LYCOS RETRIEVER 
Blindness: Vision
built 661 days ago
Blindness is the loss of vision due to physiological or psychological conditions. Total blindness is a complete lack of form and light perception. "Blindness" is ... frequently used to describe serious vision impairment with residual vision. Individual governments and jurisdictions often create their own complex definitions of "legal blindness."
Source:
Colour blindness is normally diagnosed through clinical testing and a number of tests have been devised. The most common test is the use of special test plates called "pseudo-isochromatic" plates or colour confusion plates. The plates are made up of a series of spots of varying hues and lightnesses so that a central number or letter stands out from the background. Those with defective vision are unable to distinguish these figures or will see a different figure due to the different appreciation of the hues. By changing the figure and background colours, the basic types of defective colour vision can be identified. Other more specific tests, such as the anomaloscope, can pinpoint the more subtle defects in colour vision and provide a more accurate classification.
Source:
[One] cause of night-blindness is pigmentary degeneration of the retina (retinitis pigmentosa) which, combined with partial loss of the visual field, eventually contracting down to ‘tunnel vision’, can be most disabling. This condition is mainly inherited as an autosomal recessive condition (showing itself only when both parents carry the mutant gene), but other forms occur. A high proportion of the population of the Atlantic island Tristan da Cunha was recently discovered to be affected when they were evacuated because of volcanic activity. The disorder is progressive and untreatable.
Source:
Deaf-blindness is a combination of vision and hearing loss, not necessarily complete deafness and complete blindness. There is a wide range of thinking and developmental ability among deaf-blind individuals from gifted to profoundly multiply handicapped. Deaf-blindness creates additional problems in the areas of mobility and communication, as well.
Source:
MORE ABOUT
Vision