A pyrometer is non-contact temperature measuring device, generally the term is applied to instruments measuring temperatures above 600 degrees Celsius. It is typically used to measure temperatures of glowing hot metals in a steel mill or foundry. It is also known as an optical pyrometer.
One of the most common pyrometers is the absorption-emission pyrometer which is a thermometer for determining gas temperature from measurement of the radiation emitted by a calibrated reference source before and after this radiation has passed through and been partially absorbed by the gas. Both measurements are made over the same wavelength interval.
To measure the temperature of incandescent metals, you look through the pyrometer at the glowing metal, and turn a knob or ring which adjusts the temperature of a glowing filament projected into your field of view. When the color of the filament matches the color of the metal, you can read the temperature from a scale on the filament color adjusting knob/ring.