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To understand infrared thermal imaging, we must first understand wavelengths of the electromagnetic spectrum. Infrared radiation is part of the electromagnetic spectrum and occupies frequencies between visible light and radio waves, spanning wavelengths from 0.7 micrometers to 1,000 micrometers (microns).
The amount of radiation emitted by an object increases with temperature; therefore, thermography allows one to see variations in temperature. An infrared thermometer measures temperature by detecting the infrared energy emitted by all materials which are at temperatures above absolute zero (0 degrees Kelvin). The most basic design consists of a lens to focus the infrared (IR) energy onto a detector, which converts the energy to an electrical signal that can be displayed in units of temperature after being compensated for ambient temperature variation. This configuration facilitates temperature measurement from a distance without contact with the object to be measured. As such, the infrared thermometer is useful for measuring temperature under circumstances where thermocouples or other probe-type sensors cannot be used or do not produce accurate data for a variety of reasons.
Designs for an infrared thermometer (IRT) have existed since at least the late 19th century, and various concepts on the subject were featured by Charles A. Darling in his book “Pyrometry”, published in 1911. However, it was not until the 1930s that the technology was available to turn these concepts into practical measuring instruments. Since that time, there has been considerable evolution in the design, and a large amount of measurement and application expertise has accrued. At the present time, the technique is well accepted and is widely used in industry and in research.
The appearance and operation of a modern thermographic camera is often similar to a camcorder. Often, the live thermogram reveals temperature variations so clearly that a photograph is not necessary for analysis. A recording module is, therefore, not always built in.
Thermal images, or thermograms, are actually visual displays of the amount of infrared energy emitted, transmitted and reflected by an object. Because there are multiple sources of the infrared energy, it is difficult to get an accurate temperature of an object using this method. A thermal imaging camera is capable of performing algorithms to interpret that data and build an image. Although the image shows the viewer an approximation of the temperature at which the object is operating, the camera is actually using multiple sources of data based on the areas surrounding the object to determine that value rather than detecting the actual temperature.
This phenomenon may become clearer upon consideration of the formula:
Incident Energy = Emitted Energy + Transmitted Energy + Reflected Energy
Incident Energy is the energy profile when viewed through a thermal imaging camera.
Emitted Energy is generally what is intended to be measured.
Transmitted Energy is the energy that passes through the subject from a remote thermal source.
Reflected Energy is the amount of energy that reflects off the surface of the object from a remote thermal source.
If the object is radiating at a higher temperature than its surroundings, then power transfer will be taking place and power will be radiating from warm to cold following the principle stated in the Second Law of Thermodynamics. So if there is a cool area in the thermogram, that object will be absorbing the radiation emitted by the warm object. The ability of both objects to emit or absorb this radiation is called emissivity.
The thermal imaging camera would next employ a series of mathematical algorithms. Since the camera is only able to see the electromagnetic radiation that is impossible to detect with the human eye, it will build a picture in the viewer and record a visible picture or video.
In order to perform the role of non-contact temperature recorder, the camera will change the temperature of the object being viewed with its emissivity setting. Other algorithms can be used to affect the measurement, including the transmission ability of the transmitting medium (usually air) and the temperature of that transmitting medium. All of these settings will affect the ultimate output for the temperature of the object being viewed. This functionality makes the thermal imaging camera an excellent tool for the maintenance of electrical and mechanical systems in industry.
By using the proper camera settings and by being careful when capturing the image, electrical systems can be scanned and problems can be found. Faults with steam traps in steam heating systems also are easily located.
Infrared thermometry is a mature but dynamic technology that has gained the respect of many industries and institutions. It is an indispensable technique for many temperature measurement applications. When the technology is adequately understood by the user, and all the relevant application parameters are properly considered, a successful application will usually result, provided that the equipment is carefully installed. Careful installation means ensuring that the sensor is operated within its specified environmental limits, and that adequate measures are taken to keep the optics clean and free from obstructions.
A factor in the selection process, when choosing a manufacturer, should be the availability of protective and installation accessories, and also the extent to which these accessories allow rapid removal and replacement of the sensor for maintenance. If these guidelines are followed, the modern infrared thermometer will operate more reliably than thermocouples or resistance thermometers in many cases.
Darling, Charles R.; “Pyrometry: A Practical Treatise on the Measurement of High Temperatures”. Published by E.&F.N. Spon Ltd. London. 1911.