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Keratoconus - Frequently asked Questions: Spectacles

 

General Spectacles / Eye Glasses Q&A
  
 

What are Hi-Index lenses for Spectacle glasses, and how do I know if I need them?

Having thinner, lighter prescription lenses with a higher index of refraction is the ideal lens for any prescription which requires 4 or more diopters of correction. High index spectacle lenses can accomplish this for a more stylish look and comfortable feel of your new pair of spectacles. Please note that if you need contact lenses for your Keratoconus to correct your vision adequately, spectacles will not give you the same. However if you wear spectacles for your Keratoconus and they do correct your vision adequately (which usually means that your Keratoconus is mild) then you may benefit from Hi-Index lenses.

 

What are Transition lenses?

Transition lenses work the same way as photochromatic lenses but are made of plastic. There is also a thinner Transition lens for the patient that has a stronger prescription. This lens has UV protection and a very hard scratch coating.

 

What are Photochromatic lenses?

Photochromatic lenses are spectacle lenses that change color: from light to dark with the sun. They are lightly tinted indoors, and get darker with the sunlight outdoors. The lens is made in single vision, lined bifocals, trifocals and progressive lenses. There is also the Thin and Dark photochromatic eyeglass lens that works the same way as the regular photochromatic lens, but is thinner and lighter.

 

What is the basic Terminology?

Lenses commonly used for spectacles are either spherical powered or cylindrical powered, or a combination of the two. A spherical lens is one that focus a distant object to a point of focus. As in the 'crude' ASCII image below the distant parallel light is focused at F1

                     /\
   .......>..........||..
                     ||   .
                     ||     .
                     ||       . F1
                     ||     .
                     ||   .
   .......>..........||..
                     \/


A cylindrical lens has 2 focal points corresponding to the two different powers of the lens. Distant light is focused at two focal planes, labelled F1 and F2 in the diagram below. The diagram shows the axes of the cylindrical powers at 90 degrees to each other. 

                      ^
          ------------|--------------
                      |     l         -
          ............|...l             -
                      | l   .            -
                      +      .F1          - F2
                   l  |   .              -
   ..............l .  |.               -
               l      |              -
          ------------|-------------
                      V

  

How do I read a glasses prescription?

A prescription for spectacles will generally contain the powers for lenses of each eye, possibly also a reading addition power, information on the separation of the lenses and any special requirements for the lenses (eg, type of lens, tinting, coatings etc). Lens power is written in Dioptres, which is 1/focallength(metres). For example, a 5.0D (5.0 dioptre) lens has a focal length of 0.2m or 200mm

An example of a spectacle correction is -

Patient Name : Mr Patient
OD: +1.50/-1.00 x 35
OS: +1.75/-1.25 x 135
Add: +1.50
PD 62

The terms mean
OD - oculus dextrum or right eye
OS - oculus sinistrum or left eye
Add - Near addition
PD - interpupillary distance

The prescription values are read, firstly for the right eye

+1.50 - spherical power (positive)
-1.00 x 35 - cylindrical lens of negative power with axis at 35 degrees

Note: There may be variations in the way that the prescription is written since there are different standards for the way cylinders are represented and the coordinate system of the axes.

The coordinate system for the prescription is 0 to 180 degrees with 90 at the vertical or 12 o'clock position.

                                RIGHT EYE                                         LEFT EYE

                                        90                                                      90
                                         |                                                          |
                            135       |        45                                 135       |        45
                                \        |        /                                        \        |        /
                                  \      |      /                                            \      |      /
                                    \    |    /                                                \    |    /
                                      \  |  /                                                    \  |  /
                    180 ======+====== 0 < NOSE > 180 ======+====== 0

 

 

Why the difference between the way Optometrists and Ophthalmologists write the prescription ?

There are two ways of writing the prescription; referred to as "plus-cyl" form or "minus-cyl" form. The plus cyl form, sometimes used by Ophthalmologists is written

         +1.50/+0.50 x 125

The equivalent minus cyl form, generally used by optometrists is written

         +2.00/-0.50 x 35

These are identical prescriptions, simply different ways of recording the same thing, even thought the final number in each prescription is different.
How to convert between the two forms

To convert from "plus-cyl" form to "minus-cyl" form use the following steps:

1. Add the cylinder power to the sphere 2. Change the sign of the cylinder from + to - 3. Add 90 degrees to the axis if the original axis is less than 90, or 4. Subtract 90 degrees from the axis if the original is greater than 90.

