Infrared Theory: How Infrared Imaging Works
There are three methods by which heat flows from one object to another.
They are RADIATION, CONVECTION and CONDUCTION. While the major concern in IR viewers is with radiation effects, the effects of the other two cannot be neglected.
CONDUCTION is the way that heat moves in a solid, by transferring thermal energy from molecule to molecule, heating up each adjacent area within the solid.
You may recognize this as the way of a frying pan conducts heat outside onto a piece meat inside, or the way a radiator feels hot to the touch if you 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 conditioning 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.
However, the most powerful effect is RADIATION. 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.
These three effects are not exclusive to each other but in most situations operate together.
Infrared Imagers observe and measure heat without being in contact with the source (Non-Contact).
The contact- type heat measurement devices work by conduction. A thermometer in your mouth receives the heat energy from your body by conduction. A thermocouple attached to an instrument receives heat receives heat by conduction. All non contact heat measurement devices use the radiation of an object to measure the temperature.
Visible light has color which are in different wavelengths (intensities), and it travels in a straight line whether through air, vacuum, or liquid. Visible light can be reflected and refracted, some wavelengths can be blocked, and others can be passed by suitable filters. However visible light is only a model for invisible infrared radiation, and there are times when that model breaks down.
Infrared radiation, or “beyond the red”, occurs beyond visible light. Although many guesses can be based on that model, some of the guesses may be incorrect. for instance, if we guessed that glass, which is transparent to visible light, would be transparent to infrared radiation as well, we would be incorrect. On the other hand, if we were to guess that some forms of silicon, which reflect visible light, would also reflect infrared, once again this would be incorrect. Many forms of silicon pass infrared radiation and yet inhibit visible light. In addition, infrared (or heat) radiation has some properties that are very “liquid” properties, unlike light. These properties are displayed when the convection transfer method is active. One does not shine a green light on water and then stir up.
the water to make that green light fill the entire liquid with the same color green. This can be done with paint, but not light. Yet in some cases infrared radiation acts exactly like that, It shows you the temperature of an object. In the production of molten glass from glass beads, for example, you will see the heat patterns that warm glass moving through each other. Observing them with an Infrared viewer will see the patterns of this cooling and heating.
THE INFRARED SPECTRUM
Every animate or inanimate body that exists emits energy from its surface. This energy is emitted in the form of electromagnetic waves which travel with the velocity of light through a vacuum, air, or any other conductive medium. Whenever they fall on another body, which is not transparent to the, they are observed and their energy is reconverted into heat, The difference between a COLD or HOT body is the level at which it both emits and absorbs energy. If the body absorbs more energy that it radiates, it can be considered cold. If the body tends to emit more energy than it absorbs, it is considered hot. The state of being hot or cold is a dynamic state. If a body is allowed to come to equilibrium with its surroundings, the emission and absorption will become equal and the body will be neither hot nor cold.
IR energy is emitted by all materials above 0°K. Infrared radiation is part of the Electromagnetic Spectrum and occupies frequencies between visible light and radio waves. The IR part of the spectrum spans wavelengths from 0.7 micrometers to 1000 micrometers (microns).Within this wave band, only frequencies of 0.7 microns to 20 microns are used for practical, everyday temperature measurement.
Though IR radiation is not visible to the human eye, it is helpful to imagine it as being visible when dealing with the principles of measurement and when considering applications, because in many respects it behaves in the same way as visible light. IR energy travels in straight lines from the source and can be reflected and absorbed by material surfaces in its path. In the case of most solid objects which are opaque to the human eye, part of the IR energy striking the object’s surface will be absorbed and part will be reflected. Of the energy absorbed by the object, a proportion will be re-emitted and part will be reflected internally. This will also apply to materials which are transparent to the eye, such as glass, gases and thin, clear plastics, but in addition, some of the IR energy will also pass through the object. These phenomena collectively contributes to what is referred to as the Emissivity of the object or material.
Materials which do not reflect or transmit any IR energy are know as Blackbodies and are not known to exist naturally. However, for the purpose of theoretical calculation, a true blackbody is given a value of 1.0. The closest approximation to a blackbody emissivity of 1.0, which can be achieved in real life is an IR opaque, spherical cavity with a small tubular entry. The inner surface of such a sphere will have an emissivity of 0.998.
Different kinds of materials and gases have different emissivities, and will therefore emit IR at different intensities for a given temperature. The emissivity of a material or gas is a function of its molecular structure and surface characteristics. It is not generally a function of color unless the source of the color is a radically different substance to the main body of material. A practical example of this is metallic paints which incorporate significant amounts of aluminum. Most paints have the same emissivity irrespective of color, but aluminum has a very different emissivity which will therefore modify the emissivity of metallized paints.
