What You Really Need to Know to Begin Using
Infrared Cameras

by L. Terry Clausing*

Figure 1-3
Figure 4-6
Figure 7-9

Introduction
Only about five years ago, most infrared camera systems sold for $30,000 to $60,000. A person planning to conduct infrared tests would generally begin by taking a one week class in the practice of infrared thermography. Today, infrared cameras cost under $10,000 and the price is continuing to drop. With the reduction in purchase price, infrared cameras are being mass marketed for general use.

Infrared cameras are simple to use, right? The digital readout tells you the temperature - doesn't it? Truth be told, measuring the actual temperature of a material with infrared requires detailed knowledge of materials and the properties of heat transfer. Comprehensive training is essential to meet industry requirements for personnel qualification and certification in infrared thermography, even with the simple, low cost infrared cameras.

This primer is intended to give you a basic understanding of infrared temperature measurement, so that you can begin to use this marvelous technology effectively.

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Today's infrared cameras are becoming increasingly affordable and easy to use.

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Why Are the Temperature Readings Inaccurate?
At this point, you are likely already thinking "Oh, come on. It's not that hard; it really isn't that complicated. Look at the display on the camera; it shows you the temperature." In fact, it really is more complicated. The most common use of infrared thermography is for the testing of electrical power distribution equipment. Let's look at a typical three phase fused power disconnect (Figure 1a) and the corresponding infrared image (Figure 1b).

Figure 1a shows a typical three phase fused power disconnect. The corresponding infrared image, Figure 1b, was taken with the emissivity setting at 1 on our infrared camera. The temperature span and color scale for the infrared image is set to 308 K (95.5 °F) referring to black, with warmer temperatures indicated progressively by blue (314 K [105 °F]), green (319 K [115 °F]), red (325 K [125 °F]) and white (329 K [133 °F] and hotter). We also measured the load in phases A, B and C (from left to right), at approximately 34 A each.

A simple analysis of the thermal image indicates that phase A is significantly hotter than phases B and C. The fuse clip at the top of phase A indicates 329 K (133.4 °F), while the end of the fuse, specifically the metal cap of the top of the fuse, appears much cooler with a temperature of 313 K (103.6 °F) and the fuse body just below the cap appears to be 323 K (121.9 °F).

Can this be true? Is the metal cap only 313 K (103 °F)? No. You are seeing an example of the apparent temperature and the effect of emissivity. The fuse end cap is a highly reflective metal, in this case copper. Notice that the body of the fuse also appears hotter than the metal cap. The temperature of the cap is actually as hot as the fuse body that it is in contact with.

To explain why the apparent temperature seen through a thermal imager can be significantly different than the real temperature, let's review our knowledge of physics.

 

Thermal Radiation and the Properties of Materials
All objects emit infrared (thermal) radiation. The intensity of the radiation depends on the temperature and nature of the material's surface. At lower temperatures, this thermal radiation is limited to longer wavelengths. As the object becomes hotter, the radiation intensity rapidly increases and the wavelengths of the radiation shift toward shorter values. The relationship between radiation intensity and temperature is defined by the Stefan-Boltzmann law (ASNT, 2001):

(1)

where

Q = radiation intensity

e = emissivity

= Stefan-Boltzmann constant

T = absolute temperature.

The maximum radiation is achieved when the object has an emissivity of 1. This is referred to as blackbody radiation, because with an emissivity of 1, the object is a perfect radiator. However, in our real world, there are no true blackbodies, that is, no perfect radiators. Since real materials are less than perfect radiators, the relevant issue is "how much less than perfect are they?" Emissivity is defined as the measure of how much less than perfect a material radiates when compared to a blackbody. However, emissivity is only one of three factors that cause an object to be less than a perfect radiator.

 

The Thermal Nature of Materials
Materials (objects in everyday life, whether they be solids, liquids or gases) are constantly affected by their surroundings. Thermally, all objects attempt to exchange energy with other objects in their natural drive towards thermal equilibrium with their surroundings. In this search for thermal equilibrium, heat is exchanged between objects via three mechanisms: conduction, convection and radiation.

Conduction is defined as heat transfer between two solid bodies that are in physical contact with each other. Convection is heat transfer usually between a solid material and a liquid or gas. Conduction and convection are dependent on physical contact between materials. Radiation is a process of heat transfer, characteristic of all matter (at temperatures above absolute zero). Radiation passes through a vacuum, and can also pass through gasses, liquids and even solids.

Since radiative thermal heat transfer between objects is not conditional on physical contact, and since all objects are constantly seeking a state of thermal equilibrium, the total incident energy from any object is defined by the Total Power Law:

(2)

where

r = reflectance coefficient

a = absorbance coefficient

t = transmission coefficient.

