Page 174 - Modern Optical Engineering The Design of Optical Systems

Chapter
8

Characteristics of the Human Eye

8.1 Introduction
A knowledge of the characteristics of the human eye is important to
the practice of optical engineering because the majority of optical
systems utilize the eye as the final element of the system in one way
or another. Thus, it is vital that the designer of an optical system
understand what the eye can and cannot accomplish. For example, if
a visual optical system is required to recognize a certain size target or
to measure to a certain degree of accuracy, the magnification of the
image presented to the eye must be sufficient to allow the eye to detect
the necessary details. On the other hand, it would be wasteful to
design a system with a perfection of image rendition which the eye
could not utilize.
The human eye is a living optical system and its characteristics
vary widely from individual to individual. For a given individual, the
characteristics may vary from day to day, indeed from hour to hour.
Therefore, the data presented in this chapter must be considered as
central values in a range of values; in fact, some data are useful only
as an indication of the order of magnitude of a certain characteristic.
The conditions under which the eye is used play a large role in deter-
mining the behavior of the eye and must always be taken into account.
In physiological optics, the unit of measure for the power of a lens or
optical system is the diopter, the abbreviation for which is D. The
diopter power of a lens is simply the reciprocal of its effective focal
length, when the focal length is expressed in meters. For example, a lens
1
with a 1-m focal length has a power of 1 diopter; a
-m focal length,
2
2 diopters; and a lens of 1-in focal length has a power of 40 diopters
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