Introduction

Projection systems are optical systems designed to project images of solid objects and photographic objects such as transparencies, usually with some magnification, onto an observation screen. Typical uses are as profile projectors in engineering, 35 mm photographic slide projectors, motion picture projectors, microfilm and microfiche readers and photographic enlargers. They consist of a projection lens, an illumination system and a screen on which a real image is observed. A typical projection system is shown in Figure 22.1. Some projection systems, though very few, have no screen because the image is virtual and this is projected directly into the eye using an eyepiece. Figure 22.1 shows the object being trans-illuminated, but some objects are opaque and the reflected light is used to form the projected image. In the common photographic projector, the object is commonly a piece of photographic film either in positive or negative form.

The projection lens

The projection lens is a positive equivalent power lens, usually well corrected for aberrations. It usually does not contain an aperture stop. Instead, the aperture stop is provided by the illuminating system with the image of the light source being imaged into the projection lens and acting as the effective entrance pupil of the projection lens. We will discuss the pupils of a projection system in Section 22.3.

The optics of the projection system are shown in Figure 22.1, where the projection lens is depicted as a thin lens. In reality, the projection lens will be more complex mainly because of the need to give good image quality for a wide aperture and over a wide field.