4-5
● Spherical equ.
The constant term (spherical component) in Fourier
analysis. The constant term (base line) represents the
average corneal refractive power over 360°, namely the
spherical degree.
● Regular Astigmatism
The second order term (regular astigmatism
component) in Fourier analysis. This represents the
refractive power distribution where the point with the
largest refractive power (major meridian) is 90° relative
to the point with the smallest refractive power (minor
meridian), namely the regular astigmatism.
● Asymmetry
The first order term (asymmetry component) in
Fourier analysis. This represents the refractive power
distribution where the point with the largest refractive
power is 180° relative to the point with the smallest
refractive power, namely the asymmetry.
● Higher order irregularity
The third or higher order term (higher order irregular
astigmatism component) in Fourier analysis. This is
the difference (residual) between the true result and
the approximate result obtained by a composite curve
formed by the constant, first order, and second order
terms, and is called the “higher order irregularity.”
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