TABLE OF CONTENTS A detector can be defined as a device that converts one type of signal into another as illustrated in Fig. 19.1. Various forms of input signal can be entered into the detector, which then generates the measurable output signal, such as an electrical current or voltage. There exist many different types of detectors depending on the objects or physical properties that they sense. The input signal can be mechanical vibrations, electromagnetic radiation, small particles, and other physical phenomena. Smoke detectors can sense the soot particulates caused by fire and seismometers sense the mechanical vibrations caused by the earth. The human body has various types of detectors: the eyes can sense electromagnetic radiation in the visible range, the ears detect sound from pressure variations through a medium such as atmospheric air or water, the tongue senses various types of chemicals, and the skin can detect temperature and pressure. Our natural sensory skills have been augmented through the development of advanced instruments such as the microscope and the thermometer, that were made possible thanks to the development of technology. Furthermore technology has made it possible for humans to detect things that could not be naturally sensed by the human body. For example, we can observe the infrared (IR) light emitted from warm objects and the ultraviolet (UV) light from hot objects with the help of photodetectors which will be the focus of the this and the next Chapters.
