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This essay is on vision, its malfunctions & diagnostics methods.

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Vision, its malfunctions & diagnostic methods

By Jaswinder Singh

1.0 Preface

This essay is on vision, its malfunctions & diagnostics methods. The content included is concordant with the criteria listed on the specification of the institute of biophysics at 2nd Faculty of Charles University in Prague. Additional acknowledgements are mentioned under the reference section.

1.1 Visual acuity and its measurement

Visual acuity is the measurement of the ability to discriminate two stimuli separated in space at high contrast relative to the background1; it is a quantitative measure of the ability to identify black symbols on a white background at a standardized distance as the size of the symbols is varied. It happens to be the most common clinical measurement of visual function.

Visual acuity is typically measured using optotype chart for close and distant vision (snellen test). The eye which is not under test is occluded by means of instructing the subject to place an obstacle, for example a hand in front of the eye. This prevents intentional peeking, which would adversely affect the validity of the examination results. A variety of charts convenient for the patient are used, particularly in cases where a subject is unable to read the alphabet.

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Normally read at 60 metres. 
Normally read at 36 metres. 
Normally read at 18 metres. 
 Normally read at 12 metres. 
 Normally read at 9 metres. 
 Normally read at 6 metres. 
 Normally read at 5 metres. 
 Normally read at 4 metres. 

Fig 4 – Showing distances at which letters can be read

The results from the snellen test are processed and denoted as fractions. For example 6/18 means that the third line down can be read from 6 meters away; 6/6 or 6/5 is considered to be normal distance vision. If no lines can be read from 6 meters then shorter distances are tried. For example, 3/36 means that the second line can be read from a distance of 3 meters away; 2/60 means that the top line can be read from 2 meters away1 (If the top letter cannot be read even with prescription lens or glasses then the subject is considered to be legally blind).

Alternatively a Lea test aimed at pre-school children may be used. In this case the optotypes denote an edible fruit (e.g. an apple). The Tumbling E chart (see visual acuity and its measurement) may also be used. In both cases the same principle applies as with the Snellen test.

Indirect subjective methods to access optical power include intraocular pressure (see below).

1.4 Intraocular Pressure & its measurements

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Amplification of visible light from an image can be achieved by making use of an image intensifier. This allows the image to be viewed by the naked eye.



Further acknowledgements:

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Further acknowledgements:

1 –

Chapters from Biophysics; ISBN:80-239-8173-0


Further acknowledgements:

1 –




Further acknowledgements:

1 -  

Psychology A2: The exam companion; ISBN: 0-7487-9262-7

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Further acknowledgements:

1 -  




3 -  


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