Infrared thermography has been used for everything from military applications to residential energy audits to animal science. The core user group, however, consists of maintenance professionals who use their cameras to detect thermal anomalies that might indicate potential failure points in electrical and mechanical components.
As a reliability and maintenance professional, you are probably aware of the use of infrared cameras in the field of condition-based monitoring, even if you have not used one yourself. But do you know how they work and why? Do you understand not only their capabilities, but their limitations as well? In this series of articles, we will discuss the theory behind the cameras and the importance of understanding not only what they do, but how they do it.
Many maintenance professionals use a spot radiometer to provide a digital readout of the temperature of a specific spot on an electrical or mechanical component. An infrared imager essentially combines a large number of these spots into an array of pixels, each one of which displays the level of infrared radiation as a color on the screen based on a user-selected color palette. Combining these pixels on a screen provides us with an image that makes it easy to see thermal patterns over a two-dimensional area, rather than just a temperature readout of a single spot. This can provide a significant advantage in an environment where anomalies may be found on multiple devices and over very large areas.
For example, one of our customers has facilities containing 79 miles of conveyor belts with a set of idler bearings every 12 feet. This means that there are over 30,000 possible failure points which, prior to the advent of infrared imaging technology, would have been extremely difficult to monitor. With an infrared camera, the thermographer can look along the length of a belt segment and easily spot the bearing operating at a higher than average temperature. (See Figure 2.)
In other cases, when performing diagnostic tests from a distance is an unavoidable necessity, infrared camera lens options can significantly improve the thermographer's success rate. Interchangeable lenses can effectively change the optical resolution of the camera, yielding a higher quality image and better measurement accuracy as can be seen in Figure 3.
NOTE: "Resolution" vs. Resolution:
The higher the number of pixels in the detector, the higher the quality of the infrared image. When shopping for an infrared camera, be aware of the distinction between detector resolution and display resolution. Some manufacturers advertise "640 by 480 resolution" displays on cameras whose detectors have only 320 by 240 resolution. This promotional messaging could lead the less experienced thermographer to believe that these cameras provide a superior infrared image to other 320 by 240 cameras. Imaging and measurement performance are governed by detector resolution, not display resolution.three radiant properties using the terms: transmissivity, reflectivity, and emissivity.
Of the various color palettes available on most cameras, iron or "ironbow" (Figure 3) is the most intuitive palette for new thermographers due to the way it simulates iron glowing at different temperatures. Most of the commonly used palettes follow a similar pattern; higher relative temperatures appear in brighter colors and low temperatures appear darker (although some cameras do allow the colors to be inverted). Depending on the palette and the subject, some IR images, such as in Figure 4, can appear to be deceptively similar to visible images. One of the biggest challenges to the new thermographer is understanding the differences.
For example, we do not need a visual image to see that the subject of Figure 5 is a small SUV. Most non-thermographers, however, might instinctively guess from the brightness of the headlights that they are on. The IR image indicates that the surface of the lights are warmer than most of the rest of the vehicle, but it cannot tell us whether they are actually on. By the same token, we cannot tell from this image whether the defroster and engine were running when the image was saved. All we know for sure is that the windshield and engine bay are warmer than the surrounding areas. It important for the thermographer to understand both the camera's capabilities and its limitations. Familiarizing ourselves with infrared radiation and how the cameras work will help us understand these concepts.
What is Infrared Radiation?
Infrared radiation forms a part of the electromagnetic spectrum, along with visible light, radio waves, ultraviolet, and X-rays. Visible light takes up only a small sliver of the EM spectrum. Just beyond visible light is a wide range of wavelengths we refer to as infrared. Infrared wavelengths vary from very short waves (also known as "Near IR") such as those used by the remote control in your living room and in night vision goggles, to the wavelengths detected by these longwave infrared cameras.
Three forms of heat transfer
Heat always flows in one direction--from warm to cool--and can be transferred in any of three ways: conduction (through a solid or between solids that are touching); convection (from a solid to a gas/liquid); or radiation (from any object to any other through EM energy).
Heat transfer is occurring constantly. Our own bodies, for instance, conduct heat energy to our seats, convect to the air, and radiate to the surrounding walls. We tend to think of heat transfer mostly in terms of the first two, conduction and convection. In fact, most heat transfer occurs through radiation. On a cold winter day, the amount of heat your body loses through radiation to the outside walls accounts for about 70% of the total loss. An infrared camera works by detecting that infrared energy.
Although radiation is the most important form of heat transfer, it is important to understand the concepts of conduction and convection as well. By way of example, conduction is the key to analyzing the issue in Figure 8. The temperature gradient on the wire on the left shows us that the source of the heat is near the termination, rather than along the length of the entire wire. This indicates that the issue is caused by a termination problem, rather than a load problem (which would result in a uniform heating pattern along its length, as well as on the line side).
A study by the National Fire Protection Association has found that 85% of all electrical fires are caused by electrical termination problems! Hotspots on electrical components are the thermographer's "low hanging fruit." A minimal investment of time and resources can yield a significant return in terms of safety and reliability.
Convection plays a role as well. In the oil-filled transformers in Figure 9, we can use the surface temperature to calculate the temperature of the source using convective heat transfer formulas. For our purposes, it is sufficient to understand that even a very small temperature delta in such a massive component indicates a very high temperature at the source.
