Javal's rule is a mathematical formula used to estimate
astigmatism
Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at ni ...
based on
keratometry
A keratometer, also known as an ophthalmometer, is a diagnostic instrument for measuring the curvature of the anterior surface of the cornea, particularly for assessing the extent and axis of Astigmatism (eye), astigmatism. It was invented by the ...
readings.
[Schwartz, S. Geometric and Visual Optics. pg 219-221. McGraw-Hill. 2002.] The estimate is useful for high cylinder measures, generally over -2.00
diopters
A dioptre ( British spelling) or diopter ( American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or cu ...
.
The estimate is found by multiplying the difference in power between the two meridians by 1.25 and factoring in the average lenticular astigmatism, which is -0.50 x 090.
References
Ophthalmology
Optometry
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