The ability of an object to absorb radiation is also related to its ability to emit radiation. This is defined by Kirchoff's law

(3)

where

a = absorbance coefficient

e = emissive coefficient.

Therefore, when the infrared camera observes the thermal radiation from real objects, part of what the infrared camera sees is reflected from the surface of the object, part is emitted by the object and part may be transmitted through the object. In our example of a steel part, the transmission is zero, but to the degree that the part is reflective, it is less emissive and therefore real objects will usually appear cooler than they actually are. Except when there is something hotter in the vicinity since with opaque materials, the lower the emissivity, the higher the reflectivity. The result in this case is materials appearing hotter than they actually are. Let's examine some real objects to illustrate these effects.

 

Applying Emissivity to Real Objects
In Figure 1b, not only is the fuse end cap temperature actually much hotter than the 313 K (103.6 °F) that it appears, the hot spot above it is most assuredly hotter than the 329 K (133.4 °F) that it appears.

So, how much hotter might it be? This fused power disconnect is electrically energized, so let's conduct a simple experiment with a metal part that is not electrically energized and therefore safe to experiment with. Be careful because while this experiment may not be shocking, the results can really burn you if you try to make measurements without knowledge. Infrared thermography is predominantly practiced in the testing of electrical power distribution equipment. This paper discusses the technical aspects of performing infrared analysis, especially as it relates to predictive maintenance of electrical equipment. All persons working on or around energized electrical equipment should consult NFPA 70E for OSHA safety requirements (NFPA, 2002).

We have a round stainless steel block (Figure 2a). With this block sitting on the bench at ambient temperature, we observe the block with our infrared camera (Figure 2b). The metal appears to be 299 K (78 °F).

This would seem to be fairly accurate since the ambient temperature in the room is also 299 K (78 °F). We can use a thermocouple to verify by contact that the temperature of the steel actually is about 299 K (78 °F).

Now let's take this block and place it in a 353 K (175 °F) oven and bake it for 3 h. We remove the block from the oven and test it with the infrared camera (Figure 2c). The block appears to be only 308 K (94 °F). Using the thermocouple, we measure the temperature and find that it is actually measured at 350 K (170 °F).

How can the infrared camera appear accurate when the part is at room temperature and be so wrong when the part is hot?

At room temperature, the block appears to be room temperature because the block is primarily reflecting the thermal radiation from everything around it. Since the ambient in the room is 299 K (78 °F), the reflection from the surface of the block appears also to be 299 K (78 °F). When the same part is heated in the oven, the part becomes much hotter than the surroundings, so the infrared camera is able to see an increase in radiant energy, albeit much lower in apparent temperature because of the low emissivity value of the surface. Let's modify our experiment to better demonstrate what the infrared camera sees.

We take another stainless steel block and paint half of it with a flat black paint (Figure 3a) and bake it another 3 h. When we remove the block from the oven this time, the unpainted side still appears to be 308 K (94 °F) but the infrared camera now indicates the painted sided to be 350 K (170 °F), fairly matching the thermocouple reading Figure 3b). We can make a very good estimation of the actual emissivity of this material by observing the unpainted surface with our infrared camera and adjusting the emissivity value on the camera until the reading matches the temperature observed on the painted side. In this case, the emissivity is found to be approximately 0.15.

 

Emissivity Is a Cantankerous Variable
Emissivity varies according to surface condition as seen here, and also by viewing angle, and even by temperature and by spectral wavelength. A table of common emissivity values is almost certainly included in the operating manual for your infrared camera. The table should be considered only a rough guide in estimating an emissivity value to use with any particular material. If actual temperature values are required, it is best to perform experiments as described here, to properly characterize the emissivity for the material and its application. The two most common techniques for estimating emissivity are the application of a flat black high emissivity paint to the surface (as described above) or the application of common black electrical tape to the material's surface. Black electrical tape has an emissivity of approximately 0.96 and, coupled with its low mass and strong adhesion properties, provides a very good basis for the estimation of the actual emissivity of a material.

In this experiment we see that the difference between the apparent temperature on the unpainted side and actual temperature is an error of 44 K (79 °F) degrees. If we were to conduct a similar experiment with a high temperature infrared sensor operating at 8 to 14 µm (the same as our infrared camera) and attempt to examine steel that is around 1370 K (2000 °F), the error between the actual and apparent temperatures could be more than 220 K (400 µF).

Stefan-Boltzmann's law is valid when the full spectrum of radiant energy is taken into consideration in the measurement. It is often useful to use a narrow spectral band, which is near the wavelength of peak radiant energy of an object. Wien's displacement law helps us determine the peak wavelength for an object at a certain temperature.

