Introduction

The measurement of the radii of curvature of surfaces is one of the most common optical metrological measurements. Different methods of measuring the radius of curvature of a spherical or toric surface are presented in Chapter 11. In ophthalmic optics, we are concerned with the radii of curvature of the surfaces of spectacle or contact lenses and the radii of curvature of the anterior corneal surface. Because of the potential toric nature of these surfaces any clinical instrument should be able to measure the radii of curvature in a section and hence the radii of cuvature of the principal meridians of a toric surface. There are three main types of ophthalmic instruments for this task – Geneva lens measures, radiuscopes and keratometers. The Geneva lens measure is designed to measure the surface curvature or power of a spectacle lens and is a contact method based upon the spherometer. The radiuscope is a non-contact or optical method and is based upon the Drysdale principle. The keratometer is also a non-contact or optical method and is designed to measure the surface curvature of the anterior surface of the cornea. The Geneva lens measure has been adequately described in Chapter 11 and we will not spend any more time on discussing it. We will describe the radiuscope and keratometer in the following sections.

The radiuscope

Radiuscopes are instruments designed to measure the radius of curvature, mainly of contact lenses. Because these may be soft and easily deformed, a non-contact or optical method must be used. A typical construction of a radiuscope based upon the Drysdale method is shown in Figure 11.11 and the Drysdale principle is described in Section 11.3.2.1.