To convert from "minus-cyl" to "plus-cyl" form the steps are reversed, i.e

1. Add the cylinder power to the sphere power 2. Change the sign of the cylinder from - to + 3. Add 90 degrees to the axis of the cylinder if axis < 90 4. Subtract 90 degres from the axis if axis > 90

 

What are the various Eyeglass Lens materials?

Spectacle lenses are made of two main types of materials - plastic or glass. Plastic lenses are often CR39 or polycarbonate. Both main materials come in a variety of refractive indexes, designed to minimise the thickness.

Hi Index lens materials are commonly 1.56, 1.60 and 1.67, as compared to 1.523 for common crown glass. The refractive index of plastic lenses is capped at around 1.74, whereas glass lenses range up to an index of 1.9, making them potentially thinner. However, hi-index glass lenses are very brittle and not suitable for many activities which carry a risk of lens impact.

Ultimately, the lens choice is very much a user application. Hi Index will provide a thinner edge and better cosmetic appearance but still weigh in heavy. Comparatively, plastic lenses are lighter but in high minus prescriptions will have a significantly thicker lens edge.

Furthermore the size of the frame will influence the overall edge thickness since a larger eyesize means a larger lens.

 

What are the different types of Spectacle Lenses?

 

  • Single Vision lenses

Spherical or with astigmatic correction - in plastic or glass

 

  • Bifocal Lenses

Made of a main lens and an additional segment for near vision - variety of shapes of near addition

 

  • Trifocal Lenses

Composed of a main lens and two reading ssegments; one for near and one for intermediate distances (about arm lengths)

 

  • Multifocals

Composed of main lens and continuously variable addition for various near reading positions

 

  • Aphakic Lenses

High power plus lenses used for cataract patients. - made with aspheric surfaces to minimise aberrations due to shape of the lens

 

  • Photochromic lenses

Darken when exposed to short wavelength radiation (300nm - 400nm) or ultra-violet light - rate of darkening and final transmission of the lens (saturation transmission) depends on the ambient temperature. - lenses will not become as dark when driving due to UV absorption by the car windscreen. - available in plastic lenses now - Transitions+ and Spectralite are two common lens names.

 

  • Coated Lenses:

Spectacle lenses may often be treated with a surface coating (vacuum coating) to reduce unwanted refle- ctions. - the surface reflection is related to the material index so that different lens materials will have different amounts of surface reflectance. - reflections are classified as 4 main types

(a) reflections visible by an observer from the front  

(b) internal reflections of the lens itself  

(c) reflections from behind the lens, eg overhead lighting  

(d) reflections from the corneal surface.

        

  • Hi Index Lenses:

Manufactured from a higher refractive index material that enables the lens thickness to be reduced whilst still maintaining the optical properties. That is the optical power of a lens is a combination of the two surfaces, front and back, and the thickness of the lens. - increased index of lens also means that there is a reduction in the base curve (back curve) so that there is greater thickness savings when comparing two lenses of the same center thickness.

 

  • Toughened Lenses

Either heat or chemical treated to increase impact strength - Used in industrial applications for added safety and prote- ction.

 

  • Aspheric Lenses

Elliptical curve surfaces designed to minimise lens abberations available as single vision and even some in progressive lens designs.

 

  • Polarized Lenses

Available in glass, plastic and hi-index plastic (n=1.56)


What are Ultra Violet absorption and Lenses?

Spectacle lenses, depending on the type of material will absorb varying amounts of ultraviolet light. In order of best absorbing lens the materials are polycarbonate, plastic and finally glass lenses. Different standards define the amount of absorption required, and/or the definition of the categories for spectacle lenses. The amount of UV absorption is also influenced by tinting, dying and lens coatings.

 

How can I reduce the edge thickness of the lens?

Since edge thickness is a function of lens power, centre thickness and curvature of the front and back curves (all inter-related) the edge thickness is influenced by the following features:

  •  Refractive index of the lens material
  •  Centre thickness (e.g safety lenses have a greater central thickness)
  •  Size of the spectacle frame, i.e larger eyesize means a bigger lens.





All content is intended as an informational series and should not be used as a substitute for medical advice.
 

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