(4)

where

= peak wavelength of radiant energy

b = 2897 µm/K

T = temperature (in Kelvin).

When you are working with high temperature materials, you can greatly reduce the errors due to emissivity mismeasurement by selecting infrared detectors that operate at narrow wavelength bands closer to the wavelength of peak radiant energy. Planck's function is

(5)

where

= radiated energy at a given wavelength

= emissivity

T = absolute target temperature

= wavelength

h, c, K and e are various physical constants.

While the math and physics necessary to prove this is beyond the scope of this text, it can be used to demonstrate that by choosing an infrared sensor with a wavelength band that corresponds with the wavelength of peak radiated energy, such as one operating at 1 µm for this example, the maximum difference we would observe between the 1370 K (2000 °F) actual and apparent temperatures would be closer to 28 K (50 °F) without knowing the precise emissivity of the material with better certainty.

To briefly summarize, temperature measurement without knowledge would result in an error of more than 220 K (400 °F), while making the same measurement with knowledge would result in our error being closer to 28 K (50 °F) and with no better determination of the material's emissivity.

 

Emissivity, the Variable Variable
Back to our steel block example, let's discuss another very significant phenomena. We will take our unpainted metal block and drill three holes in the body. All three holes are 3.2 mm (0.125 in.) diameter. The first is 3.2 mm (0.125 in.) deep, the second is 6.4 mm (0.25 in.) deep and the third is 9.5 mm (0.375 in.) deep. Bake the block at 353 K (175 °F) for another 3 h, then remove the block and observe it again with the camera (Figures 4a and 4b).

Interestingly, the block still appears to be 308 K (94 °F) and now appears to have three hot spots. The 3.2 mm (0.125 in.) deep hole appears to be 319 K (115 °F). The 6.4 mm (0.25 in.) deep hole appears to be 325 K (125 °F) and the 9.5 mm (0.375 in.) deep hole appears to be 333 K (140 °F).

We know that the metal block is truly soaked to 353 K (175°F) and the surface finish is uniform and has an emissivity of approximately 0.15. The reason the temperature appears to be higher in deeper holes is that a hole in a body simulates a blackbody cavity. The better the simulation, the higher the effective emissivity. By adjusting the emissivity on the camera to match the actual temperature at each hole, we find that the emissivity appears to be 0.25 for the 3.2 mm (0.125 in.) deep hole. The emissivity of the 6.4 mm (0.25 in.) deep hole appears to be 0.35 and the 9.5 mm (0.375 in.) deep hole appears to have an emissivity of 0.45.

This is an extremely important effect. Let's look at another piece of electrical equipment to see why this is so important to us.

 

Emissivity and Electrical Equipment
In Figure 5a, you see another power disconnect with the conductors bolted in place using socket head bolts. The corresponding infrared image (Figure 5b) shows a hot connection on the middle phase. Notice the apparent hot spot in the hot socket head bolt. The well of the bolt head appears hotter primarily because of the well, illustrating the blackbody effect of a hole.

In manufacturing processes, steel or aluminum rolls are often used to heat or cool a material such as in paper or plastic film processing. These rolls are usually polished metal surfaces and there is often a strong interest in understanding the thermal profile since the manufacturing process is dependent on thermal uniformity across the rolls. These rolls tend to be very difficult to image with an infrared camera because they have very low emissivities. However, there are often points where the material passes between two rolls. The tangent point between two rolls also tends to simulate the blackbody effect, allowing for effective temperature measurement in an otherwise difficult situation.

This effect is illustrated in common electrical equipment as well (Figure 6). In this case, we have another power disconnect with knife blade switches. This type of switch utilizes shiny metal blades and the proximity of the blades with narrow gaps between them simulates the blackbody effect for greatly improved effective emissivity.

The important message here is to begin to develop your understanding of apparent and actual temperature measurement. Actual temperature measurement requires an intimate understanding of physics, heat transfer and the characteristics of materials.

 

Qualitative versus Quantitative Infrared Thermography
The difficulties with emissivity are not a barrier to the effective use of infrared thermography for predictive and preventive maintenance. The practice of infrared thermography for this purpose is guided by relevant ASTM standards for conducting these tests. These standards describe the use of infrared cameras for qualitative and quantitative infrared testing (ASTM, 2005).

Quantitative infrared tests are predicated on the determination of emissivity of each component so that accurate temperature measurements are presented. This practice is of somewhat questionable value. Predictive and preventive maintenance using infrared thermography is often of greater value when practiced using qualitative approaches. Qualitative approaches allow you to leave the emissivity at 1.0 and evaluate the equipment on a relative basis. The basis for the qualitative evaluation is that you are comparing similar equipment under similar loads.

