• In this type of pyrometer, the tungsten filament of an electric bulb is used as a radiator.

• The intensity of radiation of filament is compared with the intensity of the radiation of the hot surface.

• When both intensities match, the filament disappears against the background.

• The intensity of the filament can be controlled by the current flowing through it.

• The maximum temperature of the filament is 2800 - 3000˚C at the rated voltage. The minimum visible radiation is at 600˚C. Hence, we can measure the temperature in between 600 – 2800 ˚C

• The ampere meter in the lamp circuit is calibrated in degree centigrade.

• The figure shows an optical pyrometer.

• The radiations from the source are focused onto the filament of the reference temperature using an objective lens.

• Now the eye piece is adjusted to focus the images on the hot source and the filament.

• Now the lamp current is controlled such that filament appears dark if it is cooler than the source, and appears bright if it is hotter than source and filament will not be seen if the filament and the temperature source are at same temperature.

Advantages

• It is a portable device.

• It is light in weight and can be carried at any place.

• The temperature of moving object can be determined by using optical pyrometers.

• It is a non-contact type temperature measuring device.

Disadvantages

• The instrument can be used to measure the temperature of only hot surfaces, which radiates energy.

• More chances of human error while adjusting image.

• Device is costly as compared to others.