ABSTRACT
Temperature is the most important parameter in scientific and industrial environments. The development of temperature sensors is continually evolving for more than two centuries. Any material whose property changes quickly with temperature and is reversible, can be used as a thermometer. This property can be pressure, volume, length, refractive index, color, etc. The efforts of Fahrenheit and later Celsius led to the development of the mercury-in-glass thermometer. Modern infrared (IR) thermometers scan the forehead quickly to reveal the body temperature. Some of the popular devices are thermocouples, resistive temperature devices (RTDs), thermistors,
thermostats, silicon-based sensors, and IR pyrometers [1]. Each type of device operates in a specific temperature range. They are installed in consumer appliances, process industries, and aircrafts. Thermocouples are cheap and rugged, some operate up to 1800°C, and they are installed in furnaces, combustion engines, chemical reactors, and many others. Silicon sensors operate up to 150°C and are used in electronic circuit boards. RTDs are wire windings of copper or another metal, whose resistance increases linearly with temperature, and they are useful for temperature measurement up to a few hundred degrees. Conventional thermometers are inexpensive, but they have limited applications. Though several different types of temperature probes are available there is no ideal device that works in all types of environments, for temperature measurement. Optical materials are either heated up or cooled down when a laser beam passes through them [2, 3]. Some materials require longer times to reach thermal equilibrium. Contact thermometers are not suitable for temperature measurement along the beam path. These measurements require noncontact sensing devices, which are preferred in many cases. To address these specific needs optical techniques are investigated for temperature measurement, using rare earth ion-doped solids. These are alternate techniques for temperature measurement when conventional techniques are not suitable.
