By Wayne Ruddock
There are many misunderstandings about infrared, both in the general public and amongst the practitioners of infrared in the industrial world. Hollywood portrays infrared falsely as a technology that can see through walls and windows and investigate the interior of buildings. It incorrectly depicts it as a technology that will allow you to see if there are any occupants in a building and where they are located, due to their “thermal signatures.” This has been seen in a number of Hollywood movies and now has trickled down to television programs such as “CSI.”
The reality of any infrared technology is that it “sees” infrared radiated energy only. This radiated energy comes from the first one thousandth of an inch of the surface of most solids and liquids. This form of electromagnetic radiation will travel through a gas or a vacuum only. When you look at a building wall with an infrared camera, the image that you see is a representation of the energy radiated from the paint or whatever material makes up the surface that you are viewing. You do not see into or through the wall. Those are the facts.
Even the manufacturer’s representatives often mislead the end user about the physics of infrared. Educators in the infrared thermography field have often, over the past 30+ years, seen disbelief and even shock in students’ faces when it is explained and demonstrated to them that the infrared gun or spot radiometer that they and their colleagues have been using for the past decade does not measure temperature. There is no infrared device in the world that actually measures temperature. They all work on the same principle. They view and measure radiated energy, as mentioned above. In devices such as spot radiometers and the newer generation of uncooled predictive maintenance cameras, the onboard computer or microprocessor takes the energy value measured by the infrared detector and computes or calculates a corresponding temperature.
In most predictive maintenance applications, the radiated energy comes from two distinct sources. A portion of the energy is emitted from the surface of the object, viewed as a function of the surface temperature of the object. The rest of the energy is reflected off of the object’s surface from the background. The simple formula E + R = 1 describes the total amount of energy the camera views from the surface of an object. E represents the portion of energy emitted, while R represents the portion of energy reflected. The lower the emissivity of the object of interest, the greater the effect of background reflection on the image displayed by an infrared camera.
Theoretically, if an object had no reflection it would be a perfect emitter. This theoretical object is called a black body. When it comes to the emittance of infrared energy, the concept of the black body is the gold standard to which all real world objects are compared. It would have an emission rate of 1 and a rate of reflection equal to 0. Unfortunately, in the real world there is no such thing as a black body.
To arrive at a temperature with any infrared device, we must account for the rate of emission and also the amount of reflected energy, which is not related to the temperature of the object of interest. To do this, we must enter the correct emissivity of the object of interest into the onboard microprocessor in our infrared device. Emissivity is defined as the rate at which an object emits energy as compared to that of a black body at the same temperature and in the same wavelength. Without knowing this value and correctly entering it into your infrared system, you will never arrive at an accurate temperature with any infrared device.
In order for the processor to correctly subtract out the amount of energy that is a function of reflection, we must measure the amount of energy coming from the background and reflecting off of our object of interest and striking the infrared detector(s). The infrared camera or spot radiometer sees right through the atmosphere in most cases, so the ambient air temperature is usually not a true representation of the background reflected quantity. To arrive at this value, you must measure the background reflection with your infrared device. We do not want to enter a true temperature reading, but we want to enter a false value that represents accurately the total radiated energy coming from the background. To arrive at the correct reflected value, we can use the following procedure.
1. Determine the area from which the reflection will come off your object of interest. This can be done by imagining bouncing a ball off the object of interest, then imagining where it would bounce back to.
2. Set the emissivity on your system to 1. We do not want the actual temperature, but we want to determine the amount of energy coming from the background. With the emissivity set at 1, the camera will give you a false temperature value that represents the actual total radiated energy from that background area.
3. With your system, image the area from which the reflection will come and use a measurement function, spot meter, isotherm, or area function and determine what the general average value of the background is.
4. Enter this false temperature into the ambient reflected function in your system.
The bottom line is that if you do not enter the correct emissivity and background reflected quantity into an infrared device (camera or spot radiometer), you will never get an accurate temperature displayed on the device. Example A shows how far off the displayed temperature can be, using an incorrect emissivity value and background.
Which one is correct? It is the one where the correct emissivity and background were entered into the camera.
I have often heard it said that if you are doing electrical inspections and are mainly interested in component-to-component rise calculations, you can just leave your emissivity at .9 and you will get the right phase-to-phase or component-to-component comparisons. This is a false assumption and totally incorrect. If your actual values are wrong due to using the incorrect emissivity value of .9, your comparisons will be wrong also. Example B illustrates this principle.
If your infrared device does not allow you to enter both emissivity and background reflection, it is not suited for quantitative infrared applications. These devices also cannot be relied on to do temperature trending.
It must also be remembered that we do not see temperature patterns or true thermal patterns with an infrared camera; we only see radiated energy patterns that may or may not have anything to do with the actual temperature of an object. The infrared technician must have a good knowledge of emissivity and background reflection to interpret infrared images.
Wayne Ruddock has been involved in Infrared Thermography and Infrared Thermographic Training since 1979. He is a seasoned veteran of hands-on infrared inspections, giving him the ability to teach real life thermography. He has been conducting Level 1 and Level 2 training courses throughout the world since 1980. He has written and presented many thermographic papers at conferences over the last 30 years, and he is the author of Basic Infrared Thermography Principles, available at www.mro-zone.com.