Introduction

There are a number of visual optical instruments used in ophthalmic optics, designed to measure such quantities as vertex powers and surface radii of curvature of both lenses and the cornea. The principles of these instruments have been discussed already in Chapter 11. In this and the following five chapters, we will examine these a little further and introduce some other useful ophthalmic instruments. In this chapter, we will look at the measurement of vertex power.

In ophthalmic optics, spectacle lenses are rarely specified by their equivalent power. Instead they are specified in terms of their back vertex power, front surface power, refractive index and thickness. Thus the measurement of vertex powers is perhaps the most common power measurement in ophthalmic optics, and fortunately it is far simpler to measure vertex power than to measure equivalent power. The focimeter is a simple instrument for measuring vertex powers and has already been described in Chapter 11.

Focimeters may be visual, projection or fully automatic. In the visual type shown in Figure 11.16, the image of a target is viewed and focussed through an optical system consisting basically of a collimator and a viewing telescope. The scale reading may also be presented in the eyepiece of the telescope. In the projection type, the telescope is replaced by an optical system which projects the image of the target onto a viewing screen. In both of these types of instruments, the operator has to make some judgement on the best focus of the target. In automatic instruments, the instrument automatically locates the optimum focus and the final powers are given directly on some type of display.