how does a snellen chart work

How do you use a Snellen chart step by step?

ProcedureEnsure good natural light or illumination on the chart.Explain the procedure to the patient.Wash and dry the occluder and pinhole. ... Test each eye separately – the 'bad' eye first.Position the patient, sitting or standing, at a distance of 6 metres from the chart.More items...

What does a 20 40 vision mean?

If someone has 20/20 vision, it means they can see the same amount of detail from 20 feet away as the average person. If someone has a visual acuity of 20/40, they can see the same amount of detail from 20 feet away as the average person would see from 40 feet away.

What do the numbers mean on the Snellen chart?

These numbers measure your visual acuity, so if you have 20/20 vision, you can clearly see at 20 feet what normally should be seen at that distance, according to the American Optometric Association (AOA).

What line is 20/20 on the Snellen eye chart?

The 20/20 mark is the mark that a person with normal acuity can read at a distance of six metres. The line on the eye chart is the Eighth line, that's the one that marks 20/20 vision.

What vision is legally blind?

20/200Visual acuity less than 20/200 is considered legally blind, but to actually fit the definition, the person must not be able to attain 20/200 vision even with prescription eyewear. Many people who would be legally blind without eyewear can function well in everyday life with appropriate glasses or contact lenses.

What is the legal eyesight limit for driving?

Visual Acuity ▪ Distant binocular visual acuity of not less than 20/40 by Snellen (or 0.5, 6/12) in both eyes, with or without corrective lenses. 1. A person applying for a driver's license shall be required to take a screening vision test administered by the licensing authority, in this case, the LTO.

What does 20 70 vision look like?

The ratio measurement of vision describes visual acuity, or the sharpness of vision, at 20 feet from an object. For example, having 20/70 vision means that you must be at 20 feet to see what a person with normal vision can see at 70 feet.

How many letters can you miss on a Snellen chart?

Even if you miss one or two letters on the smallest line you can read, you are still considered to have vision equal to that line.

What do the 20's mean in 20 20 vision?

The definition of 20/20 vision is the ability to see an object clearly from 20 feet away. Others with normal vision can also see an object clearly at that distance.

What does 6 6 and 6 9 mean for an eye test?

The ability for a person to read or see the last line on the visual chart is recorded (at a test distance of 20 feet or 6 metres) and defined by terminology 6/6 vision (normal), 6/9 (one line less than 6/6), 6/12 (2 lines less than 6/6) etc. as the Snellen fraction. Distance vision impairment is considered.

Is 20 50 considered legally blind?

If you're legally blind, your vision is 20/200 or less in your better eye or your field of vision is less than 20 degrees. That means if an object is 200 feet away, you have to stand 20 feet from it in order to see it clearly.

What is 0.5 on the Snellen scale?

You must also meet the minimum eyesight standard for driving by having a visual acuity of at least decimal 0.5 (6/12) measured on the Snellen scale (with glasses or contact lenses, if necessary) using both eyes together or, if you have sight in one eye only, in that eye.

Do I need glasses if I have 20 40 vision?

A person with 20/40 vision sees things at 20 feet that most people who don't need vision correction can see at 40 feet. This means that they are nearsighted, but only slightly. A person with 20/40 vision may or may not need eyeglasses or contacts, and can discuss his or her options with a doctor.

What is 20 40 vision on a prescription?

When you get a prescription for 20/40 vision, it means that you must be 20 feet from things that people with 20/20 vision can see clearly at 40 feet. In other words, your doctor may prescribe eyewear, such as contacts or glasses, to correct your vision.

What line on an eye chart is 20 40?

third lineThe third line is equivalent to 20/40, it is the driver's test line. You must be able to read most of the letters on this line in order to obtain an unrestricted drivers license in most states including Ohio. The bottom three lines represent 20/30, 20/20 and 20/10.

What's a bad vision score?

20/30 to 20/60, this is considered mild vision loss, or near-normal vision. 20/70 to 20/160, this is considered moderate visual impairment, or moderate low vision. 20/200 or worse, this is considered severe visual impairment, or severe low vision.

What is a Snellen chart?

Snellen chart is used to estimate visual acuity. A Snellen chart is an eye chart that can be used to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen, who developed the chart in 1862. Many ophthalmologists and vision scientists now use an improved chart known as the LogMAR chart .

How many lines are on a Snellen chart?

