• Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

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

Extracts from this document...

Introduction

J.Singh

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.

...read more.

Middle

image03.jpg

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

...read more.

Conclusion

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.

References

1.1:http://en.wikipedia.org/wiki/Visual_acuity

Further acknowledgements:

1 –

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12809155&dopt=Abstract

1.2:http://www.chemsoc.org/exemplarchem/entries/2002/upton/defects_of_the_eye.htm

Further acknowledgements:

1 –

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

1.3:http://www.agingeye.net/glaucoma/glaucomaeyeexam.php

Further acknowledgements:

1 –

http://freespace.virgin.net/amanda.gibson/snellen.htm

1.4:http://en.wikipedia.org/wiki/Intraocular_pressure

1.5:http://en.wikipedia.org/wiki/Color_vision_deficiency

Further acknowledgements:

1 -  

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

2 -  

http://www.visionchannel.net/colorvision/types.shtml

1.6:http://en.wikipedia.org/wiki/Binocular_vision

Further acknowledgements:

1 -  

http://en.wikipedia.org/wiki/Depth_perception

2-  

http://ahsmail.uwaterloo.ca/kin356/cues/bcues.htm

3 -  

http://en.wikipedia.org/wiki/Depth_perception

4 -  

http://en.wikipedia.org/wiki/Convergence

1.7:http://en.wikipedia.org/wiki/Night_vision

...read more.

This student written piece of work is one of many that can be found in our AS and A Level Microscopes & Lenses section.

Found what you're looking for?

  • Start learning 29% faster today
  • 150,000+ documents available
  • Just £6.99 a month

Not the one? Search for your essay title...
  • Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

See related essaysSee related essays

Related AS and A Level Microscopes & Lenses essays

  1. "An investigation into the factors affecting a lens."

    This is a virtual focal point. Because of this, a convex lens can be used to project an image onto a screen, the sort that would be used in a cinema projector, forming a "real image" which can be seen.

  2. Relationship Between U and V For a Convex Lens

    Shown by the first diagram having a smaller angle going through the convex lens than the second. Causing the image to be produced a much shorter distance than the second. You can also see that a smaller image is produced by the first diagram, due the rays having met at a smaller distance.

  1. My experiments focus is to obtain an accurate measurement for a specific lenss power.

    For this reason the average result from this graph should not be summarised as the actual power, but rather an indication of uncertainity between the two axes intercepts. However the uncertainty of 0.02 Dioptres in the measurement is rather small, indicating the final result will be quite reliable.

  2. How has the Invention of Contact Lenses affected people's Sight?

    or also the use of a new material, a softy water absorbing plastic, it is in 1971 that the soft lens became available for commercial distribution in the United States. New models of lenses, each time with more improvement, were then created and commercialised (around a new model every two years).

  1. The eye.

    those from nearby objects - will not be brought to a focus by the time they strike the retina. Eyeglasses with convex lenses can correct the problem. As people get older, the lens becomes less elastic and can't 'bulge' to focus near objects i.e.

  2. Find the separation between two cones of the same type on the fovea of ...

    * The size and shape of the light sources. I will control this by shining the light through measured apertures made in black acrylic. I will control the size of the apertures by using a 1mm drill bit to create the holes through which the light sources will be shone, this will also ensure the sources are of a constant shape.

  • Over 160,000 pieces
    of student written work
  • Annotated by
    experienced teachers
  • Ideas and feedback to
    improve your own work