Scientific Principles of Thermal Testing
Energy can come in many forms, and it can change from one form to another but can never be lost. This is the First Law of Thermodynamics. A byproduct of nearly all energy conversion is heat, which is also known as thermal energy. When there is a temperature difference between two objects or two areas within the same object, heat transfer occurs. Heat energy transfers from the warmer areas to the cooler areas until thermal equilibrium is reached. This is the Second Law of Thermodynamics. When the temperature of an object is the same as the surrounding environment, it is said to be at ambient temperature.
Heat Transfer Mechanisms
Thermal energy transfer occurs through three mechanisms: conduction, convection, and/or radiation. Conduction occurs primarily in solids and to a lesser degree in fluids as warmer, more energetic molecules transfer their energy to cooler adjacent molecules. Convection occurs in liquids and gases, and involves the mass movement of molecules such as when stirring or mixing is involved.
The third way that heat is transferred is through electromagnetic radiation of energy. Radiation needs no medium to flow through and, therefore, can occur even in a vacuum. Electromagnetic radiation is produced when electrons lose energy and fall to a lower energy state. Both the wavelength and intensity of the radiation is directly related to the temperature of the surface molecules or atoms.
Wavelength of Thermal Energy
The wavelength of thermal radiation extends from 0.1 microns to several hundred microns. As highlighted in the image, this means that not all of the heat radiated from an object will be visible to the human eye
but the heat is detectable. Consider the gradual heating of a piece of steel. With the application of a heat source, heat radiating from the part is felt long before a change in color is noticed. If the heat intensity is great enough and applied for long enough, the part will gradually change to a red color. The heat that is felt prior to the part changing color is the radiation that lies in the infrared frequency spectrum of electromagnetic radiation. Infrared (IR) radiation has a wavelength that is longer than visible light or, in other words, greater than 700 nanometers. As the wavelength of the radiation shortens, it reaches the point where it is short enough to enter the visible spectrum and can be detected with the human eye.
An infrared camera has the ability to detect and display infrared energy. Below is an infrared image of an ice cube melting. Note the temperature scale on side, which shows warm areas in red and cool areas in purple. It can be seen that the ice cube is colder than the surrounding air and it is absorbing heat at its surface. The basis for infrared imaging technology is that any object whose temperature is above 0°K radiates infrared energy. Even very cold objects radiate some infrared energy. Even though the object might be absorbing thermal energy to warm itself, it will still emit some infrared energy that is detectable by sensors. The amount of radiated energy is a function of the object's temperature and its relative efficiency of thermal radiation, known as emissivity.
(Photo courtesy of NASA/JPL-Caltech/IPAC)
A very important consideration in radiation heat transfer is the emissivity of the object being evaluated. Emissivity is a measure of a surface's efficiency in transferring infrared energy. It is the ratio of thermal energy emitted by a surface to the energy emitted by a perfect blackbody at the same temperature. A perfect blackbody only exists in theory and is an object that absorbs and reemits all of its energy. Human skin is nearly a perfect blackbody as it has an emissivity of 0.98, regardless of actual skin color.
If an object has low emissivity, IR instruments will indicate a lower temperature than the true surface temperature. For this reason, most systems and instruments provide the ability for the operator to adjust the emissivity of the object being measured. Sometimes, spray paints, powders, tape or "emissivity dots" are used to improve the emissivity of an object.