Emissivity is an essential concept to master when working with thermal imaging. It’s a bit like having one’s balance when riding a bike. A new rider can’t hope to really get the mechanical part down if he or she can’t maintain a sense of balance to stay upright while the bike moves. In general terms, emissivity involves energy projected outward from something. In the case of heat, this is known as radiant heat or radiant energy. Anyone who has been near a fireplace knows what it is in practice; the heat that one feels pushed out from the fire is radiant energy in the form of temperature change in the immediate air.
Three Types of Energy to Consider
Energy generally moves in three ways from a physics perspective. The first is transmitted. This type of energy literally moves through some materials. The most common type that people are used to is sunlight through a window. While the sun and air are outside, the heat of the sun moves through the window without resistance unless the window is treated or shaded.
The second type of energy travel is reflected energy. In this case, energy moving bounces off of something and gets focused on a different location. Mirrors are fundamental reflectors, bouncing light and heat from their surface back to another location. With sunlight, mirrors can reflect sunlight energy and heat to a different location, depending on their angle.
Radiant energy, or emitted energy, is energy coming off of an object itself. Lava from a volcano gives an immense amount of radiant energy. Before it even gets close to structures, lava oftentimes causes things to catch fire because molten rock is so hot it is sending out waves of heat from its surface. Our bodies also produce radiant energy. We have an internal temperature of about 97 degrees. When we are warmer than our surroundings, we give off heat, sometimes seen as steam.
For the purposes of true temperature measurement, the ideal setting tends to be emissive or radiant energy observation. Transmitted energy comes from an outside source. Reflected energy tends to be the same, distorted by what it is reflected off of. Emitted energy comes directly from an object itself. This is ideal when one is trying to use temperature measurements to gauge how to manage the environment for that object. For example, if a factory is going to handle red red-hot steel fabrication, the room and assembly belt moving the items needs to be able to handle the heat. Measures confirm the true heat level and what to expect.
Emissivity is a Messy Behavior
In formal mathematical terms, emissivity is measured as:
E = ε’σT4
E = total flux, ε’ = “effective emissivity” (a value within 0 and 1), σ is a constant, and T = temperature (Kelvin)
Working things out on the chalkboard is not practical for most folks, however. So, instead, a typical way to measure emissivity is to use a probe to get the actual direct temperature off an object, then one uses the thermal camera to measure temperature from a distance and, noting the variation, adjust the emissivity dial until the values are equal.
The fact is, most materials and objects don’t give off a clean amount of radiant heat. Some parts exude a very strong heat signature, while others have far less. The angle of the camera picking up the heat matters as well and so does the lens material actually receiving and measuring the energy recorded. In addition, there can be a lot of interference. Emitted energy doesn’t just push out from an object cleanly. It moves out and bounces off of nearby materials and interferes with other radiant energy, creating a bit of a stormy mess around the core object, especially if it is inside a containment area versus outside. All of these things contribute to incorrect temperature readings on thermal cameras if the operator does not compensate for them correctly.
Other Factors Affecting Emissivity Readings
The surrounding air and atmosphere can have a significant influence on the minute, detailed readings. If there is moisture in the air, for example, it will produce variations in heat signatures and readings, mainly because water tends to be a tremendous heat sink, even in microscopic form.
Environmental conditions, such as normal room temperature, will augment or resist emissivity to different levels. If a room is cold, for example, the heat energy from an object will dissipate rapidly until the surrounding air balances out. Alternatively, if the room is as hot or hotter, the radiant heat will have nowhere to release and will likely concentrate. Both will cause havoc with readings obtained by a thermal camera.
A third interference will come from other objects in proximity to the target also being hot and giving off their own radiant heat, a common problem when the overall environment is being heated up by multiple factors. Ideally, one wants to point at the particular target and measure temperature accurately, but in the distance between the camera and the object, there is a Grand Canyon of space being interrupted by radiant heat from anything else nearby as well. Alternatively, one might be interested in the temperature of a cooler object, and a hotter one nearby is throwing off the readings.
Overcoming the Problems
The combination of user skill as well as the use of a high-quality camera makes a huge difference in addressing problems caused by emissivity. Awareness is usually half the battle, producing the development of knowledge and wisdom based on experience and understanding how to adjust for problems as they begin to occur. Even the best thermal imaging cameras will be thrown off by many of the issues discussed above if the user is not expecting the issues to occur or knows how to deal with them.
Correct thermal imaging is critical for many assembly processes dealing with heat, and both equipment and personnel need to be up to speed to maintain quality production levels. Steel plants, for example, cannot tolerate sloppy temperature management, for example. The product will be of poor quality, which in turn can produce serious defects and structural concerns later on. Without dependable monitoring, many factories and plants would not be able to fabricate the extraordinary inventory they are capable of today. And it’s possible because thermal imaging applied correctly produces a powerful window in the heat-intensive creation phase that is otherwise taking a wild guess.
