What is emissivity?
Emissivity is a measure of how efficiently an object radiates heat, i.e., how well the surface of an object "tells the truth" about its temperature. Emissivity values range from 0 (a theoretically perfect mirror that reflects all energy) to 1 (a theoretical object called a black body that perfectly absorbs and radiates all energy). Note that, emissivity values of 0 and 1 can only be achieved in theory. In reality, physical objects can best be described in the range of 0.01 to 0.99 on the emissivity scale.
Highly polished metal surfaces, such as copper or aluminum, typically have an emissivity of less than 0.10. Rough or oxidized metal surfaces will have a much higher emissivity (0.6 or higher depending on the condition of the surface and the amount of oxidation). Most matte finish paints have around 0.90, while human skin and water have around 0.98.
In thermography, emissivity is a crucial parameter for obtaining accurate temperature measurements using thermal imaging cameras. Thermal imaging cameras measure the thermal radiation emitted by objects and use the emissivity to calculate the surface temperature. Therefore, it is important to know and take emissivity into account when taking thermographic measurements in order to obtain accurate results.
It is possible to determine the emissivity of an object by direct calibration or by consulting tables or databases that provide typical emissivity values for different materials. It is also possible to use specific measurement and testing techniques to determine the emissivity of a particular object.
If you are looking at a highly polished metal object with low emissivity, that surface will act as a mirror. Instead of measuring the temperature of the object itself, your camera will detect the reflected temperature. Reflected temperature (also known as background or T-reflected temperature) is any thermal radiation originating from other objects that is reflected off the target you are measuring.
An object could be cold to the touch but show a much higher temperature according to your camera if a nearby heat source (such as a thermograph) were reflected off the surface.
Or an object may have the same temperature as an adjacent object but appear much cooler.
For objects of higher emissivity, the reflected temperature has less influence. But for low emissivity objects, it is a critical factor that must be carefully understood. As emissivity decreases, what you are measuring (and seeing thermally) comes more from the surfaces of surrounding objects and not from the target you are inspecting.
If the object you are trying to measure has a high emissivity, you can adjust the emissivity and the reflected temperature in your camera settings. For example, if you want to measure a person's body temperature, you should set the emissivity to 0.98 (emissivity of human skin) and the reflected temperature to the ambient temperature (probably around 68°F / 20°C if indoors), and your camera will compensate.
For other types of surfaces, and if temperature measurement accuracy is important to you and your inspection program, you will probably need thermography training to properly compensate for emissivity when taking measurements. Emissivity can be affected by many different factors, and we recommend obtaining certification before attempting to take measurements on difficult surfaces.
In general, keep in mind that if the emissivity of the target you are trying to measure is less than 0.5, it is unlikely that you will be able to obtain an accurate temperature measurement. In these cases, you may need to consider finding ways to increase the emissivity of your target. Emissivity is one of the most challenging factors to consider when taking temperature measurements. However, it can be understood, and with the right techniques, it can be properly compensated for.
Thermography classes can be taken at the Infrared Training Center (ITC), of which Apliter is an official trainer. A certified Level I thermography course will teach you how to correctly set the emissivity and reflected temperature on a variety of components, including those that are electrically energized or difficult to access.
For more information on these certification classes, as well as upcoming training dates and locations, visit our training page.
In addition, the Level I certification is a 100% live online training with an ITC Level III certified teacher.