Just as is the case with visible light, the more highly polished some surfaces are, the more IR energy the surface will reflect. The surface characteristics of a material will therefore also influence its emissivity. In temperature measurement this is most significant in the case of infrared opaque materials which have an inherently low emissivity. Thus a highly polished piece of stainless steel will have a much lower emissivity than the same piece with a rough, machined surface. This is because the grooves created by the machining prevent as much of the IR energy from being reflected. In addition to molecular structure and surface condition, a third factor affecting the apparent emissivity of a material or gas is the wavelength sensitivity of the sensor, known as the sensor’s spectral response. As stated earlier, only IR wavelengths between 0.7 microns and 20 microns are used for practical temperature measurement. Within this overall band, individual sensors may operate in only a narrow part of the band, such as 0.78 to 1.06, or 4.8 to 5.2 microns, for reasons which will be explained later.
Theoretical Basis for IR Temperature Measurement
The formulas upon which infrared temperature measurement is based are old, established and well proven. It is unlikely that most IR users will need to make use of the formulas, but a knowledge of them will provide an appreciation of the interdependency of certain variables, and serve to clarify the foregoing text. The important formulas are as follows:
1. Kirchoff’s Law
When an object is at thermal equilibrium, the amount of absorption will equal the amount of emission.
2. Stephan Boltzmann Law
The hotter an object becomes the more infrared energy it emits.
3. Wien’s Displacement Law
The wavelength at which the maximum amount of energy is emitted becomes shorter as the temperature increases.
4. Planck’s Equation Describes the relationship between spectral emissivity, temperature and radiant energy.
Thermography (infrared; thermal scans) uses specially designed infrared video or still cameras to make images (called thermograms) that show surface heat variations. This technology has a number of applications. Thermograms of electrical systems can detect abnormally hot electrical connections or components.
Thermograms of mechanical systems can detect the heat created by excessive friction. Energy auditors use thermography as a tool to help detect heat losses and air leakage in building envelopes. Home energy audits sometimes include a thermographic inspection.
Infrared scanning allows energy auditors to check the effectiveness of insulation in a building’s construction. The resulting thermograms help auditors determine whether a building needs insulation, and where in the building it should go. Because wet insulation conducts heat faster than dry insulation, thermographic scans of roofs can often detect roof leaks.
In addition to using thermography during an energy audit, you should have a scan done before purchasing a house; even new houses can have defects in their thermal envelopes. You may wish to include a clause in the contract requiring a thermographic scan of the house. A thermographic scan performed by a certified technician is usually accurate enough to use as documentation in court proceedings.
Thermographic scans can be done inside or outside a structure. Exterior scans, while more convenient for the homeowner, have a number of drawbacks. Warm air escaping from a building does not always move through the walls in a straight line. Heat loss detected in one area of an outside wall might originate at some other hard-to-find location inside the wall. Air movement also affects the thermal image. On windy days, it is harder to detect temperature differences on the outside surface of the building. The reduced air movement and ease of locating air leaks often make interior thermographic scans more effective.
To prepare for an interior thermal scan, the homeowner should take steps to ensure an accurate result. This may include moving furniture away from exterior walls and removing drapes.
The most accurate thermographic images usually occur when there is a temperature difference of 20°F (-6.6°C) between inside and outside air temperatures. In northern states, thermographic scans are generally done in the winter. In southern states, however, scans are usually conducted during warm weather with the air conditioner on.
The cost of a thermographic inspection ranges from $200 to $500, depending on the size of your home and the services provided. Copies of the thermographic image, recorded on videotape, may cost extra.
The energy auditor may use one of several types of infrared sensing devices in an on-site inspection. A spot radiometer (also called a point radiometer) is the simplest. It measures radiation one spot at a time, with a simple meter reading showing the temperature of a given spot. The auditor pans the area with the device and notes the differences in temperature. A thermal line scanner shows radiant temperature viewed along a line. The thermogram shows the line scan superimposed over a picture of the panned area. This process shows temperature variations along the line.
The most accurate thermographic inspection device is a thermal imaging camera.
Spot radiometers and thermal line scanners are less expensive, but do not provide the necessary detail for a complete home energy audit. Infrared film used in a conventional camera is not sensitive enough to detect heat loss.
There are comprehensive training courses in the Unites States for all users of portable infrared imaging equipment.
Various Certification Program are available at an average cost of $1400. Computer software programs for use in thermographic analysis / and reporting are also available.