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Snellen charts are a device named after the ophthalmologist who invented and prepared it. Charts are used by ophthalmologists and general practitioners. Thanks to this tool, it is possible to determine visual acuity and assess how the patient sees near and far. The form of the currently used tables differs from the original, but their main idea is used, and the description of the test result is based on the initial rules.
What are Snellen Charts for?
Snellen charts is a method by which it is possible to determine the level of distinguishing the details of the image seen, i.e. – in short – they check the sharpness of our eyesight. The purpose of the test is to determine the acuity of central vision. Thanks Snellen chart it is also possible to determine if and to what extent the vision of a particular eye is impaired. If no pathology is found during the examination with the slit lamp and the fundus, it is assumed that the cause of amblyopia is a refractive error. It can be improved by wearing glasses, contact lenses or by performing refractive surgery.
Types of Snellen tables
Traditional set of Snellen tables it consists of a distance plate and a near plate. The first set consists of large charts that the patient interprets from a distance of 6 meters. Near cards are located approximately 25-40 centimeters from the person’s eye. This interval is the so-called good vision distance. A healthy eye at this distance should, without any effort, see an object near it. Snellen charts to literythat are in rows. The size of the letters gradually decreases with the successive rows. It is worth knowing that each of the lines is marked with a numerical value, represented by a fraction. The given value describes the patient’s visual acuity. Importantly, the basic order is the one whose fractional value is 1. Most often it is presented in the form of 20/20 or 6/6. The letters contained in this line should be correctly interpreted by the patient with the correct visual acuity. The test is based on the fact that people with a refractive error will only learn letters larger than the basic ones, and the smallest size recognized will reflect the size of the defect.
They also function Snellen charts intended for children. They contain pictures. Thanks to this, you can avoid erroneous test results, because the youngest may still have problems with the correct recognition of letters or numbers. Pictures are easier to describe or name. Additionally, it is easier to keep your child’s concentration and attention while exercising eye tests.
How is the test performed?
How the test is performed depends primarily on which one tables they are used. If you are measuring with tablic Snellena to the distance, it is necessary to place them at a distance of 6 meters from the patient. The test is carried out in a darkroom, and the light beam has a constant intensity. The patient covers one eye and then reads out the letters shown by the doctor. The specialist starts with the largest ones, moving on to smaller and smaller ones. The test is terminated when the patient is no longer able to read the letter indicated by the doctor. The result of the study is the fraction of the line that the patient could read last. If the test subject has strong accommodation or there is some uncertainty, a post-accommodation paralysis examination may be performed using drops with atropine. Unfortunately, the test is then more unpleasant, but the result is usually more precise. After determining the sight defect, trial corrective lenses are put on. Then the test is repeated and the patient should be able to recognize all rows with letters.
Other methods of visual acuity testing
Nowadays, more and more often, in modern ophthalmic offices, traditional Snellen charts are changed to so-called optotype projectors. They are projectors that project images of various characters on the screen, which shrink step by step. The patient’s task is to define them. Despite the new equipment, the method of calculating visual acuity is the same all the time.
Go with the times – Snellen boards online
The Internet is a treasury of knowledge, a huge database that we can use practically without restrictions and, of course, without leaving home. Snellen charts You can find them on the websites and then download them for free in PDF format. The original board looks different because it is larger, but the rules are similar. The printed sheet should be attached to the wall without decorations and windows at the level of our eyesight. It is worth asking someone from your family, a friend, to point out the letters on the board that we should recognize. If you already have a visual impairment and wear lenses or glasses, wear them. Then stand 3 meters from the wall with online printed board. As with the office examination, first cover one eye and read the indicated letters, and then do the same for the other eye.