Vision and EyeCare FAQ
Section 2: Spectacles


2.0 Terminology of Spectacles

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 90degrees to each other.
                      ^
          ------------|--------------  
                      |     l         -
          ............|...l             -
                      | l   .            -
                      +      .F1          - F2
                   l  |   .              -
   ..............l .  |.               -
               l      |              -
          ------------|-------------
                      V
Other WWW references:
URL: http://www.west.net/~eyecare/optical_lens_prescriptions.html

URL : http://www.nbn.com/youcan/glasses/eyeglasses.html
(Questions and answers on spectacles and glasses)


2.1 How to 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


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


Other WWW reference:
URL: http://www.west.net/~eyecare/optical_lens_prescriptions.html


2.2 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 D
          ---------------
            +0.50 x 35

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

              +2.00 D
         ---------------
           -0.50 x 125
2.3 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 degress from the axis if axis > 90

Other WWW references:
URL: http://www.west.net/~eyecare/optical_lens_prescriptions.html



2.4 Eyeglass Lens materials

Spectacle lenses are made of two main types of materials - plastic or glass. Plastic lenses are often CR39 or polycarbonate. Glass lenses come in a variety of refractive indexes, designed to minimise the thickness.
HiIndex lens materials are commonly 1.56, 1.60 and 1.67, as compared to XXXX for common crown glass.

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.



2.5 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
               reflections.
              - 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 centre thickness.

+ Toughened Lenses
              - Either heat or chemical treated to increase impact strength
              - Used in industrial applications for added safety and protection.

+ 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)

2.6 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.



2.7 Standards Requirements for Spectacle Lenses

International and local standards define the properties of lenses that are required for spectacle lenses. There are different standards for prescription spectacle lenses and sunglass or fashion lenses.

In the USA the standards include -

-U.S. Sunglass standard for durability is Ct. SGSTD40 (I think)
-U.S. Standard for tints and UV requirements ANSI Z80.3

You can search the ANSI Standards from the WEB using the ANSI homepage
URL: http://www.ansi.org/

The standard information is located at the ANSI site as well as other useful information on opththalmic and optometric standards
..
+ Prescription Lenses

+ Sunglass lenses
              The requirements will include some or all of the following attributes
                            + surface finish
                            + visual quality
                            + flame propagation
                            + dimensions
                            + refractive power of lens
                            + prismatic power of lens
                            + lens curvature
                            + thermal stability
                            + optical transmission (UV, visible and near Infra-Red)
                            + uniformity of colour
                            + colouration limits (red factor, violet factor)


2.8 Cost of frames

Varies depending on the type of material, eg plastic frame to titanium metal.



2.9 Cost of lenses

Like frames varies with the type of the material. Also depends if the lens requires customised grinding to fit the lens the to the frame.



2.10 Reducing 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 (eg safety lenses have a greater central thickness)
+ size of the spectacle frame, i.e larger eyesize means a bigger lens.



Index:

© Grant Sayer , email: grants@research.canon.oz.au
 

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  http://www.hon.ch/Library/Theme/VisionFaq/section2.html Last modified: Thu Jul 25 2002