Visual acuity
Visual acuity is the eye's ability to distinguish and appreciate the shape,
dimensions and details of the surrounding elements. Visual acuity is
determined using optotypes.
Visual acuity is expressed by the formula:
VA = d / D
d = the distance at which the optotype is read
D = the distance at which the letter of the respective line can be read by an
emmetropic eye.
The patient should be cooperative, understand the optotypes, be able to
communicate with the physician, and many more factors. If any of these
factors is missing, then the measurement will not represent the patient's real
visual acuity.
If the subject cannot read the optotype at a distance of 6m (the distance at
which the rays of light from an object are considered to reach parallel to the
retina), he will get close to the optotype at a distance of 1m. If in this situation
the subject reads the first line, the visual acuity is 1/50 (0,02). If the subject
cannot read even the first letter of the optotype, the visual acuity is less than
0.02 and is expressed:
-counting fingers (CF) at 30 cm, 20cm, 10 cm
-hand motion (HM)
-light perception (LP)
-no light perception
Vision testing tools
The Snellen Chart is used in most facilities for testing distance vision. They
are designed to be read at 5 metres.
The Snellen Chart comes as a free standing cardboard chart or on a light box.
Chart should be in good order If using a light box, ALL globes must be working
to ensure standard illumination.
Each eye needs to be tested separately .Use an occluder to cover the eye that
is not being tested .If glasses are worn, the occluder goes over the top of the
glasses .If occluder is not available use the patient’s cupped hand or a patch .
Avoid pressure on the eye and be aware of patients peeking through their
fingers.
For examining the close view, where the accommodation also occurs, tables
containing texts with characters of different sizes are used.
Optotypes close read like a book, from a distance of 30-35cm, being used to
correct presbyopia.
Factors than may influence the visual acuity
Objective factors: geometric form of the test; it is necessary to use tests with a
simple and easily recognizable form;
-the contrast between the test and the background: at high contrast, visual
acuity is little influence, at low contrast, visual acuity diminishes greatly
Subjective factors
- pupil diameter: the best visual acuity is obtained when the pupil diameter is
4mm;
- the age of the subject
Binocular vision
- in humans, binocular vision is a function that is gained progressively from the
first months of life;
- for the existence of the binocular view certain conditions are required:
- sensory conditions: anatomical and dioptric integrity of the eye
- motor conditions: the anatomical-functional integrity of the oculomotor
muscles and nerves, to ensure perfect motility in all directions of the eye
