Introduction

The performance of visual optical instruments cannot be fully assessed without some knowledge of the anatomy and functions of the eye, working either monocularly or binocularly. This chapter describes the optics of the eye, but its interaction with visual instruments is covered later in Chapters 36 and 37.

A cross-section of the human eye is shown in Figure 13.1, giving only the most relevant optical components. A more detailed anatomical description can be found in a number of textbooks, for example Davson (1990). Image forming light enters the eye through and is refracted by the cornea. It is further refracted by the lens, bringing it to a focus on the retina. Of the two refracting elements, the cornea has the greater refractive power. However, whereas the power of the cornea is constant, the power of the lens depends upon the level of accommodation, which is the process by which the refractive power of the eye changes to allow closer or more distant objects to be sharply imaged on the retina. The diameter of the incoming beam of light is controlled by the iris, which is the aperture stop of the eye.

The dimensions of the eye and its optical components vary greatly from person to person and some further depend upon accommodation level, age and certain pathological conditions. In spite of these variations, average values have been used to construct representative or schematic eyes. These are discussed further in Section 13.6.

The refractive components

The relaxed eye has an equivalent power of about 60 m–1. The corneal power is about 40 m–1, which is two-thirds of the total power.