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February 1, 2018
A form of visual acuity testing was employed 1000 years ago when desert Bedouins used the ability to identify double stars as an evaluation of vision. The development of the Snellen chart in the 1860s by Dutch eye doctor Hermann Snellen was an important landmark in the standardization of measuring visual acuity. Dr. Snellen also developed the “Tumbling E” chart (Random E test).
Visual acuity measures the ability of the eye to distinguish shapes and the details of objects at a given distance (or sharpness of vision). Central visual acuity is a key sign of overall ocular function. Excellent visual acuity indicates the cornea and lens are clear, the image is clearly focused on the retina, the afferent visual pathway is functioning, and the visual cortex has interpreted received signals appropriately. The exam is performed by optometrists, ophthalmologists, opticians, technicians, nurses, and now even in households with the EyeQue Insight. The central visual acuity test is used to:
Provide a baseline visual acuity recording
Determine best possible visual acuity in each eye
Aid examination and diagnosis of refractive error
Assess any changes in vision
Measure the outcomes of cataract or LASIK surgery
The Snellen test is a chart of letters or symbols, commonly used in schools and eye doctors’ offices. It usually includes 11 rows of capital letters, with the first line having one very large letter. Each row has an increasing numbers of letters that get progressively smaller in size. The individual views the chart from either 14 to 20 feet away, while sitting or standing. At an eye doctor’s office, the chart might be projected or shown as a mirror reflection. Using a mirror eliminates the need to have 20 feet of space from the patient to the chart. One eye is covered with a plain occluder, card, or tissue. Letters are read out loud with the uncovered eye, from the top of the chart down until a person can no longer accurately distinguish them. Then the test is repeated with the other eye. An easier and faster technique is to instruct the patient to read the smallest line they can see.
Often, this results in the individual going directly to the 20/20 line and reading it correctly, saving considerable time. If a person wears distance glasses, the test is done with and without glasses. In individuals age 40 and older who wear reading glasses or bifocal spectacles, visual acuity can be measured with the near card at 14 inches, which correlates well with distance visual acuity.
This test is 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 facing in different directions. While looking at the letter on a chart or projection, the person states whether the E is facing up, down, left, or right. With this chart, there is a one in four chance a person can guess the direction, therefore it is recommended they correctly indicate the orientation of most letters of the same size (e.g. five out of six).
The top number refers to the distance in feet you sit/stand from the chart. The bottom number indicates the distance at which a person with normal eyesight can read the same line you correctly read. The line recorded is the last one in which the patient correctly reads the majority of letters. The visual acuity of the right eye is typically written above the fraction for the left eye.
Visual acuity is expressed as a fraction (e.g. 20/20 or 20/40). A person with 20/20 vision can see what an average person can see on an eye chart when they are standing 20 feet away, which is considered normal. If a person has 20/40 vision, this means they need to be 20 feet away to see an object others can normally see from 40 feet away. After reading the chart, a person looks at the chart using different corrective lenses. The lenses are switched out in each eye until the person can see the chart clearly with both eyes.
Visual acuity worse than 20/40 frequently results in difficulty in reading small print. While state laws differ, visual acuity of 20/40-20/60 or better in at least one eye is generally required for driving a car. Visual acuity of 20/200 or worse in the better eye is frequently used as the parameter for legal blindness.
Although they are useful for assessing central visual acuity, eye charts do not determine if you have an eye disease such as glaucoma or problems with the retina. They also do not measure other vision problems such as loss of peripheral vision. However, with new technological advances, you can now track your visual acuity from home which can serve well for those in need of frequent visual acuity check ups. Visit www.eyeque.com/insight to learn more about the at home visual acuity screener.