Understanding Optical Prescriptions
Most people know the numbers that signify the prescription
of their eyes, but not everyone understands what they mean. To better understand your own eyes, here is a
brief guide to reading optical prescriptions:
Visual refractive disorders such as myopia, astigmatism,
hyperopia, and presbyopia are measured in units that are known as
diopters. Diopters represent the amount
of correction that is needed to achieve normal vision. The more nearsighted or farsighted a person
is, the higher their prescription is in terms of dippers.
Prescriptions are written in three numbers.
As an example, -5.00 -1.50 x 180 represents a typical
prescription.
The first digit signifies the degree of nearsightedness or
farsightedness. The sign identifies
whether the person is nearsighted (which is demonstrated by a - sign) or
farsighted (which is demonstrated by a + sign).
In the case of the example prescription above, the first number (-5.00)
shows that the person is nearsighted to a degree of 5.00 diopters.
The second number identifies the degree of astigmatism. It can be written with either a + sign or a –
sign. The example prescription shows
that the patient has astigmatism of 1.50 diopters.
The third number is the axis, which indicates the direction
of the astigmatism. In the example, an
axis of 180 degrees means that the astigmatism is in the horizontal direction.
In terms of connecting these numbers to LASIK eligibility,
most excimer laser studies have found that the average laser patient has a
prescription between -3.00 and -4.00 diopters of myopia with less than 1.00
diopters of astigmatism.
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