Optical Pyrometer:
If the temperature of the object is high enough to emit a lot of visible light, the measurement of thermal radiation can be used to confirm its temperature. This thermometer is called an optical thermometer. A thermometer consists primarily of a telescope with a red filter and a set of circuits with a small light bulb, a galvanometer, and a variable resistor.

Resistance Pyrometer:
There are metal resistance thermometers and semiconductor resistance thermometers, which are made according to the characteristics of the resistance value changing with temperature. Metal thermometers are mainly used for platinum, gold, copper, nickel and other pure metals, as well as rhodium iron and phosphor bronze alloys; semiconductor thermometers mainly use carbon, germanium and other materials, resistance thermometers are easy to use and reliable, and have been widely used in most industries.
High temperature thermometer:
Refers to thermometers specially used to measure temperatures over 500°C, including optical thermometers, colorimetric thermometers and radiation thermometers. The principle and structure of high-temperature thermometers are more complicated, and will not be discussed in depth here. The measurement range is 500°C to 3000°C and above, not suitable for measuring low temperature.