Determining Temperature with Infrared
IR detection instruments measure energy at a particular wavelength and derive a temperature based on Plank’s radiation energy equation:
To determine the temperature of an object, the self emitted radiation intensity (L) at a known wavelength (l) and the emissivity (e) of the object being measured must be known (c is a physical constant). The self emitted radiation intensity (L) is measured by the IR instrument but the emissivity (e) must be approximated and entered manually.
Emissivity is defined as the ratio of actual emitted radiation to the theoretical maximum emission. More simply, it is the radiation efficiency of an object. An object that emits the theoretical maximum amount of radiation is defined as a blackbody. A blackbody is a perfect radiator and absorber with an emissivity of 1.0. In the real world, there are no true blackbodies. All objects have an emissivity less than 1.0, although there are some materials that are very close (rubber, graphite powder and human skin have emissivities ranging from 0.95 – 0.98). Reflection, on the other hand, is the opposite of absorption. As there are no true blackbodies, there are no perfect reflectors e = 0 (polished gold has an emissivity of 0.02). Reflection efficiency (reflectivity) is defined as r = 1 – e
The measured radiation is the sum of emitted and reflected radiation. For example: If a furnace tube has an emissivity of 0.9, the radiation viewed by the IR instrument will be the self emitted radiation from the tube at 90% efficiency plus 10% of the background radiation. This complicates matters in determining the true temperature of the object. Not only do you need to know the emissivity, but also the background temperature. Fortunately, background temperature can be approximated and emissivities for certain materials are generally known. The more advanced instruments can automatically correct for background temperature. However, background temperatures and actual emissivities can not always be determined precisely, thereby affecting the accuracy of the measurement.