ABSTRACT
Figure 15.1 Camera-eye analogy. 15.2 Component of EyesFigure 15.1 also presents eye components along the optical axis, in the same order hit by light in the vision process. Light travels through air (refractive index n = 1) and hits the cornea (n = 1.376) having 12
mm f and 0.6 mm thick (near center, and thickening further at the edge). After that light strikes the aqueous humor (n = 1.336). Around this area, there is an iris giving an eye pigment and controlling light intensity via an adjustable pupil. The light then goes to the lens (n = 1.41), vitreous (n = 1.336), and retina, respectively. At the retina, there are two photoreceptors, as summarized in Table 15.1. Table 15.1 Details of photoreceptors
After retina, the light finally directs to the brain, which fuses two distinct images into one, referred to as binocular vision. 15.3 Function of EyesThere are at least three functions of eyes: accommodation, adaptation, and visual acuity. Accommodation is the eye’s ability to see objects nearby or far away (Table 15.2 and Fig. 15.2). Accommodation exists for young adults, whereas accommodation loss occurs in the early 40s. Table 15.2 Accommodation details
Figure 15.2 Near (left) and far (right) visions. NNP is normal near point. Visual acuity is the ability to see detailed image (eye resolution-the smallest readable letter on an eye chart). Vision test uses a chart of capital letters with various letter sizes on a white background, known as the Snellen chart, kept at a fixed testing distance of 20 ft (= 6 m). Visual acuity is measured in terms of the Snellen fraction obtained by dividing the testing distance of defective eyes by the testing distance of normal eyes. 15.4 Ocular Problems and Their CorrectionsOcular problems and their corrections by glasses are summarized in Table 15.3 and Fig. 15.3 and Fig. 15.4.
