Visual acuity tests
Visual acuity tests measure the ability to see fine detail. Visual acuity is determined by measuring the smallest size print that a person can read. Both at a distance and close up.
Distance visual acuity
A visual acuity chart is used to test distance visual acuity. It uses capital letters of different sizes to test vision in literate adults.
The test is conducted at 6 metres (20 feet in USA). The test result is given as a fraction that indicates the distance in metres at which that row of the chart can be read by a normal eye.
The top number of the fraction indicates the test distance (how far you are standing from the chart). It is usually 6 metres in Australia.
The bottom number represents the size of the letter seen. The larger the bottom number the larger the letter on the chart (E.g. 6/48 indicates a bigger letter than 6/12).
Normal visual acuity is recorded as 6/6 (20/20 in USA).
The diagram below is a LogMAR Distance Visual Acuity Chart
Examples of the impact of certain visual acuity levels
|6/12||Reduced vision, Australian legal driving limit.|
|6/18||Low vision (World Health Organisation definition)|
|Less than 6/60||Legal blindness (eligible for various entitlements)|
Terms used to indicate that vision is too reduced to see a chart:
|CF||count fingers at distance specified|
|HM||can see a hand moving at a close distance in front of the eyes|
|LP||can perceive light and dark but no detail|
|NLP||No light perception or totally blind|
Near visual acuity
Near visual acuity tests a person’s central vision. It is recorded in point notation, the same as for computer font, and marked with an ‘N’.
- N18 - N16 is large print.
- N18 is 18 point font
- N8 is newsprint
- N6 is telephone book print
Children’s visual acuity tests
Young children’s visual acuity is tested with symbols and pictures. Children who can’t understand these tests are shown different sized stripes.
Visual field tests check peripheral vision. These tests are performed by asking the person to look straight ahead and then changing the position of targets. Visual field tests measure the total area that is seen while the eye is directed straight ahead.
The total visual field with both eyes open is approximately 180° horizontally and 155° vertically. A visual field of less than 60° will start to significantly impact on a person’s ability to move about within the environment. A person must have at least 120° of visual field in the horizontal meridian to legally drive a car in Australia.
About 8% of males and 0.4% of females are colour blind. Colour blindness is predominantly an inherited condition, for which there is no cure. Colours may be confused and some colours may be seen more dimly. Some retinal and optic nerve diseases also affect colour vision. Colour vision can affect areas such as career choice and communication.
Testing is done by looking at figures that can only be seen when all the colours can be distinguished. The diagram is an example of an Ishihara colour vision test
Visual acuity measures the ability of the eye to resolve fine detail with maximum contrast (black on white). In real life situations, most tasks are not in black and white and may require the ability to detect small changes in shade. It is useful to determine how much contrast a client requires in order to discern an object from its background. Contrast sensitivity tests use graded levels of grey letters or stripes to determine the lowest difference in contrast discernible. The commonly used tests require the person to indicate which direction parallel lines are facing.
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