A Short Course to Optical Genius
By Dr. Jim Hess
This bit of information is for those volunteers about to go on their first VOSH trip and for those who have been on several and really didn’t know what all those numbers meant.
Let’s take an average prescription (Rx):
O.D. +2.25 – .75 x 90
O.S. +2.00 – .50 x 90
This Rx is for a farsighted person with astigmatism who needs a bifocal. Now let’s break this Rx down.
O.D. is an abbreviation for the right eye. All Rx’s are written with the right eye first and the left eye, O.S. second. Often times you will see the Rx written without either the O.D. or O.S. letters, but the right eye prescription will always be written first.
Prescriptions will either have a (+) or a (-) written in front of it. (+) stands for farsightedness and (-) stands for nearsightedness. The higher the number, the stronger the power. Our unit of measure is the Diopter, and we measure in quarter diopter steps. In our example, above, the right eye has two and a quarter diopters of farsightedness. Don’t get hung up on terminology, it’s nothing to worry about. It’s like dollars, four (.25) s make a dollar. You often will see the decimal point omitted. +2.25 will often be written +225 and mean the same thing.
You will sometimes see the word “PLANO” or “PL” written. In optical terms, this means Zero power. For example you may see the term, Plano sunglasses, this means non-prescription sunglasses. You may also see a prescription written as:
PL – .75 X 90, and this means there is no nearsightedness or farsightedness in the prescription, just the astigmatism.
Next comes astigmatism.
In the Rx +2.25 – .75 X 90 the -.75 is the amount of astigmatism. Again, our unit of measure is the Diopter and we measure in .25 steps. You always will see (-) sign in front of the astigmatism power. If you see a (+), it is marked incorrectly and the glasses should be shown to one of the opticians or doctors on the trip. In the original example Rx, the right eye has more astigmatism, (-.75) than the left eye, (-.50).
So you think you have this stuff mastered already, Huh? Good, it doesn’t get much more complicated.
Also in astigmatism corrections, after the amount, you will see an (X), this stands for axis. Astigmatism has a maximum power in a certain meridian or direction, which we call axis. Axis run from 1 to 180 and then starts all over again. Axis 0 is the same as axis 180 and we never write axis 0, it’s always 180. The unit of measure here is degrees, and here the higher number doesn’t mean it’s stronger, it’s just oriented differently. We use primarily axis 180 and axis 90 on our VOSH trips, although all numbers are possible.
Try to study astigmatism axis. Most of the questions asked on VOSH trips and most of the mistakes made in picking the correct glasses for someone deal with axis or astigmatism. Always try to get as close to what is prescribed as possible. Axis 90 is as far from axis 180 as you can possibly get. If the Doctor has prescribed axis 90, don’t pick glasses with axis 5, or 175 or 180, rather pick something close to 90, such as 80, 85, 95 or 100. If the Doctor prescribes axis 180, then axis 5 is very close, as is 175 or even 170 or 10.
You will also find some Rx’s that don’t have astigmatism in them. They will look like this -1.25 Sph or +1.75 Sph, or +200 RO. RO is Readers Only.
The (Sph) is an abbreviation for sphere, which is what a lens is that doesn’t have astigmatism in it. Then you won’t have the (X) in it to worry about.
Next comes the bifocal.
The power for a bifocal is always, always, written in (+). In the first example Rxs, the +1.75 is the power of the bifocal. For our purposes, we will assume the power of the bifocal on the right eye (O.D.) is the same as the left (O.S.), so you will only see the bifocal power written once. You may see the word (ADD) which means additional power, which is what the bifocal actually is. Again, our unit of measure is the Diopter and we measure in (.25) steps. Always substitute a stronger bifocal power then what is prescribed, if the correct power isn’t available.
Another term you will see used is Balance or abbreviated as Bal. This means that the eye where this term is noted does not see well and we can put any Rx in front of it and it won’t make any difference. The word “balance” is used because all we want to do is balance the glasses with something that will look similar to the lens in the other eye.
Now That You Know the Basics, Here Are Some Tips on Choosing the Correct Glasses for Someone:
- There will be many times we won’t be able to give the patient exactly what he/she needs, but our goal is to get as close as we can.
- In farsighted prescriptions, those starting with a (+) , if we can’t find the proper power, always give a weaker power. Giving glasses that are too strong is worse than too weak.
- In bifocals, the opposite, give a slightly stronger bifocal power and they will “grow into them” as they get older.
- In astigmatism, always get as close to axis as you possibly can and remember that axis 1 is almost exactly the same as axis 180 and axis 5 is almost as close. Axis 175 is just as close to axis 180 as axis 5 is. Again, in astigmatism power, always give less than prescribed if you can’t find the correct amount.
Here is another helpful hint that you probably won’t find easy to understand, but will be extremely helpful if you can get a handle on it: It’s called spherical equivalent. It’s used when the patient has some astigmatism and we really don’t have a good substitute pair of glasses with astigmatism in them. You take HALF of the astigmatism power and add it to the sphere power (now I lost you, right?).
Let me give you some examples:
+2.00 – .50 X 180 would change to +1.75 Sph. You are taking half of the astigmatism power, (-.50) so therefore (-.25) is added to +2.00 and you get +1.75 and forget about the astigmatism. Remember, adding (+) and (-) you get a lower number (4th grade math, I believe!). Now for nearsighted prescriptions, -2.25 – 1.00 X 90, you take half of the (-1.00, so (-.50) and add it to -2.25 and you get -2.75 Sph, and again forget about the astigmatism This takes a lot of practice and don’t worry in the least if you don’t understand it.
These are the basics for working in the dispensary at a VOSH clinic. It may seem overwhelming, but really it isn’t. Feel free to ask questions at any time. But don’t worry, after a few hours of actually doing it, you’ll be a pro.
You are about to leave on a difficult journey that will be filled with extremes of virtually every emotion or physical sensation – Jubilation, Sorrow, Pity, Excitement, Exhaustion, Frustration, Impatience, Love, Kindness, and on and on. By giving of ourselves, we will see and experience a country in a way no tourist ever will. It may be one of the hardest things we will ever do, yet one of the best. It will be a remarkable experience nearly impossible to describe, where you will receive more than you give.
Best of luck to you and thanks so very much for caring enough for your fellow man to volunteer to work on a VOSH Mission.
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