I've been playing around with writing a "general discussion" of
ophthalmic Rxs, so I'm gonna run it up the flagpole here.
LASIK Flap Forum Index » General Discussion » Primer on understanding
our prescriptions?
Posted: Thu Feb 23, 2006 2:55 am
DrMinarik
Joined: 17 Feb 2006
Posts: 6
Location: Rockford, IL
O.K., I'm game to take a shot at this. Since this forum is read by a
lot of different people with different backgrounds, some people who
have a math or physics background will be really bored by this, while
others might find it very difficult. But, I have to start somewhere, so
lets go.
For this discussion, imagine we are in a classroom, and I hold a piece
of chalk out in front of the class. I call on you, Truly, waking you up
from your nap in the back of my classroom. I ask you to tell me as
precisely as possible where this piece of chalk is, using as few words
as possible with no doubt being allowed to "chance" where the chalk is.
As it so happens, you just finished reading your homework chapters
about "three dimensional thinking", so you are ready for my challenge.
"That piece of chalk is 12 feet from the front of the room, and
thirteen feet from the side wall", you explain gleefully. But your nap
was too restful, because you only gave me a two dimensional answer. I
take the chalk and start jumping straight up and down. "That's not
enough of an answer" I explain, because all the way up and down while
I'm jumping, the chalk is still 12 feet from one wall and thirteen feet
from the other.
"I get it now!" you add gleefully, "The chalk is also 7 feet from the
floor!"
Ahh, we are finally there...any point in space can be described as long
as you have three dimensions to assign to it. The space from left to
right, or wall to wall in this example, is often called the "x" axis.
The space from the front of the room to the back, in this example,
would by the "y" axis. So, the X and y axis cross each other to form a
two-dimensional set of co-ordinates. Then, when you add the "up and
down", or "ceiling to floor" dimension, you have added the "z" axis.
When you watch a "normal" television, everything is happening in two
dimensions, since all the little moving people on the screen are moving
along an "x" or "y" axis. But, if you put on some "3-D Glasses" and the
people seem to jump off the screen and you can seem to touch them, you
have now added the "z" axis.
Now, let's relate this to opthalmic prescriptions. The exact center of
the fovea in the back of your eye, the location of your most precise
vision, is a little "divot" in the middle of your macula. That little
spot is where the chalk is. I want to bend light into your eye, and I
want that light to hit that exact little spot called the fovea. Where
do I need to send the light? How do I want to get it there? I'm going
to bend light, I'm going to "refract" (bend) light into your eye and
send it to that spot. How can I get to that one spot? I'm going to
supply three dimensional co-ordinates that explain where that spot is,
so that the light that comes along will go to the right spot.
Let's "make up" a prescription. Let's say that Truly's new right eye
prescription is +1.00 -1.00 X 180. What does that mean? The first
number, "+1.00" means that the power of her eye along the x axis is now
+1.00. Along the 180 meridian, which happens to be level with the
horizon, the power of her eye is +1.00. The next number in the
prescription is a "difference" between the x axis and the y axis. So,
if the x axis power is +1.00, the power 90 degrees away (at axis 90) is
1 "unit" less than the power at 180. So, if the power at 180 degrees is
+1.00, the power at 90 degrees is +1.00 minus 1.00, so it is a net
power of zero. So, if the first column of the prescription is the "x"
dimension, and the second column helps you get the "y" dimension, all
is left is the "z". The z number is determined by the numbers, plus or
minus numbers, that you put in those first two columns. So, the power
of Truly's eye at 180 is one Diopter, the power 90 degrees away is
Plano
(or Zero) so her prescription is +1.00-1.00X180. If I shine a beam of
light at her eye, and i bend it through a spectacle lens with the
intent to bend it to that sweet spot, the fovea, the "chalk spot" I
must specify three dimensions, and the beauty of an ophthalmic
prescription is that it takes that beam of light to the exact spot we
want it to, as long as that beam of light hits a spectacle lens that
bends light with this little formula...+1.00-1.00X180. Its all there!
Where is the chalk in relation to the two walls and the ceiling? It
takes a lot of words to describe where the chalk is in relation to four
walls and a ceiling, especially when you consider that an opthalmic
prescription squeezes all that information into a little formula like
+1.00-1.00X180.
Alright, wake up. Tomorrow's homework is due on my desk by morning, and
I expect the answer to the following extra credit question: What did
the Zen Master say to the hot dog vendor?
"Make me one with everything."
_________________
For resources, check
www.doctormyeye.com
I made my own math reguarding diopters to 20/xxx
Linear Mode