Looking back at Figures 1a and 1b, you can see that there is little value to be gained in spending time estimating or debating the emissivity of the various parts in the power disconnect. The value is in understanding that phase A is hotter than phase B and C. In addition to realizing that a phase is hotter, it is essential to measure the load of the three phases. Greater electrical load inherently means more heat is present:

(6)

where

P = power in watts (heat)

I = current in amps

R = resistance in ohms.

Comparing Similar Equipment under Comparable Loads
The first rule of thermography in predictive maintenance is to compare similar equipment under comparable loads. In electrical power distribution, comparable equipment is usually the easy part since each electrical phase is usually similar in materials to the phase next to it. Load is a very different matter: it is not uncommon to find significant load imbalances. Figure 7 shows an electrician measuring the electrical load.

So just observing that there is a hot spot does not indicate a problem. You must measure the loads and determine if the presence of a thermal anomaly indicates a problem. Infrared cameras do not identify thermal problems - trained, knowledgeable, qualified people make educated assessments of equipment. This leads to real value in preventive maintenance and reduced frequency of equipment breakdowns.

 

Total Power Law
Emissivity cannot be discussed without due consideration also for all of the components of the Total Power Law, as the three together (reflectance, absorbance and transmission) constitute total incident radiance.

 

Infrared Is Not X-ray
First, let's establish that infrared cameras do not see through metal. It has been a common practice by people who do not understand infrared thermography to not remove covers from electrical panels for them to be tested. It is essential for covers to be removed so that the infrared camera has a direct line of sight of the equipment in order to provide a relevant infrared image of the equipment. As noted previously, it is also necessary in order to have access for measuring the loads for proper assessment.

Electrical power distribution systems include bus systems with bus plugs. The bus plugs are often located overhead in generally inaccessible locations. It is accepted practice to test this equipment from the ground without opening each bus plug. This practice requires extensive training and experience and should be performed only by qualified personnel such as certified Level II thermographers. Let's examine an example of this application (Figure 8).

First we see the photograph of the overhead bus plug (Figure 8a). The corresponding infrared image shows a small apparent temperature rise on the upper left corner of the housing (Figure 8b). In practice, it is common to see apparent hot spots, many of which are determined to be reflections from other heat sources in the vicinity. In determining if a hot spot is a reflection, you would observe the hot spot as you move around the object. A reflectance from the surface will tend to follow your line of sight. A true hot spot will remain in a fixed location, as this did.

Keep in mind that we are evaluating the interior of a piece of electrical equipment based on an apparent temperature rise on the exterior of the enclosure. If there is a problem, the hot spot will be produced as a result of the radiant energy from the problem, since there is no conductive path (either electrical or thermal) from the electrical equipment. We also have the low emissivity issue of the surface of the bus and bus plug. Small apparent temperature rises on this equipment can therefore indicate significant internal problems.

When this bus plug was opened (Figure 9a), we found the fuse was so hot that the metal had begun to fail (note the sagging fuse connection, middle left). The infrared image (Figure 9b) indicated an apparent temperature in excess of 422 K (300 °F). This was an imminent failure avoided by infrared testing.

 

Infrared Transmission
It is increasingly common for clear acrylic panels to be installed over critical electrical connections inside panels and control cabinets. Just as infrared cameras cannot see through the metal enclosures, infrared cameras cannot see through acrylic. Acrylic covers, though clearly visible to the human eye, are completely opaque to an infrared camera. When testing protected electrical components such as these, it may be necessary for the electrician to remove these protective acrylic panels so the equipment can be properly examined.

One of the most difficult issues in infrared thermography is that the spectral characteristics of materials are generally very different between visible and infrared radiation. In everyday life you cannot see through walls, if you want to look outside you look through a glass (or acrylic) window, and if you want to see yourself, you look into a mirror. Infrared cameras fool people because they do not behave the way people are accustomed to behaving with their eyes. Infrared cameras generally cannot see through a glass or acrylic window - these materials look very much like a wall in the infrared. Even a regular mirror looks like a wall, not like a mirror. This is because the typical mirror is actually glass with a reflective coating on the back surface. The infrared camera never sees the reflective coating on the back because it can't see past the front surface of the glass. Infrared mirrors are often referred to as front surface mirrors, because the reflective coating is a highly reflective material on the front surface of a material. Front surface infrared mirrors are often used in process applications where it is not possible to obtain a direct line of sight, so a mirror is used like a periscope to look around obstacles at points of interest.

