This chapter will be devoted to a description of the human eye that will provide a basic understanding of how the eye functions and how it will interact with visual optical systems. Also covered will be the topic of optical design and lens design as they apply specifically to systems that are used with the eye as the final detector. Because the human eye is a living and dynamic component, it is important that the optical designer be aware of some details regarding its construction and function. Each individual is unique in terms of his or her specific eye characteristics. As a result, the visual optical system must be designed with all of those potential differences in mind.
Figure 10.1 shows a cross section view through the typical human eye. This is a simplified version, suitable for the purpose of understanding the basic function and makeup of the eye. The eye is nearly spherical in shape with a diameter of approximately 1 in. Because it is filled with a jelly-like substance, the eye is flexible (similar in feel to a tennis ball). The front of the eye contains the cornea, a convex transparent window about 12 mm in diameter that allows light to enter the eye. Because of the cornea's curvature, it is responsible for most of the lens power that is present in the eye. The volume of the eye between the cornea and the eyelens is filled with a clear fluid called the aqueous. Located just in front of the eyelens is the iris, an opaque layer with an opening at its center that will adjust in diameter, depending on the amount of light that is present in the scene being viewed. The diameter of the iris opening will vary over a range of 2 to 7 mm. The normal size, for typical daylight viewing, is about 3 mm.
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