The normal Snellen chart is printed with eleven lines of block letters. The first line consists of one very large letter, which may be one of several letters, for example E, H, or N. Subsequent rows have increasing numbers of letters that decrease in size. A person taking the test covers one eye from 6 metres or 20 feet away, and reads aloud the letters of each row, beginning at the top. The smallest row that can be read accurately indicates the visual acuity in that specific eye. The symbols on an acuity chart are formally known as " optotypes ".

What is the smallest row on an acuity chart?

The smallest row that can be read accurately indicates the visual acuity in that specific eye. The symbols on an acuity chart are formally known as " optotypes ". In the case of the traditional Snellen chart, the optotypes have the appearance of block letters, and are intended to be seen and read as letters.

What is the purpose of acuity chart?

Acuity charts are used during many kinds of vision examinations, such as "refracting" the eye to determine the best eyeglass prescription.

What is the DPI of a digital chart?

Digital chart. Since computer monitors typically have good lighting for reading and LCD/LED monitors have high DPI (between 96 and 480) they are suitable for displaying optotypes. Commonly digital chart products support randomizing optotypes displayed to prevent patients from cheating through memorizing the chart.

How tall is the 6/6 line?

At exactly 6 metres' distance from the patient, the letters on the 6/6 line shall subtend 5 minutes of arc (such that the individual limbs of the letters subtend 1 minute of arc), which means that the chart should be sized such that these letters are 8.73 mm tall and the topmost (6/60) "E" should be 87.3 mm tall. Putting it another way, the eye should be at a distance 68.76 times the height of the top (6/60) letter. The formula is

How to find the thickness of lines in a letter?

the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter " C"

What is the ratio of a Snellen chart?

All line-to-line steps are equal 10√10, which is about a 4:5 ratio. The number of letters per line varies.

What is Snellen's principle?

Snellen designed his optotypes on a 5×5 grid, on which the line thickness is one unit and the letter width and height are five units. This system is followed for most letter and number charts. For children’s tests (such as Allen pictures and other test symbols), it is often abandoned and much thinner lines are used. Precision Vision offers stylized pictures, called Patti Pics, for which Snellen’s 5×5 principle is followed. They result in less change in the measured acuity when children graduate from picture charts to letter charts. Alternative symbols that can be used for illiterate adults or in countries that are not familiar with the Roman alphabet are the tumbling E test and the Landolt C or broken ring.

What is the logmar scale used for?

The logMAR scale is widely used in scientific publications where visual acuity values need to be depicted graphically to analyze trends or need to be averaged across population groups.

Why are letter charts used?

Letter charts are not only used to measure visual acuity, they are also used as targets for subjective refraction . This is the main reason distance acuity is measured more often than near acuity. At a long distance, accommodation is relaxed, so that the refraction can be more accurate.

Is a Snellen chart a logarithmic chart?

Although the name “ Snellen chart ” should apply to any letter- or symbol-based chart, some authors use the term Snellen chart only for charts with a non-logarithmic progression.

What is the Snellen chart?

The Snellen chart is an eye chart that measures a person’s vision by how well they can read and see detail. Dr. Herman Snellen, a Dutch eye doctor, created the eye chart in 1862 for his colleague, Dr. Franciscus Donders. Dr. Donders conducted eye exams by having people look at a chart on the wall and describe what they could see.

How far away can you read a Snellen chart?

During an eye exam, patients will read the Snellen chart from 20 feet away. The farther down the chart the patient can read, the better their visual acuity is.

Can you use a Snellen chart at home?

Using a Snellen chart at home can give you an idea of your visual acuity, but it does not replace an actual eye exam. You should still schedule regular eye exams to make sure your eyesight and eye health are in good shape.

What is a snellen chart?

The Snellen chart, more commonly called an eye chart, is a common tool used by eye care professionals, driver's license exam supervisors, or anyone else interested in testing a person's eyesight. Although more advanced and complicated equipment is used in the eye care industry, the Snellen chart, which has been in use since ...

How to secure a Snellen chart?

Secure the Snellen chart to a flat surface in a well-lit room. The chart should be at a comfortable height, which may change depending on the individual's height.

What does the row for each eye mean?

The row for each eye will not necessarily be the same. Take note of the fraction shown to the left of each row: It identifies how your vision rates. For example, if you could read to the fifth line, you have 20/40 vision; if you could read to the seventh line, you have 20/25 vision.

How far can a person with 20/60 vision read?

For example, someone with 20/60 vision can read at 20 feet away what a person with normal vision could read at 60 feet away. In the United States, a score of 20/40 or better is required to drive a car without corrective lenses, vision of 20/200 or worse qualifies an individual as legally blind.

