How Does the Eye Chart Work? | Long Island Eye Doctor
The eye chart is a well-known, universal symbol used to test a person’s visual acuity. The Snellen eye chart was developed by Herman Snellen, a Dutch ophthalmologist, in 1862. Snellen’s invention created an objective standard for measuring visual acuity. This, in turn, transformed the role of eye care specialists in vision testing and prescribing of spectacle prescriptions and is still used today.
Have you ever wondered what 20/20 actually stands for? A patient achieves 20/20 visual acuity when he can read the bottom line of the acuity chart at a distance of 20 feet. Similarly, a patient achieves 20/40 visual acuity when he can read no lower than the fourth line at a distance of 20 feet. In order for a patient to be considered “legally blind”, he is unable to read even the largest letter on the acuity chart WHILE wearing his updated spectacle correction.
Not all eye charts are composed of standard Snellen letters. Visual acuity charts for children often have shapes, numbers, or the letters HOTV which can be used as a matching game. Additionally, the letter E or C can be used in a “tumbling” format, rotated in increments of 90 degrees. In this case, an illiterate patient does not need to verbalize which letter he sees, but rather which direction the letter is facing.
The eye chart is a critical device used during all vision exams and visual acuity screenings. Its one of those inventions you may wish YOU had come up with!
Dr. Meghan Schiffer