High temperature thermometer refers to a thermometer specifically used to measure temperatures above 500°C. There are optical pyrometer, two color pyrometer, radiation pyrometer, and infrared pyrometer. They are all non-contact measuring instruments. Their common characteristics are: they do not destroy the temperature field of the measured object, nor are they affected by the corrosion and poisoning of the side medium; the measurement range is wide and the accuracy is high; it is convenient for automatic recording, remote measurement, and remote control. However, it is easily affected by the radiation of surrounding objects, and the measured temperature is not the true temperature of the object, and the object's blackness correction is required. However, these blackness corrections often cause large errors due to the various surface conditions of the objects.

The optical pyrometer measures the temperature of an object as a function of the wavelength of a certain monochromatic wavelength and the temperature. In the specific temperature measurement, it uses the principle of brightness balance. That is, the body is imaged by the telescope system on the filament surface of the thermometer with the tungsten filament lamp inside, and then the filament current is adjusted visually so that the brightness of the two is the same. Finally, the temperature of the object is indicated by the current in the filament loop. The instrument can be divided into two basic parts: optical system and electrical measurement system.

The radiation pyrometer uses lenses or concave mirrors to gather the radiation intensity of all wavelengths of the measured object onto sensitive components (such as thermopile, photoresistor, photocell, etc.), and measure the temperature of the object by the signal size of these components. It is a thermometer made according to the law of total radiation. The radiation pyrometer is different from the optical pyrometer in that it relies on sensitive components for temperature measurement without the need for human eyes to balance the brightness, so there is no subjective error. It can automatically, quickly, indicate and record temperature values, so it is widely used in industry.

Two color pyrometer is also called color pyrometer, because it determines the temperature of the object according to the "color" of the radiation of the object. The temperature measurement is usually performed by using the ratio of the "red" and "blue" radiant energy in the spectral components of the radiating object to the energy ratio of the black body radiation at the same two wavelengths. For example, if the energy ratio of an object (generally called the red-to-blue ratio) is exactly the same as the energy ratio of a black body at 1000°C, the color temperature TC of the object is called 1000°C.

Infrared pyrometer is a new technology developed in recent years. It is a thermometer that uses the infrared radiation of an object to determine the temperature of an object. Its main feature is that with the advent and development of infrared components, it can be used to measure 700°C; low-temperature radiation below makes the non-contact pyrometer extend to the low-temperature direction. According to reports, the infrared pyrometer can even measure the temperature of objects below 0°C with a resolution of 0.1°C or even 0.001°C. In addition, it is less affected by the intermediate medium.