Broadcast Engineer's Reference Book

Paul Cameron

3.3 Optical Sensors

3.3.1 Early Image Sensors

3.3.1.1 Selenium detectors

Selenium was the first photoelectric material to be found, in 1873. It was used in the first mechanical experiments in television, like the Nipkow disk system. Selenium is classed as a photoconductive material because its resistance changes when exposed to light.

3.3.1.2 The Ionoscope

The Ionoscope was the first image sensor of any commercial importance. It consisted of an evacuated glass enclosure with a tube fixed to it enclosing an electron gun (Figure 3.3.1). The main enclosure had a screen made from a sandwich of photosensitive particles, called a mosaic, a thin mica insulation layer and a conductive sheet backing. The plate acted like a capacitor.