 |
|
| Knowing your eyeglass or contact lens prescription is an important part of knowing if Lasik is right for you. |
|
| |
|
Reading your eyeglass or contact lens prescription is often the
first step to understanding if conventional or custom wavefront Lasik, Bladeless Lasik, PRK, LASEK, Epi-Lasik, RLE, or any refractive surgery procedure is appropriate for you. You should
understand your glasses and contact lens prescription even before
you select a Lasik Surgeon.
Bending Light
Refractive error, or the degree from which you do not have normal
vision, is commonly measured in diopters and indicates the amount light bends within your eye to be focused on the retina and be "seen". A diopter is often represented by a capital "D" in
a prescription. No refractive error is referred to as "plano", often
represented as "pl". The greater the refractive error, the larger
the number for both sphere,
representing the amount of myopia or hyperopia, and and cylinder, representing astigmatism. This known as a spherocylinder or spherocylindrical
prescription.
Spherocylindrical Prescription
A typical spherocylindrical prescription would look like:
| |
sphere |
cylinder |
axis |
| OD: |
-2.75 |
-1.25 |
x15 |
| OS: |
pl |
-0.75 |
x85 |
OD is an abbreviation for the Latin oculus dexter, meaning
right eye. OS is an abbreviation for the Latin oculus sinister,
meaning left eye. The first number after the determination of which
eye is the sphere. A negative number indicates myopia. A positive
number indicates hyperopia. The second number in this prescription
is the cylinder (astigmatism), and the third number is the axis
of the cylinder component. The axis of the astigmatism does not
relate to the amount of cylinder, just the location of the irregularity.
If the patient has no cylinder, then the last two columns may remain
blank, or "DS" for "diopter sphere" may be used.
This prescription shows that the patient has 2.75 diopters of
myopia with 1.25 diopters of astigmatism at an angle of 15 degrees
in the right eye, and the left eye is plano with 0.75 diopters of
astigmatism at an angle of 85 degrees.
| |
sphere |
cylinder |
axis |
| OD: |
-2.75 |
-1.25 |
x15 |
| OS: |
pl |
-0.75 |
x85 |
The above prescription shows that the patient has 2.75 diopters of
myopia with 1.25 diopters of astigmatism at an angle of 15 degrees
in the right eye, and the left eye is plano with 0.75 diopters of
astigmatism at an angle of 85 degrees.
Minus Cylinder or Plus Cylinder
Spectacle prescriptions can be written in two value sets, minus
cylinder or plus cylinder, which are mutually exclusive of each
other but provide the same information. As a rule, ophthalmologists
write scripts in minus cylinder whereas optometrists write scripts
in plus cylinder. Why these two professions cannot get together
and decide on a common method of reporting refractive error is impossible
to explain, but if you have ever had competing siblings in your
family, you may get an idea why this has not yet occurred.
To convert a minus cylinder form prescription into plus cylinder,
or to convert the plus cylinder form into minus cylinder, do the
following:
- Add the sphere and cylinder powers together; this becomes
the new sphere power.
- Change the sign of the cylinder power, from minus (–) to plus
(+) or from plus (+) to minus (–).
- Change the axis value by 90?, remembering that the axis must
be a number from 1 to 180.
The following lens prescriptions, therefore, are equivalent and
interchangeable:
These two prescriptions
are exactly the same
presented in both minus cylinder and plus cylinder form. |
| |
sphere |
cylinder |
axis |
|
|
|
sphere |
cylinder |
axis |
| OD: |
-2.75 |
-1.25 |
x15 |
|
|
OD: |
-4.00 |
+1.25 |
x105 |
| OS: |
pl |
-0.75 |
x85 |
|
|
OS: |
-0.75
|
+0.75 |
x175 |
| minus cylinder form |
|
|
plus cylinder form |
Spherical Equivalent
The spherical equivalent power of a lens prescription is the
average of the dioptric powers in all meridians of a lens. To obtain
this value, add half of the cylinder power to the sphere power.
In other words, do the following:
- Divide the cylinder power by 2.
- Add this value to the sphere power; the result is the equivalent
sphere power of the lens.
For the glasses prescription...
| |
sphere |
cylinder |
axis |
| OD: |
-2.75 |
-1.25 |
x15 |
| OS: |
pl |
-0.75 |
x85 |
| minus cylinder form |
...the equivalent sphere powers of each lens would be calculated
as follows:
OD: –2.75 D + (–1.25 D ÷2) = 2.75 D + 0.625 D = –3.375
D
OS: 0.00 D + (–0.75 D ÷2) = 0.00 D + 0.375 D = –0.375 D
Eye Prescription Terms
| Table of Eye Prescription Terms |
OD |
|
Right Eye |
OS |
|
Left Eye |
BC |
|
Inside curve of
your contact lens (8.0, 8.1, 8.2, etc.) |
Diameter |
|
Size of your contact
lens (13.8, 14.0, 14.2, etc.) |
Power |
|
[Pow] Strength
of your corrective lens (-1.00, -2.75, +2.25, etc.) |
Cylinder |
|
[Cyl] Strength
of your astigmatism (-0.75, -1.00, -1.25, etc.) |
Axis |
|
Orientation of
your astigmatism in degrees (170, 160, 090, etc.) |
Plano |
|
No refractive error |
Pl |
|
No refractive error |
Add |
|
Bifocal plus power
for near distance (+1.00, +2.00, etc) |
Looking For Best Lasik Surgeon?
If you are ready to choose a doctor to be evaluated for conventional
or custom wavefront Lasik, Bladeless Lasik, PRK, or any refractive surgery procedure, we recommend you consider a doctor who has been evaluated and certified by the USAEyes nonprofit organization.
Locate a USAEyes Evaluated & Certified Lasik Doctor.
Personalized Answers
If this article did not fully answer your questions, use our
free Ask Lasik Expert patient forum.
