Page 186 - Modern Optical Engineering The Design of Optical Systems

Characteristics of the Human Eye 169

lenses which effectively flatten or strengthen the curvature of the outer
surface of the visual optical system.
Radial keratotomy is a surgical technique where radial cuts are made
in the cornea (through most of its thickness). This weakens the cornea,
and the internal pressure of the eye causes it to bulge in the region of
the cuts, thus changing the shape and the power of the cornea. Two obvi-
ous drawbacks to this procedure are light scattering from the corneal
scars left by the cuts, and the fact that the power of the eye tends to
change as one ages, so that the correction may not be permanent.
Another technique (PRK) involves a change in corneal shape by sculpt-
ing using laser ablation. LASIK slices a thin flap of the cornea off and
then ablates the cornea to change its shape; the flap is then replaced.
The chromatic aberration of the eye was discussed in Sec. 8.3; many
eyes have some undercorrected spherical aberration as well. The lens of
the eye has aspheric surfaces and a higher index of refraction in the cen-
tral core of the lens than in the outer portions; both of these factors reduce
the power of the system at the margin of the lens and tend to correct the
heavy undercorrected spherical from the cornea. A few persons have over-
corrected spherical. In most people, the spherical tends toward overcor-
rection with accommodation, since the lens bulges more at the center
than at the edge when the eye focuses on a near point. As much as 2
diopters of spherical have been measured; however, like chromatic aber-
ration, spherical seems to have little effect on the resolution of the eye.
Presbyopia is the inability to accommodate (focus) and results from the
hardening of the material of the lens which comes with age. Figure 8.11
indicates the (typical) relationship between age and the power of
accommodation. When the eye can no longer accommodate to reading

Figure 8.11 The variation of accommodation power
with age (solid line). The dashed line indicates the
time in seconds to accommodate to 1.3 diopters.