Introduction |

Two principal theories are employed by pyrometry: Planck's law and the Stefan-Boltzmann law. Planck's law is used in narrow-band pyrometers, where only one or a few specific wavelengths are targeted. The Stefan-Boltzmann law is used in broad-band pyrometers, where a wide range of wavelengths are measured. |

Planck's law | |||||||||||||

The radiation intensity and wavelength from a blackbody at a given temperature
| |||||||||||||

If the radiation intensity W at a specific wavelength l is measured, the temperature _{b}T is the only unknown in the equation.
If l As shown, the radiation intensity is a function of wavelength l
_{p}T = 2891 µm·Kwhere l This equation tells us what the most effective wavelength should be for measure a certain temparature |

Stefan-Boltzmann law |

In broad-band pyrometers, the photodectors are collecting radiation intensity over a wide range of wavelengths. In other words, they are summing the radial power intensity W-l curve at a particular temperature _{b}T, resulting in the Stefan-Boltzmann law,
where s is the Stefan-Boltzmann constant (5.6697×10 |

Emittance and Emissivity |

When electromagnetic radiation impinges upon a surface of an object, the radiation is partially absorbed, partially reflected, and partially transmitted. If all of the electromagnetic radiation is absorbed by the object, the object is called a Planck's law and the Stefan-Boltzmann law are derived assuming blackbody properties. In reality, most common materials are not 100% T.
The If the emittance is independent of wavelength (i.e. e Another term called |

Error Correction | ||||||

As a result of differences in emittance, different objects at the same temperature may radiate differently. The emissivity of a single object may change over time, due to oxidation for example. Thus, the same object at the same temperature may still radiate differently at two different times, and the emittance should be considered in the calibration of pyrometers. Including the emittance in the above equations results in,
| ||||||

Finally, the geometry of the probe tip and/or the detecting angle may also affect the radiation reading. In that case, a geometric correction factor f should be introduced in the above equations as a multiplier.
_{g} |