What is Astigmatism?
What is Hyperopia?
What is Myopia?
What is Emmetropia?
What are the different types of Spectacle Lenses?
Can I store my lenses in tap water?
Solutions required for contact maintenance
What are the different types of Soft Contact Lenses?
Q: What is Astigmatism?
A: 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. 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.
Q: What is Hyperopia?
A: 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.
Q: What is Myopia?
A: 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.
Q: What is Emmetropia?
A: Emmetropia is just another name for an eye that has no optical defects and a precise image is formed on the retina.
Q: What are the different types of Spectacle Lenses?
A: + 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.
+ 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)
Q: Can I store my lenses in tap water?
A: No. There are a number of micro-organisms, which like 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.
Q: Solutions required for contact maintenance
A: 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.
Q: What are the different types of Soft Contact Lenses?
A: Spherical - Single prescription power 360 degrees around the lens.
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.
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).
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
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