The Technique of measuring high temperature is known as pyrometry and the instrument employed is called pyrometer. Pyrometer is specialized type of thermometer used to measure high temperatures in the production and heat treatment of metal and alloys. Ordinary temperatures can be measured by ordinary thermometer, instead pyrometer is employed for measuring higher temperature.
Any metallic surface when heated emits radiation of different wavelengths which are not visible at low temperatures but at about 5400C radiations are in shorter wavelength and are visible to eye and from colour judgement is made as to probable temperature, the colour scale is roughly as follows.

Dark red - 5400C
Red - 7000C
Bright red - 3500C
Orange - 9000C
Yellow - 10100C
White - 12050C and above

The Technique of measuring high temperature is known as pyrometry and the instrument employed is called pyrometer. Pyrometer is specialized type of thermometer used to measure high temperatures in the production and heat treatment of metal and alloys. Ordinary temperatures can be measured by ordinary thermometer, instead pyrometer is employed for measuring higher temperature.
Any metallic surface when heated emits radiation of different wavelengths which are not visible at low temperatures but at about 5400C radiations are in shorter wavelength and are visible to eye and from colour judgement is made as to probable temperature, the colour scale is roughly as follows.

Dark red - 5400C
Red - 7000C
Bright red - 3500C
Orange - 9000C
Yellow - 10100C
White - 12050C and above

When a substance receives heat, change in pressure, electric resistance, radiation, thermoelectric e.m.f and or colour may takeplace. Any of these change can be used for measurement of temperature. Inorder to exercise provision control over the heat treatment and melting operation in the industry temperaturemeasuring device known as pyrometers are used. Also accurate measurement of temperature of Furnaces, molten metals and other heated materials