Eye Chart Still The Standard For Vision
EFPTOZ . . . Wasn't that a disco hit back in '74?
Not by a sight. Those letters form the first lines on the most popular poster of all time: the eye chart.
Q. The eye chart in the doctor's office is the all-time top poster?
A. That's right: More copies have been printed and sold in America than any other poster. You find it - and its electronic descendants - in the offices of optometrists, eye physicians (ophthalmologists) and all variety of school infirmaries.
Q. What exactly are the charts used for?
A. "It's our basic tool for checking central visual acuity - checking the sharpness of the vision that's directly in the center of the retina," says Judy Hustead, an ophthalmologist with Charlotte Eye Associates, in Charlotte, N.C. "That's all it does check. But is it standard? Absolutely."
The chart can be important as a diagnostic tool for pediatric ophthalmologists. "There are many problems that glasses will not help, so it is very important for children to have a complete eye exam early in life," according to Jennifer Nichols, a certified orthoptist (eye muscle specialist) at Charlotte Eye, Ear, Nose & Throat Associates, in Charlotte N.C. "This test will help with the diagnosis."
Q. So the eye chart is pegged to 20/20 sight?
A. It's the other way around. The standard eye chart is the yardstick that established 20/20. If, from a distance of 20 feet, you can read a line that "normal" sight would enable you to read at 20 feet, your sight is termed 20/20.
Q. Does that big E at the top have a title?
A. Yes: 20/400. And if that's all you can make out on the chart, that's what your vision is considered.
Q. What's the smallest line?
A. "Most charts go to 20/15; some go to 20/10," Hustead says. "If you can read that, your sight would be considered `better than normal,' though a lot of people can see 20/15. By the way, a lot of people can see better with both their eyes open."
Q. Whose idea was all this?
A. Dr. Herman Snellen (1831-1908), a professor of ophthalmology at the University of Utrecht, in the Netherlands, is credited with inventing it in the 1860s. However, an American doctor - Ezra Dyer of Philadelphia - is thought by some to have made the first test card for vision.
Dyer had worked in Europe with Snellen and Snellen's teacher, F.C. Donders. He overheard Donders tell Snellen that what the world really needed was an eye test based on letter recognition. Dyer created a test based on Snellen's principles in 1862. History gave us the test and Snellen the recognition: The test is commonly known as the Snellen eye chart.
Q. How did the letters get picked?
A. The 1935 "Pocket Ophthalmic Dictionary" says Snellen held that "in order to distinguish one letter from another the eye must be able to distinguish the spaces between the lines which correspond to a visual angle of pi." In other words, can you tell an O from a C? An E from an F?
Q. Can any letter be used?
A. We asked Nichols, who thought for a moment before answering: "Well, I haven't seen a Q, but pretty much all of them have been used."
Hustead: "What's important is the size and contrast of the letters, not what the individual letters are."
Q. So do some charts have other alphabets?
A. Franel Optical Supply of Maitland, Fla., sells Snellen-style charts that have the alphabet in Hebrew, Chinese and Greek as well as clocks, aprons and T-shirts bearing it. (Sorry - the firm sells only to others in optical industry.)
Q. How do they test children who don't know the alphabet?
A. The same way they did when you were a kid: With a Snellen chart made entirely of E's: Some are upside down, others are on their sides. The child holds a cardboard E and turns it to match the E's on the line he or she is "reading." It's called the "tumbling E chart."
You also can use the Allen Preschool Chart with children who can't read letters, says Nichols. "With that one, kids identify pictures - such as a hand, a birthday cake, a horse, a truck, duck, etcetera."
And there are hand-held picture charts.
Q. How come some doctors have you look at a mirror that reflects an eye chart - or part of one - that's behind you?
A. Ah, the "projection chart." Again, the eye-care field is pegged to what the "normal" person can see at 20 feet. Nichols says, "If you have an examining room where the `lane' isn't 20 feet, you just use the mirror to get that 20-foot effect."
A patient doesn't have to see the whole chart, either. The Snellen concept - not the actual chart - is used at Charlotte Eye Associates offices. "We just roll to one individual line and ask the patient to read it," says Hustead.
The other lines don't appear on the wall. Sometimes, patients see even less, says Dr. Timothy Saunders, a pediatric ophthalmologist with Charlotte Eye, Ear, Nose & Throat: "For some problems, you just have to present one letter at a time."
You can get projector charts that have both numbers and letters: "You can just roll to whichever one you want to use," says Nichols. "In our office we have all of them. Which one you pick depends on the patient."
And changing charts can throw a curveball at patients who may try to cheat by memorizing the letter sequence. That does happen.
Nowadays, computerized charts can select an assortment of numbers and/or letters at random and put them up on a video display screen.
The verbal refrain is the same: "Would you please read . . ."
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