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11 Variations in prescription writing
There is a surprising amount of variation in the way prescriptions are written; the layout and terminology used is not uniform.
When no correction is needed, the spherical power will sometimes be written as "0.00" and sometimes as "Plano" or "Pl" (because the lens, although not flat, is optically equivalent to a flat piece of glass).
When cylindrical correction is needed, the mathematics and optics of the way lenses combine mean that there are two different ways to write the same correction. One is called the plus-cylinder form and the other the minus-cylinder form. These two prescriptions are equivalent:
| Spherical
| Cylindrical
| Axis
|
| 2.00
| 1.00
| 90
|
| Spherical
| Cylindrical
| Axis
|
| 3.00
| −1.00
| 180
|
Both of them specify a power of 2.00 diopters at the 180 degree axis and 3.00 diopters at the 90 degree axis.
The first one specifies a 2.00 spherical component, which, by itself, would give a power of 2.00 diopters along both the 180 and 90 degree axis, and adds a 1.00 cylindrical component at 90 degrees. The result is 2.00 diopters at 180 degrees and 2.00 + 1.00 = 3.00 diopters at 90 degrees.
The second specifies a 3.00 spherical component, which by itself would give a power of 3.00 diopters along both the 180 and 90 degree axis, and subtracts a 1.00 cylindrical component at 180 degrees. The result is 3.00 − 1.00 = 2.00 diopters at 180 degrees and 3.00 diopters at 90 degrees.
12 See also
Ophthalmology
