By this point, just about everyone in the U.S. has seen a visual acuity chart.
The Snellen, or "Big E" charts projected or mounted onto the wall of an optician's office or DMV measures the sharpness of an individual's vision at varying distances. Arranged as a pyramid, the letters of the Snellen chart test the clarity of a patient's vision from the baseline distance of 20 feet.
A Dutch eye doctor named Hermann Snellen in the 1860s
, developed The Snellen eye chart - which tends to be the classic and most recognized option for vision acuity tests. Though there are many variations, the chart often shows eleven rows of capital letters, with the top row containing a single letter and the other rows featuring letters that grow progressively smaller.
The History of Visual Acuity
As you might imagine, complications arose in diagnosing eye problems and purchasing glasses in the 1700s. Not only did you have to ascertain the cause of the issue yourself, but you also had to find the right lenses to match your condition. Some vendors tried to provide assistance by scratching age-ranges into spectacles - leading to the misconception that everyone's vision deteriorated at the same rate as they aged. However, in the mid-19th century, doctors came to realize that patients need varying levels of eye care. As a result, Dr. Franciscus Donders developed a concept for diagnosing vision issues - asking his colleague, Dr. Snellen, to create a chart for patients to look at and report what they saw.
Initially, Snellen printed a chart using various symbols and signs of disparate sizes. Though a reasonable idea in theory, the symbol-based chart wasn't effective, as some people describe symbols differently to others - leading to varying results. After some research, Snellen decided that letters would be the best choice, and his chart became a roaring success.
The Snellen chart spread across Europe rapidly, with the first large order from the British Army coming in during 1863. As a simple solution to a complicated problem, the chart has remained popular as a cheap, and easy-to-use tool. Known to some as the big "E" test, the Snellen chart earned the name because it usually featured an "E" as the initial letter, following from the original design. Snellen also created a test for illiterate individuals which used a rotating "E"
The resulting accomplishment was a way for eye care providers to express the sharpness of an individual's vision in the form of a fraction. Although they are the standard today, Snellen's tests were not the first, or the last. Before Snellen, Heinrich Kuechler invented a chart in 1843
, and after Snellen, several charts emerged based on his fraction system.
Measuring Visual Acuity
Visual acuity tests
measure the central vision of an individual - otherwise known as their ability to distinguish the details and shapes of varying objects. In regards to the Snellen test, people who can read the second line of letters from the bottom have 6/6 vision - originally referred to as 20/20 vision
. Regardless of common misconceptions, it's important to note that 6/6 vision does not translate to perfect eyesight, or indicate other important aspects such as color perception, depth perception, or peripheral vision. Rather 6/6 vision is "normal" vision - meaning that you can read letters from a twenty foot distance.
Within the United States, an individual is "legally blind" if their best visual acuity including the use of contact lenses or eyeglasses is 20/200 or worse. This would mean that you may be able to read the big "E" at the top of the chart, but would not be able to read letters any lower than that. 20/200 vision indicates that the patient can read a letter at twenty feet that individuals with "normal" vision would be able to recognize from 200 feet. Visual requirements for a driver's license is determined state by state. Many states require visual acuity of of at least 20/40 to obtain an unrestricted driver's license; however, patients with reduced vision who meet other criteria as determined by the DMV may still be eligible to drive under certain conditions. Not many people have visual acuity that is better than 20/10 - however many animals do.
The Size of the Letters
To indicate the letter sizes used on his visual acuity chart, Snellen determined size by the distance in feet or meters at which the letter subtends 5 min of arc. Louise Sloan simplified this - commenting that M-Unit was the size that subtends 5 min of arc
at a single meter. Today, most eye charts used for vision testing carry the letter size designation in M-units, and this makes it easier for professionals to calculate the visual acuity of a patient for charts used at any distance alternative to the original design.
Visual acuity values find the magnification requirement of the person to achieve "normal" 6/6 or 20/20 clarity. Often, these examinations make up part of a routine physical examination or eye exam - particularly if a problem with sight is present. Unfortunately, as useful as the acuity chart is, it is not capable of determining the complete scope of a person's vision capacity.
Limitations of Visual Acuity Charts
Snellen eye charts, or acuity tests, only measure the sharpness of an individual's vision - or their ability to perceive small details. Though they can help doctors determine the need for contact lenses or prescription eyeglasses, they do not measure a person's ability to perceive contrast, color, depth, or peripheral sight. Visual acuity charts also do not measure factors relating to the overall health of a person's eyes - such as the pressure of their eye fluid, their chances of acquiring glaucoma, and retina shape.
Visual acuity tests are an important part of measuring a person's sight for various purposes and functions, but they are not the only thing worth considering in comprehensive eye care. Rather, visual acuity charts make up a single component of a complete eye exam, which patients should have every one to two years to ensure they take optimum care of their vision.
Where do you think that visual acuity charts are most useful? Do you think other letter and symbols may be more helpful on a Visual Acuity Chart? Comment below!
This article was provided by Veatch Ophthalmic Instruments.