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?/Answers

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1. Name the two photoreceptor cells of the retina and describe their distribution.

Rods and cones. Rods are distributed in the periphery, cones are distributed most densely around the macula and decrease in density in the periphery.

2. Briefly describe the process of “vision”.

Light strikes the photoreceptors, this is converted into an electrochemical signal and it travels down the visual pathway to the visual cortex of the brain for interpretation.

3. Define the term “visual acuity”.

Visual acuity (VA) is determined by the smallest retinal image which can be clearly distinguished at a certain distance (usually 6 meters).

4. What is the most common chart used to test visual acuity?

Snellen Chart (Or Tumbling E chart for someone who is illiterate or doesn’t recognise letters).

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?

Sit patient 3 meters from a mirror with a backwards facing Snellen chart behind them reflected in it. Or use a specially adapted chart made for 3 meters.

6. Write down the formula we use to record visual acuity.

Distance from the chart
Number next to lowest line read

7. What value is considered normal vision?


8. What does visual acuity of 6/18 mean?

It means that the patient can see at 6 meters what a normal eye can see at 18 meters.

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.

Refractive error (need to wear glasses) or pathology in the eye.

10. What should you do if a patient is not able to see the top letter on the vision chart?

Move the patient closer to the chart, progressively until they can see a letter, or if the still cannot see it you check if they can Count Fingers, Hand Movements or Light Perception.