Mirror, Mirror on the Wall… The Vision Test Mystery Solved

It’s interesting that SNELLEN is very nearly a palindrome; a word spelled the same forward and backward. But before we get to that, what’s a Snellen, you might ask…? A Snellen is Herman Snellen, a 19th-century Dutch ophthalmologist and the inventor of the renowned Snellen Eye Chart.

And why is it interesting that his name is nearly a palindrome, well, the answer to that question comes a bit later…

Visual Acuity: What is 20/20 vision?

The Snellen Eye Chart is only one of the tools your eye doctor uses to test your visual acuity, or simply, how well you see. When we think of “perfect vision,” we often think of what’s referred to as 20/20 vision, when in fact, 20/20 vision is actually “normal vision” and it means the ability to see an object clearly at a distance of 20 feet.

Beyond just the ability to see an object clearly from 20 feet away, your visual acuity is also concerned with your peripheral vision, eye coordination, depth perception, ability to focus, and color vision. A comprehensive eye exam will test all of these faculties in determining your true visual acuity.

F E L O P Z D – D Z P O L E F

When Herman Snellen was developing his eye chart for testing visual acuity it was already understood within the field of ophthalmology that clear vision at 6-meters, or 20 feet, was a normal and healthy vision. As such, the Snellen Eye Chart was designed to be read at a distance of 20 feet, and while it may not have been an issue at the time, it’s not every eye doctor that has an exam room 20 feet or greater in length.

To solve this intriguing problem of space, an ingenious solution was developed whereby a projector and mirror are used to artificially increase the size of the room. By projecting and inverting the letters of the Snellen Eye Chart so they become legible the right way around on a mirror in front of the patient, a person sitting only 8 feet away from a mirror is now artificially sitting 20 feet away from the eye chart itself.

So, it’s interesting that Snellen being nearly palindrome in itself is now having the universally familiar letters of his eye chart folded and flipped around backward to simulate distance where it can’t be found otherwise.  

Ophthalmology Innovations

The projector and mirror are not as necessary as they once were though the technique is still widely in use. With innovations in technology, there are now digital and computer simulations that can be used in place of smoke and mirrors. One hundred years from now that will be as ingenious and fascinating as the simple mirror trick was when it was developed as well.

20/20 Vision at Berlin Optical Expressions