Thermal Imaging is a method of constructing an image of the scene using infrared radiation rather than visible. Thermal imaging is both non-contact and nondestructive. Since it is non-contact, it is useful for inspecting energized electrical systems as well as mechanical systems and rotating equipment. Since the infrared energy emitted from a surface is proportional to its temperature, imaging radiometers are capable of providing surface temperatures as well as images.
In nearly all manufacturing processes, temperature is the most measured variable. The three methods by which heat flows from one object to another are RADIATION, CONVECTION and CONDUCTION. While the major concern in infrared thermography viewers is with radiation effects, the effects of the other two cannot be neglected.
- CONDUCTION is the way that heat moves in a solid object, by transferring thermal energy from molecule to molecule, heating up each adjacent area within the solid. You may recognize this as the way a frying pan conducts heat from the outside heat source into a piece of meat inside, or the way a radiator feels hot to the touch if a human hand is placed on it. This is a relatively slow operating effect when compared to the other two.
- CONVECTION is a faster transfer effect, and moves the way heat does in a liquid or in a gas. In convection, the thermal energy uses a medium to carry it and actually develops a current in the medium to move it along more rapidly. This effect is seen in most houses as heat rises or air conditioned air cools the house. The air that is heated up moves through the house, warming other things as it goes. This is a faster operating and more powerful effect of thermal transfer than conduction.
- RADIATION, however, is the most powerful effect. This moves with the speed of light and is observed in the way that heat transfers from glowing coals or from the sun to the earth. It is the primary way that your hands are warmed near a fireplace.
Theory of Thermal Imaging
Absolute zero is 0°K / -459°F / -273°C. The theory of thermal imaging is simple. All objects above absolute zero emit infrared radiation. While infrared energy is invisible to the human eye, infrared imagers detect and convert these invisible wavelengths into visible light images that are displayed on a screen. Images can be either monochrome or multicolored, where the shades of gray or color represent temperature patterns across the surface of an object. These thermal images can be viewed in real time or stored on videotape, computer disk, or PC card. Thermal images then can be recorded onto photographic film or paper; those images are called thermographs or thermograms. Infrared radiation increases as a function of object temperature increases. As an object becomes hotter, its molecular activity increases, and it radiates more energy.
Electromagnetic (EM) spectrum includes both visible and invisible radiation. Radiated heat is electromagnetic radiation in the infrared band of the spectrum. EM spectrum ranges from X-rays (a relatively high-energy photon having a wavelength in the approximate range from 0.01 to 10 nanometers) and Gamma (very penetrating rays not appreciably deflected by a magnetic or electric field, emitted by radioactive substances) rays to Radio waves (an electromagnetic wave with a wavelength between 0.5 cm to 30,000 m) and includes Ultraviolet (of or relating to the range of invisible radiation wave-lengths just beyond the violet, about 380 nanometers in the visible spectrum, having a wavelength shorter than those of visible light and, at 4 nanometers, a little longer than X-rays) Visible (discernable by the human eye, is 400 to 700 nanometers), and Infrared (of or relating to the range of invisible radiation wavelengths from about 750 nanometers, just longer than red in the visible spectrum, to 1 millimeter, on the border of the microwave region).
Thermal energy (radiated heat) is transmitted in the infrared wavelength (slightly less than1 micron to 100 microns). The bulk of thermal energy, at ambient temperatures, is in the 5 to 14 micrometer (micron) region.
The image on the left shows two adults and a child through an infrared thermal imager. After a minute of sitting on the couch the thermal infrared energy of the people is transferred and stored in the couch until they get up. The image on the right illustrates the fact that all objects radiate heat. The heat from their bodies that transferred to the couch is now being emitted from the couch and displayed on a thermal imaging device. No visual light technology can record this type of data. The properties of heat transmission are more than an interesting novelty. This information can prove useful in a variety of applications.
Emissivity is not related to color.
The colored labels above are all at the same temperature.
Emissivity is a variable that makes it very difficult to obtain exact temperature readings with an infrared camera or spot thermometer. This is due to the fact that it is highly impractical to measure the emissivity of every object in your field of view. For example, if you are scanning an electrical panel in a predictive maintenance application you will be imaging wires, fuses, nuts, bolts, and other materials all of which will have a different emissivity value. So how do we deal with this?
In most infrared applications exact temperature measurement is not necessary. For example, if a circuit has a fault limit of 160° F and your instrument measures 110° F and the Є value skews the temperature reading by 5° F you are left with a ± 5° F variance, which in this case is negligible. Additionally, most thermal infrared applications rely on temperature difference (delta T) rather than exact temperature readings. To use our previous example of the circuit we measured, there would most likely be more than one circuit next to each other. If you use the same Є value for both circuits they will both be skewed by the same amount. If the one circuit was reading 110° F (which we will assume is normal operating temperature) and the adjacent circuit reads 160° F we are left with a delta T of 50° F, which would most likely indicate a problem and as you can see negates the emissivity problem.Emissivity values become even less of a problem when trending an area over time. If the same circuit with the reading of 110° F has a reading of 120° F the next time you scan it and a reading of 125° F the next time, with the same emissivity setting, we know a problem is developing regardless of the error introduced by emissivity.
Dealing with emissivity is not as hard as it would seem. The important things to remember are that exact temperature measurements are difficult to obtain, temperature difference (delta T) is more important than exact readings in most applications, and that trending an object can reveal problems regardless of Є value error. In the real world you pick an emissivity value that approximates the scene you are imaging and then you would record it and maintain that same setting every time you scan that object.