Visual acuity test

A visual acuity test verifies whether you can read letters of different size from a certain distance. Visual acuity is the ability to distinguish two separate points located close to one another. To conduct a visual acuity test the Snellen’s charts are used both for short and long distance. The charts consist of black matt symbols on white matt background – letters, numbers, circles, hooks, pictures for children – gradually decreasing in size from the top to the bottom of the charts.

The symbols are of certain thickness and the spaces between them are of the same length. There is a certain distance in metres attributed to every row of symbols of the same size, which is the distance from which a healthy eye perceives the whole symbol at a 5-minute angle and its details at a 1-minute angle. The degree of the patient’s visual acuity is represented with the proportion of the distance between the patient and the chart to the distance from which a given symbol is perceived by a healthy eye. So if a patient can read the bottom row, marked as D=5, from a 5-metre distance, it means that their visual acuity is 5/5 (full visual acuity), and if they can only read the thickest letter marked as D = 50, then their visual acuity is 5/50. It may be expressed with a decimal fraction, with 5/5 being equal to 1,0 and 5/50 equal to 0,1. Correct visual acuity is written as: visual acuity in the right eye - visus oculi dextri (V.o.d.) = 1,0, visual acuity in the left eye - visus oculi sinistri (V.o.s.) = 1,0, and visual acuity in both eyes - visus oculi utriusque (V.o.u.) = 1,0.

The test evaluates the degree of a decrease in visual acuity, and such a decrease means we should search for its reason – a visual defect or an eye disease.

THE DECRIPTION OF THE TEST

Adult patients are tested with the use of illuminated charts with symbols or a projector. A long distance visual acuity test is conducted separately for each eye, with the other eye carefully covered. The patient remains in the seated position with the face towards the Snellen’s chart at a particular distance from it, which depends on the chart structure, the usual distance being 5 m. The patient reads out loud the letters from the chart, beginning with the top rows where the symbols are the biggest. If the patient cannot read the biggest letter, the distance from the chart is shortened or the patient is asked to count the doctor’s fingers from various distances. The result is written down as follows: the patient counts fingers from a 1-metre distance (or 1/50, i.e. 0,02), from 2 m (2/50, i.e. 0,04) etc. If the patient cannot count the fingers, they are then tested for hand movement perception. If their visual acuity is even worse, the doctor uses a powerful flashlight to verify the patient’s light perception (l.p.) It may occur that there is not even any light perception in the tested eye, which is written down as l.l.p. (lack of light perception, which is considered total blindness).

A similar test is performed to evaluate short distance visual acuity. During the test the patient has to read a standardised text from the short-distance Snellen’s chart. The distance for this test is 30 cm and each eye is tested separately. The chart’s structure is the same as it is in case of the long-distance one, yet the text begins with the smallest letters and ends with the largest ones. The way the results are written down is similar: the ability to read the smallest font is regarded as full short-distance visual acuity (Sn.o.d. = 0,5) and the largest one - Sn.o.d. = 3,0.

Visual acuity tests in small babies are performed through observation of their eye movements (e.g. with Teller’s acuity cards, which consist of black stripes of different thickness). Babies also have their eyes covered alternately, one at a time, and when the stronger eye is covered, the baby begins to protest. Another method of testing visual acuity in the youngest children is to trigger optokinetic nystagmus. When a child is at least two years old or more, pictorial tests can be used.

Visual acuity in children depends on their age and intelligence. Up to the age of six difficulties in distinguishing between symbols may be observed; therefore, an averagely intelligent two- or three-year-old child has visual acuity within the range of 0,5 to 1,0, whereas four- to six-year-olds usually have the acuity of approximately 0,8 to 1,0. Full visual acuity should develop around the age of six.

In case of abnormal visual acuity, thorough examination is conducted in order to explain its cause (such as a refractive error, a disease of the eye bulb disease or the central nervous system).

If you complain of any eye conditions or you have experienced vision deterioration, we encourage you to make an appointment for a specialist ophthalmologic examination in our Ophthalmologic Centre. The cost of the examination is 120 PLN.

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