Page 187 - Modern Optical Engineering The Design of Optical Systems

170 Chapter Eight

distance (2 or 3 diopters), it is necessary to wear positive lenses to read
comfortably.
Keratoconus is a conically shaped cornea and can be corrected by
contact lenses which effectively overlay a new spherical surface on the
cornea.
An opaque or cloudy lens (cataract) is frequently removed surgically
to restore vision. The resultant loss of power can be made up by an
extremely strong positive spectacle lens. But better solutions are a
contact lens or by surgically implanting a plastic intraocular lens near
the iris. Such an aphakic eye, lacking a lens, cannot accommodate.
Also, the change in retinal image size due to the shift in refractive
power from inside to outside the eye (if due to the strong spectacle
lens) will preclude binocular vision if only one eye is lensless.
Aniseikonia is the name given to a disparity in retinal image size
from one eye to the other, occurring in otherwise normal eyes, and
results in lack of binocular vision if the disparity is larger than a few
percent. Aniseikonia can be corrected by special thick meniscus lenses or
airspaced doublets which are effectively low-power telescopes the mag-
nifications of which balance out the difference in retinal image size.
In instrument design, a number of additional factors should be taken
into consideration, especially for binocular instruments. An adjustment
must be provided for the variation in interpupillary distance, so that
both sides of the instrument can be aligned with the pupils of the eyes.
1
This distance is typically about 2
in, but it ranges from 2 to 3 in. Both
2
halves of a binocular instrument must have the same magnification
1
(within
to 2 percent, depending on the individual’s tolerance) and both
2
1
halves must have their axes parallel (to within
prism diopter vertically,
4
1
diopter divergence, and 1 diopter convergence). Each side must be
2
independently focusable to allow for variations in focus between the two
eyes. A focus adjustment of 4 diopters will take care of the requirements
of all but a few percent of the population; 2 diopters will satisfy about
85 percent. The depth of field of the eye (the distance on either side of the
1
point of best focus through which vision is distinct) is about
diopter.
4
The Rayleigh quarter wave (see Chap. 11) depth of focus is 1.1/(pupil
2
1
diameter) diopters, which for a 3-mm pupil works out to
diopter. For
8
biocular devices, such as head-up displays (HUDs), the angular disparity
between the eyes should be less than 0.001 radians.
Bibliography
Adler, F., Physiology of the Eye—Clinical Applications, St. Louis, Mosby, 1959.
Alpern, M., “The Eyes and Vision,” in W. Driscoll (ed.), Handbook of Optics, New York,
McGraw-Hill, 1978.
Blaker, W., “Ophthalmic Optics,” in Shannon and Wyant (eds.), Applied Optics and Optical
Design, Vol. 9, New York, Academic, 1983.