Optical glossary

 

Main technical terminology

Optical medical prescriptions comprise complicated technical terms, which are not always easy to understand for a novice. However, we need this information in order to produce your glasses.

Below, you find a glossary, providing you with the main technical terminology, in a simple manner.

If you desire to obtain additional information, please contact our Client Support.

The basic anatomy of the eye

The cornea

This is the first lens of the eye. The cornea helps to process our vision.

The crystalline lens of the eye

This is the interior lens of the eye which allowing us a neat, clear vision from closeby as well as from far. This focus is established thanks to the flexibility of the crystalline lens, like the autofocus of a camera. This phenomenon is called 'accommodation'.

The retina

The retina is the inner coat of the eye, and is made out of several layers of nerve cells (rods and cones), which transform the image shaped by the eye via a nerve impulse to the brain.

The defects of our sight

Myopia

Myopia is a visual defect which essentially affects the vision of objects that are far away. It means that the image of objects is formed in front of the retina instead of on the retina. Therefore the person has a blurred vision of objects that are far away, but has a good near vision.

The higher the level of myopia, the shorter the distance from which the myopic has a clear vision, becomes.

Myopia or nearsightedness is corrected with a negative lens (divergent or concave). The negative lens is thick at the edges and thin in the centre. The principle is to push the image of the object on the retina in order to re-establish a clear vision.

Hyperopia

Hyperopia is a defect of vision caused by an imperfection in the eye, affecting the vision of nearby objects. The image of objects is formed at the back of the retina instead of on the retina, which causes a blurred vision of objects that are closeby.

In weak cases of hyperopia, the person has a good near-vision, but the continuous effort can cause tired eyes.

Hyperopia is corrected with a positive lens (convergent or convex). Positive lenses are thicker in the centre and thinner at the edges. Thanks to this principle, the image of the objects is placed again correctly on the retina.

Astigmatism

Astigmatism is a relatively common visual defect. With astigmatism, the corneal curvature is slightly more oval, while the curve should be round. It causes a blurred vision from close-by as well as from far.

Astigmatism can be associated to myopia, hyperopia or presbyopia.

Astigmatism can be corrected by cylindrical lenses or toric lenses. The thickness of the lens varies according the strength needed.

Presbyopia

Presbyopia is not a defect of the vision. Rather it is part of a natural aging process. Presbyopia occurs from the age of forty and is inevitable. This phenomenon is due to the aging of the crystalline lens, which causes a decline in the accommodation abilities of the eye (causing difficulties of focus). The nearby vision becomes gradually more challenging.

Presbyopia can be corrected with different kinds of lenses :

  1. Simple reading glasses will correct your near-vision, a distance of circa 40 centimetres. To see far, you constantly need to lower or take off your glasses.
  2. Proximity glasses, also called office lenses, allow you to have a clear vision from close-by, while keeping a clear vision of the people around you. They are generally used for computer and office work.
  3. Multifocal lenses : here we speak about progressive lenses.

The elements on the prescription

Sphere (SPH)

The sphere is a dioptric value.

The sphere describes the degree of myopia when preceded by the symbol -, and the degree of hyperopia when preceded by a +.

Cylinder (CYL)

The Cylinder is a dioptric value. It compensates astigmatism.

The cylinder value is not required in all cases. It is only necessary when there is a deformation of the cornea. If an eyeglass prescription includes cylinder power, it also must include an axis value. The cylinder value is generally the second value on the left side of your prescription and can have a negative or positive symbol.

Axis (AXIS)

The Axis (AXIS) is important when you choose a lens.

We need the value of the axis to indicate the position of the cylinder on the lenses of your glasses.

The axis value indicates in what direction the lenses should be positioned in the frame, where the lens should be thicker and where it should be thinner. The axis is defined with a number from 1 to 180. It is not preceded by a negative or positive symbol, and is indicated in degrees, for example: 178°. This value has been communicated to you by your ophthalmologist or your optician. The axis value is generally the third value on the left side of your prescription.

You only have to fill out an axis value if you also have a cylinder value.

The axis value is always indicated in degrees (°) and in a number from 1 to 180. There is always two values: one for the right eye and one for left.

Prism

Prisms are prescribed to compensate for eye alignment problems that can bring about diplopia (double vision).

Caution : If there is a prism value on your prescription, you can not buy glasses on Glasses24. It signifies that there is a defect in the angle of your vision, which demand accommodations that are very specific for your eyes.

Diopters (D)

A diopter is a unit of measure allowing to indicate the power of the lenses to converge the image on the retina.

The diopter equals the inverse of the focal length in decimal form.

The focal length is the distance needed to see an object clearly.

For example, a nearsighted person with a diopter of -1 can see clearly, without glasses, objects that are situated within a distance of one meter. Everything that is further away, will be blurred. He or she needs a lens with a diopter of -1 for a clear vision. A nearsighted person with a diopter of -2 can only clearly see objects if they are at a distance of 50 centimetres or less. He needs a lens with a diopter of -2 (focal length = 0,5m, diopter = inverse of the focal length = 1 divided by 0,5m = 2).

Oppositely, a person with hyperopia who needs a lens with a diopter of +1 can neatly see objects at a distance of one meter or more, whereas everything that is closer, is blurred. A person with hyperopia who needs a lens with a diopter of +2 can see clearly the objects at a distance of more than 50 cm, but not those closerby.

Pupillary distance (PD)

The pupillary distance (PD) is the distance between the pupils measured in millimetres (mm). It is also possible to indicate the total interpupillary distance, of for example 65mm, instead of the individual values for each eye, as in R32,5 mm L32,5 mm (32,5 mm + 32,5 mm = 65 mm).

You will find this distance on the certificate of your glasses or on your prescription under the section (PD). If you do not know this distance, we can measure it from a frontal photo of your face, without glasses and with the use of a credit card, or a card of the same dimensions, that you will have to position against your forehead.

Caution : this measure is essential for the production of your glasses. It allows us to position your lenses correctly.

An illustration to facilitate the understanding of the information on the certificate of your glasses :

If you find the information « PD: 30/34 », this indicates a distance of 30 mm for the right eye and a distance of 34 mm for the left eye. The total PD corresponds in this case to 64 mm.

The cylinder value is required only when there is a deformation of the cornea. If you have selected a cylinder value, you are obliged to filled out the axis value. The cylinder value is to be found in the second position at the left of your prescription and has a negative or a positive symbol.

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