A Pyrometer is an instrument which measures very high temperatures. Objects at very high temperatures cannot be measured by contact type thermometers. Hence, non contact thermometers are used. Today, the most widely used pyrometer is the infrared thermometer. The infrared thermometer measures the wavelength of the radiation emitted by a hot body. As the temperature increases, the wavelength increases
The infrared thermometer has a lens which focuses the thermal radiation from the hot object on to a detector which converts the radiation into an electric signal. 
Another type of pyrometer which was popular before the advent of the infrared thermometers was the disappearing pyrometer. In the disappearing pyrometer, a filament in the device is heated by an electric current. As it gets heated up, it glows hot. The instrument is pointed at the hot object whose temperature is to be measured such that the filament and the object are in the same line.
The filament is now heated so that the filament disappears in front of the hot object. This happens when the hot filament and object are have the same color. This will happen when both are at the same temperature. Thus the temperature of the filament is now the temperature of the hot object.
The pyrometer has a small dial which indicates the temperature of the hot object. It does this by measuring the current flowing into the filament.
(See article on Infrared Thermometer)