, of one of the two main meridians of the prescribed cylinder. The main meridian is indicated by cylindrical correction, which can be expressed in cyl “+” or cyl “-“. The axis is measured on a virtual semicircle with a horizontal line that starts at 0 °, located at 3 a.m., and ends at 180 °, at 9 a.m.

The columns sphere And cylinder indicate the power of the glass in diopter .

• Values prism And base are not always present on all prescriptions. THE prism Referring to a movement of the image through the glass, and is used to straighten an eye muscle or a defect in vergence (see vergence) which causes disorders in the orientation or fixing of the eye. The prismatic correction is measured in “prismatic dioptry”, and the base refers to the direction of the trip.
• Pupillary gap ( EP ) is the distance in millimeters between the two pupils. Terms interpupillary gap or Pupillary half-gauges , or monocular pupillary gap are used indifferently, especially when prescribing bifocal or progressive glasses. These measures indicate, in millimeters, the distance between each pupil and the middle of the nose, that is to say the place where the center of the frame rests. Not to be confused with the eye convergence which is reflex when an object is close to the eyes. The inter-pupillary distance in far vision, when the eyes are fixed endlessly in the primary position of gaze, is different from that in near vision when the subject looks at nearby objects. These pupillary differences are essential for the sale of any optical equipment, the monocular pupillary differences essential for the sale of progressive glass or high correction glasses. Pupillary differences can be measured by the optician with a corneal reflective pupillometer, a simple ruler or a digital measurement device.
• Glass/eye distance ( Double ) is the distance between the back of the glass and the top of the cornea (the front of the eye). It is essential for corrections greater than ± 4.00 D, because a change of distance between the eye and the glass can cause in the carrier a perception of vagueness or other symptoms.

A blurred vision is the subjective feeling or the perception of a defect by the eye. The vagueness may appear differently depending on the importance and the type of amengerient. The fuzzy vision is corrected by focusing light on the retina by the use of corrective optical glasses or contact lenses, or using refractive surgery that modifies the shape of the cornea.

Corrective glasses sometimes have undesirable effects such as magnificent, decrease, distortion of images, the presence of colored rings, the modification of the direction of depth, etc. The glasses improve vision first by reducing the blur.

Visual acuity (AV) is the ability to distinctly see the details of an object and is measured by an acuity scale. The acuity scale, or optotype tables, is the tool used by vision professionals to compare the patient’s visual acuity with that of another individual. Although many acuity scales exist, the most used in France are the monoyer scale for far vision and the Parinaud scale for near vision. Other current opotypes are the rings of the Landolt scale and the Snellen scale, which was developed by a Dutch optometrist Hermann Snellen (in) in the 1860s ^{[ first ]} . Usually these scales have 11 rows of letters in capital letters and the first row has only one letter and the other rows include letters which are increasingly small.

With people unable to read the letters, for various reasons, such as children still not educated, or who do not know the alphabet or who have a handicap, vision professionals can use the Snellen scale so -called “Trident de Snellen”, which shows the letter “e” in capital letters, in various sizes and oriented by stages of 90 degrees. In this case, the refractionist will ask the patient to use his hand (with the tense fingers) to indicate the direction in which the “fingers” of the “e” point: right, left, at the bottom, bottom ^{[ first ]} . But also the children’s monoyer scale, with animal drawings.

An average visual acuity varies between 12/10 and 10/10. Few individuals have 20/10 or more visual acuity, unlike many animals, especially birds of prey, whose AV is estimated at 20/5 or better ^{[ first ]} .

The scales do not give information on peripheral vision, the perception of depth or the perception of colors and in fact are insufficient to qualify vision, or assess ocular health. A full view of view will include other tests. However, acuity scales are useful to decide which patient needs corrective lenses or contact lenses and assess what the effective correction is necessary.

A standard human eye without refraction abnormality (= emmetrope) has a refractive power of around 60 diopters.
The values ​​expressed in the columns sphere And cylinder specify the optical power of the diopters glasses, abbreviated “d”. The higher the number of diopters, the more the glass converges or diverge light. A dioptrie (d) is the opposite of the focal length in meters. If a glass has a focal length of ^first ⁄ ₃ meters, then the glass has a power of 3 diopters.

The glasses may have positive (more sign) or negative (signal sign) powers. Since a positive power can grow the image of an object and a negative power can decrease it, it is often possible to say if a glass is positive or negative simply by looking through.

The convex glasses force the light rays to converge and the concave glasses to diverge. A negative glass attached to a positive glass gives an optical system with a power equal to the sum of the two glasses, thus a glass of −2.0 D combined with a glass of +5.0 D forms a dioptric system of +3, 0 D.

By convention, positive glasses are generally drawn as convex on each side; Negative glasses are generally drawn as concaves on each side. In a real optical system, the best optical quality is usually reached when most of the light rays are approximately perpendicular (i.e., at the right angle) on the surface of the glass. In the case of a corrective glass, it means that the glass should be cut more or less with the more hollow side to the eye, which is why the shape of most corrective glasses is that of a meniscus glass.

The most important characteristic of a glass is its main focal length, or its opposite which is called glass power. The main focal length of a glass is determined by its refractive index, the curvature rays of the two surfaces, and the middle where the glass is located. For two thin convex glasses attached, all parallel rays should be focused on a point known as the main home. The distance between the glass and this point is the main focal length of the glass. For two adjoining thin glasses where the rays diverge, the main focal length is the distance to which the rays projected behind should enter together and is a negative sign. For a thick glass made with spherical surfaces, the focal distance will be different due to different rays, and this change is called spherical aberration. The focal length for different wavelengths will also be slightly different, and this will be called chromatic aberration ^{[ 2 ]} .

