Human vision and function/Part 2: Understanding vision: visual acuity, colour vision and 3D perception/2.1 What is vision and how is it measured?

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2.1 What is vision and how is it measured?

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This topic will enable you to describe simply, the steps that occur when light travels from an object being viewed, through the eye, to the brain for perception to occur. You will also be able to define vision and visual acuity and describe how visual acuity is tested in an eye clinic.

Sample Snellen chart

Task 1

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Begin by watching the What is vision? playlist.

Task 2

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The Centre for Eye Research Australia has compiled an excellent resource called Low Vision Online. Please access this resource and complete the reading. The aim of the resource is to enable you to:

  1. Understand how distance and near visual acuity is tested,
  2. Understand the purpose of testing vision with a pinhole
  3. To identify people with low vision as distinct from those who have normal vision and those who are blind
  4. To test near and distance visual acuity

After you have completed the reading, there is a quiz to check if you have understood the material. Good luck!

Supplementary reading

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1. Kalloniatis M & Luu C (2014) Visual Acuity.

2. Visual Acuity of the Human Eye (NDT Resource centre) see link below.

The eye has a visual acuity threshold below which an object will go undetected. This threshold varies from person to person, but as an example, the case of a person with normal 20/20 vision can be considered. As light enters the eye through the pupil, it passes through the lens and is projected on the retina at the back of the eye. Muscles called extraocular muscles, move the eyeball in the orbits and allow the image to be focused on the central retinal or fovea.

The excerpt above is from the NDT Resource Centre. To read the whole section please view this supplementary reading link.

Revision questions

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Discuss the answers to these questions on your group LMS forum.

  1. Name the two photoreceptor cells of the retina and describe their distribution.
  2. Briefly describe the process of “vision”.
  3. Define the term “visual acuity”.
  4. What is the most common chart used to test visual acuity?
  5. How can you adapt a clinic testing room so that the distance the patient sits from the chart is 6 meters when the room is not 6 meters long?
  6. Write down the formula we use to record visual acuity.
  7. What value is considered normal vision?
  8. What does visual acuity of 6/18 mean?
  9. List 2 things that could be the problem if a patient is not able to read the line on a visual acuity chart for normal vision.
  10. What should you do if a patient is not able to see the top letter on the vision chart?