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- From: grants@research.canon.com.au (Grant Sayer)
- Newsgroups: sci.med.vision,sci.med,sci.answers,news.answers
- Subject: sci.med.vision: Frequently Asked Questions (FAQ), Part 2/5
- Supersedes: <faq-part2_843777040@research.canon.com.au>
- Followup-To: sci.med.vision
- Date: 19 Feb 1997 08:31:50 +1100
- Organization: Canon Information Systems Research Australia
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- Approved: news-answers-request@MIT.EDU
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- Reply-To: grants@research.canon.oz.au
- NNTP-Posting-Host: mama.research.canon.com.au
- 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
-
- Archive-name: vision-faq/part2
- Posting-Frequency: monthly
- Version: $Id: faq-part2.txt,v 1.25 1995/11/22 22:38:38 grants Exp $
-
- +============================================================================+
- 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
- .
- + 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.
-
- 2.11 Internet Ordering of Prescriptiions and Sunglasses
- ------------------------------------------------------
- The are a wide variety of companies that advertise on the internet for
- dispensing of prescriptions and sunglasses. The list is too long
- to list here and the reader is advised to use one of the Web search
- engines to locate any potential sites.
-
- +============================================================================+
- + Section 3: Contact Lenses +
- +============================================================================+
- 3.0 WWW Sites on Contact Lenses
- --------------------------------
- URL: http: //www.sna.com/etonline/vision
- - currently under construction and has potential for useful information.
-
- 3.1 Difference between soft and hard contact lenses
- -----------------------------------------------------
- Soft lenses are manufactured from a plastic hydrogel polymer,
- HydroxyEthylMethacrylate (HEMA) which has a varying water content
- (38% - ~70%). Lens size is between 13.00 and 14.50mm. Centre thickness
- from ~30um
-
- Hard contact lenses are manufactured from a rigid material,
- PolyMethylMethacrylate (PMMA). This material can be combined with other
- plastics to increase the oxygen permeability. Lens size is between
- 8.0mm and 10.00mm. Centre thickness from ~100um.
-
- 3.2 How to Read a Contact Lens Description:
- -------------------------------------------
-
- A contact lens prescription differs from a spectacle prescription primarily
- by the addition of lens parameters. It should be noted that the power of a
- contact lens will not necessarily be the same as that of a spectacle
- prescription due to the optics of a lens being closer to the eye. The
- technical term is "effectivity" of the lens.
-
- The other information that is often given with a contact lens prescription
- will be the diameter of the lenses, basecurve, possibly additional curves
- lens material, design or manufacturer name.
-
- For example,
-
- OD: 8.6/14.0/-1.00DS
- OS: 8.6/14.0/-1.50DS
-
- The data is
- BaseCurve: 8.6mm
- Diameter: 14.0mm
- Power: -1.00D and -1.50
-
- 3.2 Types of Soft Contact Lenses
- ----------------------------------
- Soft contact lenses vary depending on either the refractive correction that
- they are prescibed for or the design and type of material.
-
- Major types include :
-
- 3.2.1 Spherical
- -----------------
- Single prescription power 360 degrees around the lens.
-
- 3.2.2 Toric
- -------------
- Contains both a spherical and cylinder component to correct prescriptions
- which have astigmatism. Lenses may be thicker in one meridian or have
- modified thickness profiles to enable the lens to maintain correct orientation
- on the eye.
-
- 3.2.3 Disposable
- ------------------
- Spherical or toric contact lenses which are designed to be worn for a certain
- time period, eg, weekly, two- weekly, monthly. Lenses are generally
- sold in a "blister" combination pack, eg 3 months supply with intention
- that lenses are "disposed" of at the end of the time period.
-
- 3.2.4 Extended Wear
- --------------------
- Lens manufactured from a high-water content material and/or with a very
- thin centre thickness to enable maximum oxygen transmission. Lenses can
- be worn overnight, or for a number of days without removal (see
- comments on complications for further information).
-
- 3.2.5 Bifocal Lenses
- ---------------------
- There are a variety of designs in bifocals, essentially all trying
- to provide a transition or reading zone for use at near. The different
- designs include;
-
- + aspheric multifocal -
- + simultaneous vision concentric - has either a central near or
- distance zone with surrounding zone of opposite type to centre
- (eg, centre near,distance surround)
- + diffraction/holographic - based on diffraction grating principles
-
- 3.3 Types of Rigid Contact Lenses
- ---------------------------------
- Rigid contact lenses also vary depending on the material and the design of
- the shape of the contact lens. Rigid lenses provide a "new" front surface
- to the eye and help in the elimination of astigmatism because the tear film
- fills the gap between the lens and the astigmatic cornea.
