Biomedical Engineering Reference
In-Depth Information
Figure 15-16.
The patient is not looking through the optical centers of the lenses, but
above them. Depending on the lens material's Abbe number, strength of the prescrip-
tion, and distance from the optical center, this may result in lateral chromatic aberra-
tion that reduces visual acuity.
Lateral chromatic aberration is an important consideration when prescribing
lens materials with low Abbe numbers, such as polycarbonate. As the patient looks
through more peripheral regions of the lens, the prismatic power increases, result-
ing in greater lateral chromatic aberration. This may be seen by the patient as
colored fringes.
Occasionally, a patient who has received polycarbonate lenses may report a
reduction in visual acuity even when looking straight ahead. As illustrated in
Figure 15-16, this can occur when the optical centers of the lenses are not aligned
with the patient's pupils, forcing the patient to view through a noncentral region
of the lens that has significant lateral chromatic aberration. When prescribing
polycarbonate lenses, particularly in high prescriptions, it is advisable to ensure
that the patient's pupils are not too misaligned with the lens optical centers.
Longitudinal Chromatic Aberration
Consider Figure 15-17, which shows white light from an infinitely distant
object incident upon a converging lens. Because the lens's index of refraction is
different for each of the wavelengths constituting white light, its focal length is