Usually:

• the component spherical is the main correction
• the component cylindrical is “a fine adjustment” of the correction.

In a simple spherical glass, each surface is a portion of sphere. When a spherical glass acts like a magnifying glass, it grows in all meridians in an identical way. The gross letters are enhanced both in height and in width. Likewise, when a spherical glass puts an optical system in defecting and generates blur, the vagueness is present in all meridians.

Cylindrical glasses and cylindrical power [ modifier | Modifier and code ]

The surfaces of a cylindrical glass are portions of cylinder. Therefore, this glass refracts light. When a cylindrical glass acts as magnifying, it grows in one direction. For example, magnification shows gross letters only in height and not in width. Similarly, when a cylindrical glass puts an optical system in defecting and causes blurring, the vagueness occurs only in a single meridian.

When a refractionist does an examination of sight – a procedure called refraction – It usually starts by determining the best spherical correction. If there is an astigmatism, the next step is to compensate for it by adding the right cylindrical correction.

Spherical power glasses have the same power in all meridians, such as +1.00 D, or −2.50 D. A cylindrical glass has a refractive power in a single direction. The orientation of this power is indicated on the prescription by a axe .

The axis describes the orientation of the axis of cylindrical glass in degrees and varies from 1 to 180 °, from the horizontal line when the refractionist takes the measures, that is to say when we look at the patient, therefore from the On the external side of the glass.

The total power of a glass with a sphero-cylindrical correction changes accordingly: in the meridian indicated by the axis, the power is equal to the known value as “sphere”. When you move as on a clock dial, the power in a given meridian will be very close to the sum of the values ​​given for the sphere and the cylinder until reaching the 90 ° meridian from the meridian indicated by the axis, Or the power is equal to the sum of the sphere and the cylinder.

Refractionists use the term spherical equivalent To refer to the effective focus power of the eye if only spherical aberrations exist. It can be defined as follows:

For example, the spherical equivalent of a prescription giving a sphere of -4.0 D and a cylinder of -2.0 D to 180 ° is equal to -5.0 D.

Graphics retracing the maximum amplitude of accommodation according to the age after Duane ^{[ 3 ]}

The VL indicated on the prescription indicates the correction necessary to see clearly in the distance. For most individuals under the age of forty, only this part is filled. The VP is used for the prescription of double focus or progressive lenses, necessary to correct presbyopia.

Up to forty years, the crystalline of the eye is quite flexible and elastic to accommodate, dilating or retracting, at all distances. With age the lens hardens and becomes less and less capable of accommodating. This phenomenon is called presbyopia.

The tightening of the lens is a continuous phenomenon, and not something that suddenly appears between 40 and 45 years old. It is perceptible at this age because it is the moment when the process progresses to the point that it penalizes vision for reading. Everyone naturally needs corrective glasses to read from 40–45 years.

Because young children have better accommodation than adults, they sometimes examine objects by holding them closer to their eyes than an adult would. The accommodation amplitude decreases by around 14.0 D for an individual of 10 years, up to 0.50 D to 60 years.

The theoretical correction of the addition can be rough depending on the age of the patient, when it is less than 3.0 D, as in the table below but remains an approximation.

When an individual accommodates, his eyes also converge. Anomalies of this accommodation/convergence function can lead to consulting an orthoptist.

The optical axis is the center of the glass where the light passes and where it is not deflected. The visual axis is the place where light passes through the eye to the retina and where it is admitted that it is not deflected.

Sometimes the corrective glasses are given with the different optical axis from the visual axis. This creates a prismatic effect. Prisms can be used to diagnose and deal with the problems of binocular vision and other orthoptic problems that cause diplopia such as:

Variants in writing a prescription [ modifier | Modifier and code ]

When no correction is required, spherical power can be transcribed as 0.00 or flat ( pl. ) or afocal . In this case the glass has no refractive power.

When a cylindrical correction is necessary to correct astigmatism, two ways of writing are possible, either in “cyl +” rating, or in “cyl -“, depending on whether the chosen axis is in positive or negative cylindrical correction.

For example, these two prescriptions are equivalent:

You must read :

• The CYL +rating gives a correction (hyperopia) of +2.00 D and a second correction (astigmatism) of +1.00 D along an axis of 150 °
• The notation in cyl- gives a correction (hyperopia) of +3.00 D and a second correction (astigmatism) of −1.00 D along an axis of 60 °

It can be noted that the two ratings lead to the same spherical equivalent of +2.50 D.

The conversion from one notation to another, is an operation called transposition ^{[ 4 ]} . By noting ‘the values ​​in the other notation, the relationship is as ^{[ 5 ]} (Available online here ^{[ 6 ]} ) :

• Sphere ‘= Sphere + Cylindre
• Cylinder ‘= – cylinder
• Ax ‘= ax + 90 °

In the previous example, the transposition into cyl- is (+2.00) +(+1.00) = +3.00 d and a axis of 150 ° +90 ° = 240 ° (or 60 °) and The cyl + transposition is (+3.00) + (-1.00) = +2.00 D and an axis of 60 ° + 90 ° = 150 °.

In practice, the refractionist uses the two notations and the optician commands cyl+ glasses to his glassmaker.