VISION DEFECTS

How normal eye works?

• In the case of a normal eye, the rays of light from the object fall on the eye and converge on the retina 

Vision Defects

• It is the impairment of the sense of sight . • Any abnormality of the eye that

causes less-than-perfect sight.• Two most common defects are

myopia or nearsightedness and hypermetropia or farsightedness.

Myopia or Nearsightedness

• distant objects appear out of focus, while objects that are within close range can be seen clearly.

It is a condition where the eye is too long in relation to its refractive power. The light rays therefore intersect in front of the retina instead of on it. Blurred vision results. The further away the object observed, the more blurred it appears. Clear vision is limited to nearby objects. 

Causes of Myopia• the elongation of the eye ball,

that is, the distance between the retina and eye lens is increased• decrease in focal length of the

eye lens

• A myopic eye forms the image of a far off object in front of the retina because of the increase in converging power of the eye lens. Therefore myopia can be rectified by using a suitable divergent or concave lens. The ray diagram shows how a concave lens helps in focusing the light on the retina.

Hypermetropia or Farsightedness

• distant  objects  can  be  seen  clearly,  while objects that are within close range appear out of focus. 

• in  the  case  of  a  hypermetropic eye,  the  light  rays  are  focused behind the retina.

Causes of Hypermetropia

• Shortening of the eyeball, that is, the eyeball becomes smaller• Increase in focal length of the

eye lens

•  long  sightedness  is  due  to  the  decreased converging  power  of  the  lens.  Therefore hypermetropia can be rectified by making the eye  lens  more  convergent.  This  is  done  by placing a convex  lens of  suitable  focal  length before the eye lens as shown in the figure.

Astigmatism• It  is  another  common  vision  defect  in  which eyesight  may  be  blurred  at  all  distances. People with minor astigmatism may believe to have a clear vision, but find they are squinting from  time  to  time  or  suffering  from headaches,  eye  strain,  fatigue  and  blurred vision depending on.

Causes of Astigmatism

• Normally  the cornea  is spherical  in shape but in  this  case,  cornea  is  shaped  more  like  an oblong Rugby ball

Astigmatism’s Test

Close one eye and then the other one , if you do not see all the lined squares, in the same black color , if you do see one or more squares grey, you than have an astigmatism.

• When light rays hit the steep sides of the cornea,  they  are  focused  at  two  points instead of at one point on the retina (the nerve-rich lining inside the eye, where we “see”),  producing  blurred  or  distorted images. 

• This  can  be  corrected  by using  cylindrical  or asymmetric lenses.

Color Blindness• is another common vision defect in which colors are not seen normally. Most color-blind people find it difficult to see or distinguish greens from reds and vice versa. Some may not recognize any colors at all. 

Causes of Color Blindness

• The  various  degrees  of  color  blindness  are caused  by  the  state  of  the  color  receptors found at the back of the eye. 

• While color blindness cannot be cured, color-blind people can develop personalized color identification systems for their everyday needs, or learn to recognize colors by their brightness or location. Visual aids may also be used to help color-blind people distinguish colors to some extent. 

Cataract

•   a clouding of the lens in the eye that affects vision. Most cataracts are related to aging. Cataracts are very common in older people. 

Types of Cataract

• Secondary cataract. Cataracts can form after surgery for other eye problems, such as glaucoma. Cataracts also can develop in people who have other health problems, such as diabetes. Cataracts are sometimes linked to steroid use.

• Traumatic cataract. Cataracts can develop after an eye injury, sometimes years later.

• Congenital cataract. Some babies are born with cataracts or develop them in childhood, often in both eyes. These cataracts may be so small that they do not affect vision. If they do, the lenses may need to be removed.

• Radiation cataract. Cataracts can develop after exposure to some types of radiation.

Causes of Cataract

• as  we  age,  some  of  the  protein  may  clump together  and  start  to  cloud  a  small  area  of  the lens.  This  is  a  cataract.  Over  time,  the  cataract may  grow  larger  and  cloud  more  of  the  lens, making it harder to see.

• Researchers suspect that there are several causes of  cataract,  such  as  smoking  and  diabetes.  Or,  it may be  that  the protein  in  the  lens  just  changes from the wear and tear it takes over the years.

How Cataract can be treated?

• The  symptoms  of  early  cataract  may  be improved  with  new  eyeglasses,  brighter lighting,  anti-glare  sunglasses,  or  magnifying lenses. If these measures do not help, surgery is  the  only  effective  treatment.  Surgery involves  removing  the  cloudy  lens  and replacing it with an artificial lens.

Presbyopia

•  is a common vision condition in which it becomes increasingly difficult over time to focus on objects that are within close range. 

Causes of Prebyopia

•  is caused when the centre of the eye lens hardens making it unable to accommodate near vision. This condition generally affects almost everyone over the age of 50 - even those with myopia. 

•  Eyeglasses with bifocal or progressive lenses are prescribed to correct or improve the condition. The contact lenses used by presbyopes are multifocal lenses.

How are color vision defects diagnosed?

• Specialized color vision tests can easily detect color vision defects. Pseudoisochromatic plate tests are commonly used to screen for inherited color vision defects. In this group of tests, a pattern of colored dots forms a number or letter against a background of other colored dots. Persons with normal color vision can discern these patterns but those with color defects cannot.

Pseudoisochromatic Plate Test

A Snellen chart is  an eye  chart used  by eye  care professional and  others  to  measure visual  acuity.  The normal height  for  the  letter G  is 88 mm, and the viewing distance is 6 meters.

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