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Worksheet 3: Common Visual Problems and the use of optics for their treatment.

 

VISION

Vision is the act of perceiving and interpreting visual information with the eyes, mind, and body.
Nearly every single daily activity requires the use of sight. It is truly amazing just how much we rely on our eyes, and how much we take our vision for granted.

When certain visual problems do arise, the loss is great. Thankfully though through the clever use of optics (and more recently laser surgery), many of these obstacles can be overcome.

Firstly though, what defines perfect vision ?

Emmetropia (PERFECT SIGHT)

Structure of normal eye

An eye is said to be emmetropic when an image is perfectly focused on the foveal point of the retina.

Myopia (SHORT SIGHTEDNESS)

Structure of Short Sighted Eye

An eye is said to be myopic when the point at which light from an object converges at a finite distance IN FRONT of the eye.
Myopia can be due to either an eye which is too long relative to normal standards (axial myopia), or because the optical power of the eye's lens is too strong relative to the normal eye length (refractive myopia).
The focus is correctly adjusted with a "minus" power lens, or concave lens.

Hyperopia (FAR SIGHTEDNESS)

Structure of Far Sighted Eye

An eye is said to be hyperopic when the point at which light from an object converges at a finite distance behind the eye.
Generally the hyperopic eye is too short with respect to normal standards (axial hyperopia) or because the optical power of the eye's lens is too low relative to the normal eye length (refractive hyperopia).
The focus is correctly adjusted using a "plus" lens power or convex lens.

Astigmatism

Astigmatism is a condition in which the surface of the cornea is not spherical, it causes a blurred or slightly twisted image to be received at the retina. It is quite common to have only one eye affected, whilst the other is normal.
Condition can be corrected with glasses.

Presbyopia

Presbyopia is the gradual loss of the eye's ability to change focus (accommodation) for seeing near objects. Caused by the lens becoming less elastic. The condition is associated with aging & occurs in almost all people over age 45.
Condition can be corrected with glasses.

Colourblindness

Colourblindness affects one in 12 men. But because of the way it is passed down through our genes, very few women have the condition. The most common form of colourblindness is red-green (Deuteranamolous and Protonamolous) , after which comes blue-yellow (Tritanamolous). There are many different types and degrees of colorblindness - more correctly called color deficiencies, and the severity varies from person to person.

In most cases, a color deficient person is able to see certain colors normally and unable to distinguish certain other colors and shades normally. It is extremely rare to be totally color blind - achromatopsia is the inability to see any colour and is reported to affect only one person in about 33,000.

How does it occur?
Although the majority of people with defective color vision have inherited it, it has been reported that some people can also acquire defective color vision over the course their life, the cause of this is unknown.

Why does it occur?
One very important compenent of the eye, in terms of colour detection are the cones. Cones can perceive color because of a light sensitive pigment they contain which is sensitive over a range of wavelengths (different colours of light have different wavelengths). Genes are responsible for the coding instructions to make these pigments, and if the coding instructions are wrong, then the wrong pigments are produced, and the cones will be sensitive to different wavelengths of light (resulting in a color deficiency).

Is there any cure?
Congenital colorblindness means you're born with it, and therefore it is permanent, and unfortunately there is no cure or treatment.
Tinted lenses for colorblindness have been recently introduced. Yet, although coloured lenses would help you pass some color vision tests, so would looking through "any" red glass lens or piece of red tinted cellophane.

Click here to complete an online colourblindness test

 

Binocular vision disability:

A visual defect in which the two eyes fail to work together as a coordinated team resulting in a partial or total loss of binocular depth perception and stereoscopic vision. At least 12% of the population has some type of binocular vision disability. Amblyopia and Strabismus are the most commonly known types of binocular vision disabilities.

Human binocular vision develops during the first few years of life.
Any interruption to the motor, sensory or central components, can lead to sensory or central defect for example: nerve or muscle defect.

Vision Therapy:

Vision therapy can be described as physical therapy for the visual system which includes the brain and eyes. Through a series of progressive eye exercises, patients develop or recover normal visual skills.
Vision therapy is remarkably successful in rehabilitating all types of binocular vision impairments including amblyopia (lazy eye), strabismus, esotropia, exotropia, hyperphoria, or loss of binocular fusion due to hyperopia (farsightedness), myopia (nearsightedness) or astigmatism in one eye.
In regards to the development or recovery of binocular vision, vision therapy is much more successful than surgery or glasses alone. Patients of all ages can benefit from vision therapy.

Click here to view some fun "Magic Eye" 3D images used in vision therapy


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