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.

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. The 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. Before use, establish a relationship between the temperature of the filament for different brightness matches and the galvanometer reading. When in use, connect the telescope to the object to be measured, and adjust the resistance so that the brightness of the bulb is similar to the brightness of the object to be measured. At this time, the temperature of the object to be measured can be read through the galvanometer.