Biomedical Engineering Reference
In-Depth Information
Figure 15-3.
An aspherical surface can be used to reduce lens thickness.
the eye myopic. This can be one contributing factor to
night myopia
—myopia that
is present only under low illumination.
4
Clinically, consideration should be given
to prescribing lenses with slightly more minus power (or less plus power) for those
patients who do considerable nighttime driving.
Coma
Both spherical aberration and coma occur because the refractive power of a spher-
ical surface is not uniform. When light rays are parallel to the optical axis, the
result is spherical aberration. Coma, in comparison, results when the light rays are
oblique with respect to the optical axis. Such rays may originate from objects that
do not fall along the optical axis. Because of coma, an off-axis point source results in
an image with a comet-like shape as illustrated in Figure 15-4. When the tip of the
comet is pointed toward the optical axis, the coma is said to be positive, and when
it is pointed away, the coma is negative. The asymmetrical nature of coma is espe-
cially detrimental to image formation. While coma is most often associated with
Positive coma seen on a screen
Off-axis point object
Figure 15-4.
Coma results when an off-axis point source is imaged by a spherical
lens.
4. Empty-field accommodation probably contributes to night myopia.