-
- For information on lens conditions see the URL listed below.
-
- Major Material types include;
-
- 3.3.1 PMMA
- ------------
- Original material used in the construction of "hard" contact lenses.
-
- 3.3.2 RGP
- ----------
- Combination of PMMA and other polymers to increase the oxygen permeability
- and allow longer wearing time. Lenses are also often larger in diameter
- than PMMA to increase the comfort of wearing the lens
-
-
- Major design types include;
-
- 3.3.3 Spherical
- ---------------
- same as for soft contact lenses
-
- 3.3.4 Toric
- -----------
- Can be either toric periphery, which is used for fitting reasons, or
- bi-toric used to correct residual astigmatism.
-
- 3.3.5 Bifocal
- -------------
- The different types include;
-
- + multifocal aspheric - light from all distances focus without interruption
- by lines or zones of the lens
- + concentric - different zones of the lens for distance and/or near
- + zone based - parallel bands of focal zones for various distances
- + segmented - three basic types
- (a) lens divided into two roughly equal parts like an
- executive bifocal
- (b) lens divided into two unequal parts with near part like
- a segment of bofocal spectacles, either flat-top or
- crescent shaped
- (c) internally fused segment of different refractive index.
-
- Research photos of RGP Lenses on the eye include
-
- URL: http://www.route-one.co.uk/route-one/scotlens/
-
- This site maintains some nice research photos of RGP lenses with conditions
- such as
- - Drying
- - Surface Wetting
- - Deposits
- - Mucous Deposits
-
- 3.4 Costs of Contact Lenses
- ----------------------------
- Varies depending on the type of contact lenses
-
- Lens Type FittingFee MaterialFee Total
-
- In Australia
- Spherical Soft ~A$200
- Toric Soft ~A$240
- Disposable ~A$85 (3months supply)
- RGP Spherical ~A$240
-
- In UK:
- Spherical Soft ~UK$20
-
- In USA:
- Spherical Soft 100 60 160
- Toric Soft 155 170 325
- Disposable 85 100/6mths 185 (6mths)
- RGP Spherical 115 85 200
-
-
- 3.5 Solutions required for CL maintenance
- -----------------------------------------
- In general all contact lenses, whether soft of rigid type, will
- require a cleaning, disinfecting and rinsing solution. Enzymatic agents
- (protein remover) may also be required to reduce build up of protein on
- the lens surface.
-
- 3.6 Common CL Wearer Questions
- ------------------------------
-
- 3.6.1 Why do I have to clean my Contact Lenses ?
- --------------------------------------------------
- Cleaning removes surface debris and bacteria that may adhere to the contact
- lenses.
-
- 3.6.2 Why is there a limit to the length of wearing time ?
- -----------------------------------------------------------
- The cornea the "clear part of the eye", is avascular or without a blood supply.
- It is avascular otherwise it wouldn't transmit light without distortion.
- As a result of this living tissue being avascular it is necessary to
- obtain oxygen from the atmosphere. The wearing of a contact lens interrupts
- the flow of oxygen to the cornea and due to changes in the metabolic pump of
- the corneal cells the tissue thickens, called oedema.
-
- Contact lenses, as described in Section XXX, are manufactured from material
- that allows maximum oxygen transmission. But this is still not exactly the
- same as the 20.4% therefore the lens wearing time must be controlled to
- reduce oxygen deprivation to the cornea.
-
- 3.6.3 Can I lose the lens behind my eye ?
- ------------------------------------------
- Short answer is no. The conjunctiva, the tissue that covers the white part
- of the eye (the sclera) forms a cul-de-sac between from the edge of the cornea
- to the eyelid margin. Sometimes a contact lens, especially a soft
- contact lens may roll up and become difficult to find. An eyecare practitioner
- will be able to locate the lens and remove it.
-
- 3.6.4 How do I tell if I've lost the lens from my eye?
- -------------------------------------------------------
- Cover the other eye to determine if vision is still clear from the eye where
- you suspect that you've lost the lens. If vision is blurred then more than
- likely the lens is either dislodged from the cornea or has fallen from the
- eye.
-
- 3.6.5 How can I tell if I've put them in the wrong eye ?
- ---------------------------------------------------------
- Check the vision of each eye by covering alternate eyes with your hand.
- If the vision is unclear then try swapping the lenses and then recheck the
- vision.
-
- 3.6.6 How to tell if the lenses are inside out ?
