The Snellen chart, invented in the 1860s by Dutch ophthalmologist Dr. Hermann Snellen, measures visual acuity, or clarity of vision. The clarity of vision with the help of an eye test chart has remained one of the most recognized and trusted methods for determining how well a person can see from a distance. The Snellen chart is essential in routine eye exams and helps one understand its purpose, how it works, and what terms like “6/6 eyesight chart” really mean.
What is a Snellen Chart?
The Snellen chart is a standardized tool for specifying visual acuity during eye exams. The chart has 11 rows of capital letters, also known as optotypes, which decrease in size from top to bottom. The top row features the most significant letters, while the bottom row has the most miniature letters that a person with normal vision should be able to read from a specific distance.
With the help of the chart you can check your visual acuity and can detect eye disorders or visual impairments like myopia (nearsightedness), hyperopia (farsightedness), or astigmatism.
How the Snellen Chart Works
The Snellen chart is placed at a specific distance from the patient, which is usually 20 feet (6 meters) in countries using feet or 6 meters in metric systems. The patient is asked to read the letters on the chart, starting from the largest at the top and working down to the smallest at the bottom. While reading the eye chart, one eye needs to be covered while the other reads, and then the process is reversed with the opposite eye.
Each line on the chart is associated with a specific fraction, like 6/6, 6/9, or 6/12.The first number refers to the distance from the chart which is 6 meters in most of the cases), while the second number indicates the distance a person with normal vision can read that particular line of letters. If you can read the 6/6 line from 6 meters away, your vision is considered normal, often referred to as “20/20” in the feet-based system.
What is 6/6 Vision?
Many people hear “6/6 vision” and assume it means perfect vision. In reality, it signifies normal vision at a distance of 6 meters. If you can read the 6/6 line on the Snellen chart, it means you can see what an average person with normal eyesight can see from that distance.
However, having 6/6 eyesight doesn’t necessarily mean your vision is flawless. The Snellen chart only reflects how well you can see at a distance; it does not test other aspects of your vision, like color perception, depth perception, and near vision.
What if Your Vision Is Less Than 6/6?
If you cannot read the 6/6 line, the optometrist or ophthalmologist will continue moving up the chart until you can read a larger line. Typical results include 6/9 or 6/12, which indicate that you need to be at a closer distance (9 meters or 12 meters, respectively) to see what a person with normal vision can see from 6 meters.
For instance:
- 6/9 vision: You can read at 6 meters what someone with normal vision can read at 9 meters.
- 6/12 vision: You can read at 6 meters, while someone with normal vision can read at 12 meters.
These measurements help in diagnosing whether you need corrective lenses, either for nearsightedness or farsightedness. If your vision is worse than 6/60 (or 20/200 in the foot-based system), it may be classified as legally blind, depending on local regulations.
Variations of the Snellen Chart
Snellen eye test chart has different types and versions designed for specific populations:
Children’s Charts: Instead of letters, these charts use symbols or pictures that children can recognize, such as houses, stars, or animals.
Tumbling E Chart: This variation is often used for people who are illiterate or non-verbal. The chart displays only the letter “E” in different orientations, and the patient is asked to indicate which way the legs of the “E” are pointing.
LogMAR Chart: The Logarithm of the Minimum Angle of Resolution (LogMAR) chart is considered more accurate especially in patients, who are considered in clinical studies. It has evenly spaced letters and offers a more accurate measurement of visual acuity.
The Role of Snellen Chart in Eye Care
The Snellen chart remains one of the most useful and widely used tools in routine eye examinations. Snellen eye test chart is the first possible eye test which is done when you meet an eye expert while going for your routine eye checkup.
Snellen chart alone doesn’t provide a full picture of your eye health. Regular comprehensive eye exams, including testing for other vision conditions, are vital for maintaining healthy eyes.
When Should You Get an Eye Test?
It’s recommended that adults with normal vision have their eyes tested every two years. Those who wear glasses or contact lenses or have conditions such as diabetes that may affect your vision, you should have more frequent exams. Children should have their first eye test around age three, followed by regular checkups as they grow.
Conclusion
The Snellen chart can help you to be more informed about your vision and when to seek corrective lenses or further examination. However, relying on something other than this chart, comprehensive eye exams are crucial for diagnosing broader eye health issues that the Snellen chart doesn’t measure. Consult your optometrist or eye care professional regularly for more information on eye health, tests, and preventive care.
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