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From: [email protected] (Grant Sayer)
Newsgroups: sci.med.vision,sci.med,sci.answers,news.answers
Subject: sci.med.vision: Frequently Asked Questions (FAQ), Part 2/5
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Date: 19 Feb 1997 08:31:50 +1100
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Summary: This posting contains a list of Frequently Asked Questions concerning
                vision and eyecare.
Xref: senator-bedfellow.mit.edu sci.med.vision:22074 sci.med:202852 sci.answers:5924 news.answers:95061

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+============================================================================+
                 FREQUENTLY ASKED QUESTIONS: Vision and EyeCare
                              Part 2/5
                   (Copyright (C), 1995  Grant Sayer)
+============================================================================+

+============================================================================+
+ Section 1: Optics of the Eye and General Information                       +
+============================================================================+

1.1: What is Myopia ?
---------------------
Myopia  is often referred to as "short-sightedness" or "near-sighted".

An eye is myopic when the "far point"; a point at which light from an object
is focussed on the retina, is located at a finite distance in front of the eye.
Myopia can be due to either an eye which is too long relative to the optical
power of the eye (axial myopia), or because the optical power of the eye is
too high relative to the length of the standard eye (refractive myopia).  The
focus is correctly adjusted with a "minus" power lens, or concave lens.

WWW resources:
URL: http://www.eyenet.org
                 - American Academy of Ophthalmology.

URL: http://www.ozarksol.com/medsource
                 - contains nice pictures explaining the focal point for each
                       condition

URL: http://www.west.net/~eyecare/myopia.html
                 - details and pictures to explain myopia.

URL: http://www.execpc.com/~warren/optweb/errors.html
         - John Warren OD home page

URL: http://www.web-xpress.com/vhsc/eyedz.html
                 - informational brochures on eye conditions

1.2 What is Hyperopia ?
-----------------------
Hyperopia is often referred to as "long-sightedness" or "far-sighted".

An eye is hyperopic when the far point is at a virtual point behind the eye.
Generally the hyperopic eye is too short with respect to the refractive state
of the standard eye (ie an emmetropic eye or eye requiring no optical
correction) or because the optical power of the eye is too low relative to
the length of the standard eye.  The focus is correctly adjusted using a
"plus" lens power or convex lens.

WWW resources:
URL: http://www.eyenet.org
                 - American Academy of Ophthalmology.

URL: http://www.ozarksol.com/medsource
                 - contains nice pictures explaining hyperopia and showing the
                       focal point of image formation in this condition

URL: http://www.west.net/~eyecare/hyperopia.html
                 - details and pictures to explain the condition.

URL: http://www.execpc.com/~warren/optweb/errors.html

1.3 What is Emmetropia ?
------------------------
Emmetropia is just another name for an eye that has no optical defects and
a precise image is formed on the retina.

1.4 What is Astigmatism ?
-------------------------
An astigmatic eye generally has two different meridians, at 90degrees to
each other, which cause images to focus in different planes for each meridian.
The meridians can each be either myopic, hyperopic or emmetropic.
The correction for astigmatism is a lens power at a particular direction
of orientation [ see section 4.1 ]

Astigmatism causes images to be out of focus no matter what the distance.
It is possible for an astigmatic eye to minimise the blur by accommodating,
or focusing to bring the "circle of least confusion" onto the retina.

URL: http://www.west.net/~eyecare/astigmatism.html
                 - details and pictures to explain the condition.

URL: http://www.web-xpress.com/vhsc/astigm.html
                 - diagrams and questions and answers about the causes, symptoms
                       of astigmatism.

URL: http://www.sna.com/etonline/vision
               - still under construction but contains a large amount of eyecare
                 information

1.5 What is Presbyopia ?
------------------------
Presbyopia describes the condition whereby the amplitude of accommodation,
or ability to focus on objects at near, decreases with increasing age.
It is corrected by a different prescription for reading, which is additive to
the  normal spectacle correction used for distance vision.

Some recent research indicates that presbyopia may be caused by structural
changes in the tendons and elastic fibres of the posterior ciliary body.  The
age related increase in fibrillar material could cause decreased compliance
of the posterior insertion of the ciliary muscle.

For more details see
Tamm E., Lutjen-Drecoll E., Jungkunz W., & Rohen J.W., "Posterior attachment
of ciliary muscle in young, accommodating old presbyopic monkeys."
Invest. Ophthal & Vis. Sci. 1991 Apr;32(5):1678-92

URL: http://www.west.net/~eyecare/presbyopia.html

1.6  How is Visual Acuity Measured
----------------------------------
Visual acuity is the measure of the sensitivity of the visual system.
It is expressed in Snellen notation, expressed as a fraction, where the
numerator indicates the test distance and the denominator denotes the
distance at which the letter read by the patient subtends 5 minutes of arc.
Normal vision is expressed as 20/20 (or 6/6 in countries where metric
measurements are used).

An acuity of 20/60 means that the patient was tested at 20feet but could
only see letters that a person with normal vision could read at 60feet.

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

+============================================================================+
+ 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-sci.lib.uci.edu/~martindale/Medical.html#Oph
Contains references to other web sites on glasses and how they work

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 <Person F Bar>

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 references:
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