An eyesight chart or eye chart is a visual aid used by eye doctors (known as ophthalmologists) to measure a person’s visual acuity.
The most common is known as the Snellen chart, named after its inventor Herman Snellen, a Dutch ophthalmologist.
It was developed in 1862 and is being used for eye tests up to this day.
Typically charts generally display several rows of optotypes or test symbols with each row in a different size.
What is visual acuity?
Typically, visual acuity refers to the ability to resolve separated lines or points.
A 20/20 vision is considered the maximum acuity of a human’s eye.
Visual acuity is the only thing an eyesight chart can measure which is why they are only a part of a complete eye examination.
How to use an eyesight chart
The eye chart is composed of letters and numbers which are printed boldly in black, against a white background.
The letters and numbers are arranged in rows and decrease subsequently in size.
These printed characters on the chart are called optotypes. They have a unique font and a simple geometric pattern which is exclusive to eye charts.
When a patient uses an eyesight chart, he or she is asked to stand at a distance of 20 feet, cover one eye and read each letter or number of each row aloud. Visual acuity is measured and based upon the smallest letter or number a patient can reliably read.
In the case of people who have myopia or near-sightedness, the visibility of the characters on an eye chart become more visible and readable the closer they get to the chart, and vice versa for those who have hyperopia or long-sightedness.
Other types of eye charts
Other types of eye charts, aside from the Snellen chart, are also available and used to test vision.
1. Lea Test or Kindergarten Eye Chart – contains pictures instead of letters and numbers; used to test children’s vision. It was developed in 1976 by a Finnish ophthalmologist.
2. The Tumbling E – where the letter E is printed in different orientations and a patient has to identify in which direction it is facing.
3. The Landolt C Chart – it is similar to the Tumbling E chart but has a broken circle instead of the letter E. With this type, a patient is asked to identify where the broken pieces of the circle is located on the chart.
4. The LogMAR Chart – developed in 1980 by Bailey and Lovie is a more standardized eyesight chart.
The different types of eye charts mentioned above have their own limitations and some people think that these manual charts sometimes have examiner-induced bias. For this reason, some people opt to have their vision tested with digital computer eye charts which are widely available today or even make their own.
Another important chart is Amsler’s Chart to test for macular degeneration.
See also Pelli Eye Charts which as well as diagnosing eye problems, also shed light on how the brain recognizes objects, like letters, words, and faces.
Using the chart
The person who is undergoing the eye test is generally asked to identify the numbers or letters on the chart.
The identification is generally started from the large rows and then going ahead to the smaller rows until the individual is not able to identify the optotypes reliably.
The process of testing visual acuity with the help of an eye test chart is a psychophysical measurement.
This helps in determining a sensory threshold.
Additionally, there are some eye examination charts which are also available for very young children as well for illiterate adults.
This type of chart have pictures or patterns.
Other types of charts may have letter “E” turned in different orientations.
Most of these charts are usually designed for use at 6 meters or 20 feet.
Optometrists and health care workers often carry out examinations at a persons home or work place.
In these cases a small portable eye testing chart is used at a shorter distance of 3 meters.