 

Thin Films - Complex Thermal Analysis
Compared to Planck's function, the Total Power Law looks simple and, in fact, it is. The problem is its application in real life when a material's incident radiance is shared among all three components.

This applies to thin film materials such as papers and plastics. These materials illustrate complex thermal analysis applications because the spectral constituents of the materials vary greatly and they have significant transmissive as well as emissive and even reflective spectral components.

As infrared cameras become more affordable, companies often attempt to justify their purchase on the basis that they can be used both for predictive maintenance and for thermal evaluation of their products and manufacturing processes. Paper and plastic manufacturers are especially vulnerable in this regard. While there are thousands of possible examples, let's examine some popular plastics to illustrate the complexities of thermal assessment of plastic films in the manufacturing process.

The first property to deal with is the thickness of films. Generally, thinner films are more transmissive while thicker films are less transmissive. In the case where the objective is to examine the thermal uniformity across a web, the issue is complicated partly by variations in thickness.

Acrylic and polyvinyl chloride are generally opaque in the infrared region. Popular plastics in packaging, including the clear "blister pack" plastic package, are visibly transparent, but completely opaque in the infrared. Infrared cameras can, however, provide good thermal analysis of these in the thermal forming process.

Polystyrene, polypropylene and polyester are fairly transmissive in the infrared. But polystyrene and polypropylene are completely opaque at 3.43 µm, while polyester is opaque at 7.95 µm. A common error is to attempt to thermally image these plastics in process. The thermal imager does display some thermal data. However, when viewed with 8 to 14 µm cameras, the data are confusing. You may see the reflections of other parts of the process as the radiant energy passes through the plastic and is reflected from the shiny metal rollers.

Some plastics, then, are opaque and some are transmissive. The transmissive plastics are more transmissive when they are thin films and less transmissive as they get thicker. At some thickness they will become opaque. However, these transmissive plastics are very opaque at certain specific wavelengths and there are special detectors and filters for infrared cameras that allow these materials to be thermally analyzed.

Thermal analysis of thin films such as these requires extensive training and experience and should be performed only by qualified personnel such as certified Level II or Level III thermographers or qualified engineers.

 

Infrared Tricks of the Trade
One of the most popular plastic materials that you are familiar with is the common black or dark green garbage bag. Professional thermographers will often have a plastic garbage bag (along with a roll of black electrical tape) in the infrared camera case because while these bags are completely opaque to the eye, they are nearly completely transmissive to an infrared camera. Thermographers will often use this plastic bag as a protective cover for their infrared camera in inclement weather.

 

Summary
Predictive maintenance with an infrared camera can be effectively performed by utilizing qualitative analysis of equipment. Qualitative techniques allow the emissivity setting on the infrared camera to be kept at 1.0 and apparent temperatures used for comparisons between similar equipment under similar load. This type of basic analysis requires proper training to understand (and not be misled) by the apparent thermal images produced by the infrared camera.

Quantitative infrared analysis refers to the attempt to measure actual temperatures of materials using infrared thermography. Actual temperature measurement involves more than simply adjusting for emissivity. Total incident radiance requires dealing with the effect of reflection and transmission in addition to emissivity.

Whether you are doing qualitative testing or a quantitative thermal analysis, it is very important that it be done correctly. ASTM publishes standard practices describing the process and procedure for performing infrared inspections of electrical equipment, mechanical equipment, buildings, roofs and much more. These standards are our road map to high quality work and meaningful results.

Today's infrared cameras are becoming increasingly affordable and easy to use. But what does easy mean? The practice of infrared thermography looks straightforward and simple, but it is not. It takes a truly trained eye to understand the thermal image displayed by an infrared camera. It is much like most endeavors in life: the more you learn, the more you discover there is to learn.

 

Acknowledgment
An earlier version of this article was published as an application note by the Fluke Educational Partnership Program.

 

References
ASNT, Nondestructive Testing Handbook, third edition: Volume 3, Infrared and Thermal Testing, Columbus, Ohio, American Society for Nondestructive Testing, 2001.

ASTM, ASTM E 1934: Standard Guide for Examining Electrical and Mechanical Equipment with Infrared Thermography, West Conshohocken, Pennsylvania, ASTM International, 2005.

NFPA, NFPA 70E: Standard for Electrical Safety in the Workplace, Quincy, Massachusetts, National Fire Protection Association, 2002.

 

* Drysdale & Associates, Inc., PO Box 44055, Cincinnati, OH 45244-0055; (513) 831-9625; fax (513) 831-9627; e-mail <terryc@virtualspectrum.com>.

 

Copyright © 2006 by the American Society for Nondestructive Testing, Inc. All rights reserved.

 

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