What principle does an optical pyrometer use to measure temperature?

An optical pyrometer measures temperature using the principle of brightness comparison with a filament. This device operates by comparing the brightness of an object being measured to the brightness of a heated filament within the pyrometer.

When a temperature is to be measured, the operator adjusts the filament's brightness until it matches that of the object. Since the amount of light emitted by an object increases with its temperature, this comparison allows for an accurate determination of the object's temperature without the need for direct contact. The relationship between color or brightness and temperature is governed by Planck’s law of black body radiation, which the optical pyrometer utilizes to provide a reliable temperature reading.

This method is particularly advantageous for high-temperature measurements where contact thermometers would be impractical or may alter the temperature of the object being measured.