The device which is used for measurement of very high temperature is known as Pyrometer. The meaning of ‘Pyros’ is fire. So Pyrometer is used to measure the temperature above 500°C. Platinum resistance thermometer, Gas thermometer, Thermocouple all are the Pyrometer.
Based on radiation, Radiation Pyrometer is mainly used. Such Pyrometer need not be placed in contact with the body whose temperature is to be measured. This type of device is constructed based on Stefan’s law of radiation, E = 
(T4 – To4), where T=Temperature of a black body. To= Temperature of surrounding, = Stefan’s constant = 5.672 x 10-8 wm-2K4 & E = Rate of loss of heat energy per unit area by radiation.
There are two kinds of Pyrometer. They are-
- Optical : The optical pyrometer was designed by Holborn, Kurlbaum and In this type of pyrometer, the colour or brightness of the body whose temperature is to be measured is compared with the colour or brightness of the filament of an electric lamp. The brightness of radiation of a body is directly proportional to temperature.
- Radiation : The hot bodies emit radiation, light radiation or electromagnetic The radiations depend on their temperatures. This device is used for measured the temperature of a body or radiating body by means of its radiation is called radiation pyrometer. It is used to measure the temperature above 3000°C or 3000 K
