Introduction

Ophthalmoscopes are optical instruments designed for the visual inspection of the internal structure of the eye, but most commonly the retina. However, because the amount of light reflected from the subject's eye is very low, the observer will only see an image if the subject's retina is well illuminated and thus an auxiliary illuminating system is an essential component of an ophthalmoscope. Thus ophthalmoscopes consist of two main components: a viewing system and an illuminating system.

There are a number of different designs for the viewing system, and these can be divided into two groups, as follows:

1. (a) Direct ophthalmoscopes. These are the simpler of the two types. They are discussed in detail in the next section, Section 29.1.

2. (b) Indirect ophthalmoscopes. The viewing system is more complicated than for direct ophthalmoscopes, having extra lenses between the subject's and observer's eyes. The extra complexity allows independent control over field-of-view and magnification. Indirect ophthalmoscopes are discussed in detail in Sections 29.2 and 29.3.

In this chapter, we will concentrate on the viewing system and only make a brief mention of the illumination system of the direct ophthalmoscope.

The magnification in direct ophthalmoscopy is often quoted as 15. In contrast, the magnification of indirect ophthalmoscopy is quoted as being much lower, usually in the region of about (–)3. However, once we look at the construction of indirect ophthalmoscopes, we will see that there is some potential ambiguity in the way that the magnification of an indirect ophthalmoscope is defined, but for the moment we will leave that problem aside and begin by looking at the properties of the direct ophthalmoscope.