- Medically Reviewed by Mr Mfazo Hove Consultant Ophthalmologist
- Author: Chris Dunnington
- Published: February 13, 2025
At Blue Fin Vision®, London’s premier eye clinic, we understand the importance of accurate vision testing. One of the most common tools used in eye examinations is the Snellen chart, a fundamental component of vision assessment. In this comprehensive guide, we’ll explore what a Snellen chart is, its history, how it’s used, and how to interpret the results.
The Snellen Chart: A Brief Overview
The Snellen chart, also known as the eye test chart, is a standardised tool used by eye care professionals to measure visual acuity. Named after its inventor, Dutch ophthalmologist Herman Snellen, this chart has been a staple in eye clinics worldwide since its creation in 1862.
Key Features of the Snellen Chart
- Design: The chart typically consists of 11 rows of capital letters.
- Layout: The top row features a single large letter, usually ‘E’.
- Progression: Each subsequent row contains more letters that decrease in size.
- Letter Selection: Only specific letters are used (C, D, E, F, L, N, O, P, T, and Z) to maintain consistency and avoid easy recognition.
The History Behind the Snellen Chart
Dr. Herman Snellen developed the chart to provide a standardised method for measuring visual acuity. Prior to its invention, eye doctors relied on less precise methods to assess patients’ vision. The Snellen chart revolutionised eye care by offering a consistent, reproducible way to measure and compare visual acuity across patients and over time.
How to Use a Snellen Chart
Using a Snellen chart is straightforward, but it’s essential to follow the correct procedure for accurate results. Here’s a step-by-step guide:
- Position: Stand 20 feet (6 metres) away from the chart. In some clinics, mirrors may be used to simulate this distance in a smaller space.
- Cover one eye: Use an eye patch or your hand to cover one eye completely.
- Read aloud: Starting from the top, read each row of letters, progressing to smaller rows.
- Record the smallest line: Note the smallest line you can read accurately.
- Repeat: Cover the other eye and repeat the process.
- Both eyes: Finally, test both eyes together.
At Blue Fin Vision® in London, our experienced optometrists guide you through this process, ensuring accurate results and a comfortable experience.
Snellen Chart Interpretation
Understanding your Snellen chart results is crucial for assessing your visual acuity. Here’s what you need to know about Snellen chart interpretation:
The Snellen Fraction
Results are typically expressed as a fraction, such as 6/6 or 20/20. In the UK, we use metres, so 6/6 is considered ‘normal’ vision. Here’s what these numbers mean:
- The top number (6) represents the testing distance in metres.
- The bottom number indicates the distance at which a person with ‘normal’ vision could read the same line.
For example:
- 6/6 vision: You can see at 6 metres what a person with normal vision sees at 6 metres.
- 6/12 vision: You can see at 6 metres what a person with normal vision sees at 12 metres.
Beyond 6/6
It’s possible to have vision better than 6/6. Some Snellen charts include lines for 6/5 or even 6/4, indicating exceptionally sharp vision.
Legal Requirements
In the UK, the minimum standard of vision for driving is between 6/9 and 6/12. This equates to being able to read a number plate from 20 metres away.
The Snellen Chart in the UK
While the Snellen chart is used globally, there are some specifics to its use in the UK:
- Metric System: UK charts use metres instead of feet, hence 6/6 instead of 20/20.
- NHS Standards: The NHS uses Snellen charts as part of routine eye examinations.
- Professional Guidelines: The Royal College of Ophthalmologists provides guidelines for using Snellen charts in clinical practice.
Limitations and Alternatives
While the Snellen chart is widely used, it’s important to note its limitations:
- It primarily tests distance vision.
- It doesn’t account for contrast sensitivity or other aspects of vision.
- Results can vary based on lighting conditions and chart design.
At Blue Fin Vision®, we complement Snellen chart testing with other advanced diagnostic tools to provide a comprehensive assessment of your eye health.
The Importance of Regular Eye Tests
Regular eye examinations using tools like the Snellen chart are crucial for:
- Detecting vision changes early
- Identifying potential eye health issues
- Ensuring your prescription is up to date
- Maintaining optimal vision for daily activities
We recommend adults have an eye test at least every two years, or more frequently if advised by your eye care professional.
Conclusion: Trust Your Vision to the Experts
Understanding the Snellen chart and its role in eye examinations is just the beginning of maintaining good eye health. At Blue Fin Vision®, London’s leading eye clinic, we combine traditional methods like the Snellen chart with cutting-edge technology to provide comprehensive eye care.
Whether you’re due for a routine check-up or have concerns about your vision, our team of expert optometrists is here to help. We offer personalised care, state-of-the-art diagnostics, and tailored treatment plans to ensure your eyes remain healthy and your vision clear.
Don’t leave your eye health to chance. Book an appointment with Blue Fin Vision® today and experience the difference that expert care can make. Your eyes deserve the best – trust them to London’s premier eye care specialists.