All materials have three properties with respect to incoming ("incident") infrared radiation. They can transmit it (i.e. allow it to pass through), reflect it, or absorb it. Due to the laws of thermodynamics, anything that was absorbed by the material will have to be emitted as well. Thus, the infrared radiation leaving the surface of the same material ("exitant" radiation) is either reflected, transmitted, or emitted energy. So we can summarize the three radiant properties using the terms: transmissivity, reflectivity, and emissivity.
The key concept here is that the camera cannot distinguish between the energy that is emitted versus that which is transmitted or reflected. It simply displays the sum of all three, which we refer to as "apparent" temperature. One of the most important tasks of a thermographer is to adjust the settings on the camera to compensate for the transmitted and reflected radiation to yield a "true" emitted temperature.
In the visible spectrum, glass is nearly 100% transmissive. In the longwave infrared, however, it's completely opaque. In fact, materials that transmit infrared energy are very rare, most notably, silicon and germanium (from which most infrared camera lenses are made) and a few plastics.
NOTE: X-Ray Vision?
Contrary to popular belief, longwave thermography cameras cannot see through walls (or clothes or even windows). No matter how many times Jack Bauer has done it on "24," it is not possible to detect human bodies inside a building using infrared from the outside. Images that seem to simulate X-ray vision, such as shots of a wall that clearly show the stud locations, are merely showing minute temperature differences on the surface of the material. It is usually the conduction properties of the materials behind the surface that create this illusion.
Since so few materials transmit infrared energy, the thermographer is concerned mostly with the other two radiant properties: reflectivity and emissivity.
Most metals are highly reflective in the infrared spectrum. The block of aluminum in Figure 11 reflects almost as well in the infrared spectrum as a glass mirror does in the visible. Highly reflective surfaces such as this are very difficult to measure accurately.
Figure 12 illustrates how reflectivity might fool a less experienced thermographer. Based on the image, the connection on the left appears to be significantly warmer than the other. In the image on the right, however, the relative temperatures seem to have reversed. In fact, both are emitting the same amount of energy. It is only the reflection of the thermographer's warm body that has changed. The experienced thermographer is always aware that heat patterns that follow human movement are most likely reflections.
The term emissivity refers to a material's ability to radiate; the more efficient a radiator, the higher its emissivity. For "opaque" materials that do not transmit in the infrared, emissivity is inversely proportional to reflectivity.
Since it is the emitted radiation that can be used to calculate a material's temperature, the higher the emissivity, the more accurate the temperature measurement. Conversely, the higher its reflectivity, the more difficult it is to get an accurate temperature measurement. Some thermographers use the adage, "Low emissivities lie!"
The stainless steel cups pictured in Figure 14 illustrate this principle. The cup on the left is filled with cold water, the middle with no water, and the right with hot water. Stainless steel, while a good conductor, is highly reflective in the infrared, i.e. a very poor emitter. Thus, the steel surface of all three cups appears to be very similar in temperature.• Polyvinyl-chloride tape (otherwise known as electrical tape), on the other hand, has a known emissivity level of .95 (95% emissive). By affixing electrical tape to the cups and using a camera set to the same emissivity level, we create targets that provide much more accurate temperature measurements.
• In this case, the tape on the leftmost cup appears significantly colder than the ambient temperatures whereas the tape on the right appears much warmer. Interestingly, the tape on the middle cup is almost invisible since it is at the same ambient temperature level that is being reflected by the cup.
• Thermographers frequently use electrical tape to provide accurate measurement targets on objects that otherwise might "lie" about their temperatures.
Note: Use common sense and prudent safety practices when approaching electrical and mechanical components.
One compelling example of the impact of emissivity in an industrial environment involves a mixer motor at a large food processing plant. This motor was problematic from the day it was installed. In compliance with EPA regulations, it was constructed of stainless steel, which is highly reflective and a poor emitter. Thus, any heat generated inside of the motor stayed inside the motor as is shown in the thermal image on the right in Figure 15, instead of being radiated into the environment.
By adding a coat of high emissivity paint which allowed the motor to radiate more efficiently, the operator was able to reduce the operating temperature by over 10 degrees - enough to bring the motor to a fully functional state as shown in the thermal image on the right in Figure 16. This example also illustrates the relationship between conduction, convection, and radiation.
Emissivity is probably the single most important concept for the thermographer to understand. In future articles, we will discuss factors other than surface material that can affect emissivity.Summary
The key points to remember are:
• Infrared is the portion of the electromagnetic spectrum just beyond visible light. All objects radiate infrared energy. For any given material under fixed conditions, the higher its temperature, the more infrared energy it radiates. This is the principle that allows infrared cameras to calculate temperatures.
• Radiation is one of three forms of heat transfer. The other two are conduction (within or between solids) and convection (within a gas or liquid).
• The three exitant radiant properties of a material are transparency (rare in the IR spectrum), reflectivity, and emissivity. The primary function of a thermographer is to compensate for any transmissivity or reflectivity in order to measure the emitted radiation as accurately as possible.
Operating a typical infrared camera is relatively simple, but accurately interpreting the images requires knowledge and practice. When budgeting for an infrared camera, consider including a Level I Thermography class with a reputable provider.