Does the Snellen chart show nearsightedness?

While invaluable for determining nearsightedness, the Snellen chart does not identify a number of other vision disorders.

Does the Snellen chart identify vision disorders?

AMA Journal of EthicsDecember 2010. While invaluable for determining nearsightedness, the Snellen chart does not identify a number of other vision disorders. Home use of a Snellen chart is not a replacement for professional vision testing.

What is the Snellen chart?

It is the eye chart used in nearly every eye doctor’s office. Typically, a giant letter E sits proudly on top, with a number of rows of letters underneath that become progressively smaller. If you have ever strained ...

How many lines are there in the Snellen chart?

When Dr. Snellen created the chart, he did so with 11 lines of block letters descending in size. The first line is one large letter, and is usually either E, H, or N. The next rows have an increasing number of letters that become smaller in size with each row. Only the nine letters C, D, E, F, L, O, P, T, Z are used in the common Snellen chart.

What is the smallest row of the Snellen chart?

The smallest row you can read with accuracy is how your eye doctor determines the visual acuity of that eye. In fact, the term 20/20 vision is a result of the Snellen chart, and it means you can clearly see the letters at 20 feet. By reading the chart, you help your eye doctor determine if you have problems with seeing items at a distance ...

How many people are shortsighted in 2012?

Children are now twice as likely to be shortsighted compared to 50 years ago, thanks to peering at electronic devices at a much higher rate. As of 2012, 4.2 million Americans aged 40 years and older suffer from uncorrectable vision impairment.

What is a slit lamp?

Slit lamp examination – A slit lamp is a combination of a microscope and a bright light that allows your doctor to see clearly in detail both the external and internal structures of the eye to ascertain eye health.

Why do people use eye charts?

Patients used their hand to show which direction the letter was facing. It was generally used with young children or patients who were illiterate. Eye charts became invaluable during the late 1800s and early 1900s when the industrial revolution was in full swing.

What is a refraction test?

A refraction test — This test uses a device called a phoropter. Have you ever seen the big machine with the various lenses and dials? That’s the phoropter. As you gaze at the letters on a wall through the phoropter, the doctor will present a series of lens pairs. Based on which two lens options work best, your doctor can determine if you have an astigmatism, are nearsighted, or are farsighted.

How to use Snellen chart?

The Snellen chart usually shows 11 rows of capital letters. The first line has one very large letter. Each row after that has increasing numbers of letters that are smaller in size. You stand 20 feet away from the Snellen chart, and read from it without your glasses or contacts.

Who created the Snellen chart?

Dutch eye doctor Hermann Snellen developed the Snellen eye chart in the 1860s. He was a colleague of Dr. Fransiscus Donders, who began diagnosing vision problems by asking people to look at a chart on a wall and tell him what they could see. According to The New York Times, he asked Dr. Snellen to make the chart.

What is the tumbling E chart?

Dr. Snellen also created a chart called the “Tumbling E” chart, which can be used by people who cannot read, or by young children who don’t know the alphabet. Instead of using different letters, the “Tumbling E” eye chart uses a capital letter E that faces in different directions.

What is an eye chart?

When you visit the eye doctor for a checkup, you may be asked to read an eye chart. The chart measures your visual acuity, or sharpness of vision. If you don’t wear glasses or contacts, your eye doctor will use the results to find out whether you need them. If you already wear corrective lenses, ...

What is the purpose of eye charts?

Eye care providers might use certain charts for measuring distance vision and others for measuring near vision. There are eye charts developed especially for children, and others that can be used for both children and adults.

What does a visual chart tell you?

The chart measures your visual acuity, or sharpness of vision. If you don’t wear glasses or contacts, your eye doctor will use the results to find out whether you need them. If you already wear corrective lenses, the test results will tell your doctor if your glasses or contacts prescription needs to change.

Do eye charts help with glaucoma?

Eye charts do not help the eye doctor tell whether you have an eye disease such as glaucoma or a problem with your retina. They also do not measure other vision problems such as loss of peripheral (side) vision.

Overview

History

Snellen developed charts using symbols based in a 5×5 unit grid. The experimental charts developed in 1861 used abstract symbols. Snellen's charts published in 1862 used alphanumeric capitals in the 5×5 grid. The original chart shows A, C, E, G, L, N, P, R, T, 5, V, Z, B, D, 4, F, H, K, O, S, 3, U, Y, A, C, E, G, L, 2.

Description

Snellen fraction

"6/6"(m) or "20/20"(ft) vision

Electronic chart

Digital chart

See also