- -------------------------------------------------
- Soft lenses will appear as a smooth dish shape when placed on the end of a
- finger and when the lens is the right way around.
-
- 3.6.7 How do I know when to dispose of my disposable lenses ?
- -------------------------------------------------------------
- The lenses will not automatically self-destruct at the end of the wearing
- period :-) The lenses should be discarded when the lens wearing time has
- elapsed, as advised by your eyecare professional. This wearing time/period
- has been chosen to minimise complications with contact lens wear so it should
- be adhered to !
-
- 3.6.8 Can I store my lenses in tap water ?
- -------------------------------------------
- Short answer - No !. There are a number of "nasty" micro- organisms,
- especially acanthomeba (sp) which likes feeding on corneal tissue.
- Storing lenses in water also defeats the purpose of using a disinfecting
- solution as there is no disinfection occurring. If anything you are
- exposing your lenses to more potentially dangerous microorganisms.
-
- 3.6.9 Can I store my lenses dry ?
- ----------------------------------
- Soft lenses - definitely not !. Soft lenses stored dry will turn into
- "corn-flakes" and only go "snap-crackle and pop" when you touch the lens.
- Hard lenses should also be stored in a soaking/disinfecting solution to
- reduce lens contamination.
-
- 3.6.10 How long does it take to adapt to new lenses ?
- ------------------------------------------------------
- Soft lenses are generally worn for 2-4hrs on the first day and the wearing
- time is increased by about 2hrs per day, up until 8hrs of daily wear is
- achieved. The rate at which the wearing time is increased and the maximum
- number of hours that the lenses can be worn will depend on the recommendations
- of your eyecare practitioner.
-
- Rigid lenses are generally worn for 2-4hrs on the first day with an increase
- of 1-2hrs each day until 8 hours of daily wear is obtained. Again the rate
- at which the wearing time is increased and maximum number of hours of
- wearing time will depend on the person and the recommendations of your
- eyecare practitioner.
-
- 3.6.11 Why do I have to use protein cleaner ?
- ----------------------------------------------
- Contact lenses are exposed to a complex environment whilst on the eye.
- The tearfilm contains a number of different proteins which potentially
- adhere to the surface of the contact lens. The buildup of protein
- reduces the wettability of the surface of the contact lens which causes
- a "smeary" or "fogginess" to vision. A protein cleaner, often an enzymatic
- agent, will help in reducing the potential for protein to adhere to the
- contact lens surface. One of the benefits of disposable contact lenses is
- that protein cleaners are not necessary because the lenses are disposed of
- before the is a large build up of proteinateous material.
-
- 3.6.12 Can I use eyedrops with my contact lenses ?
- ---------------------------------------------------
- In general eyedrops shouldn't be used with contact lenses because the lens
- can absorb the eye drop and result in a concentrated buildup of the solution.
- There are special, "in-eye" lubricants that many manufacturers/pharmaceutical
- companies produce for use with contact lenses. Check with your eyecare
- professional if any doubts about the solution.
-
- 3.6.13 How do I get around dryness with contact lenses ?
- ---------------------------------------------------------
- Try using an "in-eye" lubricant. There can be some dryness if you work in
- an air-conditioned environment. If the problem persist consult a professional.
-
-
- 3.7 Risks and Benefits of Contact Lenses
- ------------------------------------------
- Benefits
- - no need to wear glasses
- - no spectacle scotoma - ie "blind-spot" due to frame edge
- - overcome problems of spectacle magnification, especially when large
- difference in spectacle prescription between the two eyes.
-
- Risks
- - corneal odema
- - corneal ulcers
- - contact lens induced conjunctivitis
-
-
- 3.8 Mail order contact lenses
- -------------------------------
- Easiest method to locate the contact lens suppliers on the internet is
- to run a Web Search request.
-
- In Aust. this practice is not allowed under current Govt. legislation.
-
- There are a number of sites that offer ordering of contact lenses on the
- WWW. Some examples include:
-
- [TBD list of contact lens suppliers on the net ]
-
-
- 3.9 Making your own saline - risks
- ------------------------------------
- Greatest risk in manufacturing your own saline is the acanthamoeba.
-
- Reference:
- Phares RE, Microbiology and hygenic care of hydrophilic lenses,
- Contacto, 16(3):10-12, 1972
-
-
-
- --
- Grant Sayer
- EMAIL: grants@research.canon.oz.au PHONE: +61-2-805-2937
- SNAIL: Canon Information Systems Research Australia
- 1 Thomas Holt Drive, North Ryde, Australia 2113
-