20/20 vision…What Does It Mean?
So what’s the first thing you do during the vision part of your eye health examination? “Read the Chart with the Big letter “E” right?
The typical Eye chart has 11 rows of capital letters. The top row has one famous letter…the letter “E”. Then the letters are random and get smaller as each line goes down.
Why the letter “E”? Well, on an eye chart we call the letters “optotypes”. Dr. Snellen invented the first Eye Chart back in 1862, using Optotypes looking like block letters with a specific geometry.
-the thickness of the lines equals the thickness of the white spaces in-between the lines of the optotypes.
-the height of the letter is 5 times the thickness of the Optotypes’ line.
Dr. Snellen decided and defined ‘standard vision’ as the ability to see one of his block optotypes when is “subtended 5 minutes of arc”. OK, so what exactly does that mean in simpler terms? Well arcminutes is a unit of measure, like miles used to measure distance, or liters used to measure liquids, an arcminute is a measure of “angle”. When rays of light come together, they form an angle.
An Arcminute is equal to 1/16 of one degree. Getting confused yet? Dr. Snellen stated that “standard vision” is the ability to recognize one of his block optotypes when it subtends 5 minutes of arc, or more simply stated, when the space pattern is separated by 1 minute of arc. In other words, he felt perfect human vision is when you can distinguish a space pattern separated by 1 minute of arc.
OK! So where does the “20” in 20/20 come from?
Once again, Dr. Snellen decided that the standard distance you should be able to discriminate space of 1 minute of arc is at 6 meters, or at 20 feet.
20/20 is simply defined as Perfect vision (being able to separate 1 min of arc when is ‘subtends” 5 minutes of arc) at 20 feet.
If you is measured at 20/60 vision this simply means that another person can see 5 minutes of arc from being 60 feet a way…but you would have to go upto 20 feet to see the same thing that “miss perfect eyes” can see as far as 60 feet away.
There are many types of eye carts available at the eye doctors office.
Computer Vision Tester
This state of the art. Since many of us work on computer screens, testing more accuate Optotypes is what we use for prescribing the latest in High Definition Vision Lenses for the eyeglasses you are prescribed, the contact lenses you wear and even laser eye surgery. Our office is equiped with 2 of these testers!
Projector Eye Chart
This is also a more accurate way of vision testing and measuring, most modern offices are equiped with this (our office has 3 of these). Projectors provide an excellent measure, resulting in very good vision from your new prescribed eyewear contact lens or laser eye surgery.
Standard "OLD SCHOOL" Snellen eye chart
While this is where "it all started from" it is not an accurate measure as lighting and testing distance may not be as accurately calibrated. We suggest if your doctor is using this type of testing for your eyes, it is more a "screening" rather than an accurate vision measuring and prescribing device.
Be sure to Tune into our Next Blog where I will talk about the 20-20 Rule for 20/20 Vision ! Enjoy your day!
Dedicated to your eye health and best vision,
Dr. S. Steven Chander, O.D.