Visual acuity test

What is the Snellen Table and the visual acuity test?

The visual acuity test is used to determine the smallest letters that you can read on a standardized chart (Snellen chart for example) or a chart held 6.1 meters away.

 

Special tables or specialized equipment are required for testing at distances less than 6.1 meters.

 

The tables can be of the frame type, in which they are placed on a wall, being able to be seen directly (at 6.1m) or of the inverse type, visualizing it through a mirror , with the table at its side, with a distance of 3 , 05m from the mirror.

 

They can also be of the TV monitor type in which the characters (letters, images, patterns) are reproduced on the screen and through optotypes, which are specialized equipment for the examination in small rooms and places, or without adequate lighting , for these we recommend the EVOS-1000Plus, EVOS-3000Plus and the EVOS-5000Plus models.

​

​

​

How is the test performed?

This test can be done in the office, at school, at the workplace or elsewhere.

​

You will be asked to remove your glasses or contact lenses and stand or sit 6.1 meters from the table. You will keep both eyes open.

​

After that, you will be asked to cover an eye with the palm of your hand, a piece of paper or a small paddle (ocular occlusion) while reading aloud the smallest line of letters you can see in the table. Numbers, lines or images are used for people who cannot read, especially children.

​

If you are not sure of the letter, you can guess . This test is done on each eye and one at a time. If necessary, it is repeated while you wear your glasses or contacts .

 

You may also be asked to read letters or numbers on a card 14 inches (36 centimeters) from your face. This will test your vision closely .

​

In the case of equipment such as optotypes, this examination is carried out using only one equipment. Without the need for an occluder .

​

Remember to use equipment that puts your eyes directly on the equipment to ask for cleaning with Alcohol 70% or similar, to avoid infections and other problems with your vision .

​

​

​

How to prepare for the test

No special preparation is required for this test. If you wear glasses or lenses , don't forget to take them!

​

​

How it will feel to take the test

There is no discomfort.

​

​

Why the test is performed

The visual acuity test is a routine part of an eye examination or general physical examination , especially if there is a change in vision or a problem with vision.

​

In children , the test is performed to detect vision problems. Vision problems in young children can usually be corrected or improved. Undetected or untreated problems can cause permanent damage to eyesight.

​

There are other ways to check vision in very young children or in people who do not know their letters or numbers.

​

Scratchs

There are no risks with this test.

​

​

Normal results

Visual acuity is expressed as a fraction.

​

The top number refers to how far you are from the graph . This is usually 6 feet.

The lower number indicates the distance that a person with normal vision can read the same line that you read correctly.

For example, 20/20 is considered normal . 20/40 indicates that the line you read correctly from 6 meters away can be read by a person with normal vision from 12 meters away.

 

Visual acuity can also be expressed as a decimal number . For example, 20/20 is 1.0, 20/40 is 0.5, 20/80 is 0.25, 20/100 is 0.2 and so on.

​

Even if you miss a letter or two on the shortest line you can read, it will still be considered that you have the same view as that line .

​

​

Abnormal results

Abnormal results can be a sign that you need glasses or contact lenses . Or it could mean that you have an eye condition that needs further evaluation by a doctor or optician.

​

​