This article was originally published in the July/August 1993 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online July/August 1993
Selecting an Infrared Imaging System
by John Snell
John Snell is president of John Snell & Associates, which provides training and support to thermographers worldwide.
Over 150 infrared imaging systems are now used in U.S. weatherization programs and their numbers are growing. Selecting the right equipment can be confusing. What do you need to know before making a purchase?
Many of us in the energy conservation business are techno-junkies. We're fascinated by new gadgets. Like Toad in Wind in the Willows, who was left delirious with envy when his wonderful newer horse cart was run off the road by an even newer and more wonderful horse-less car, we find new tools seductively attractive. Infrared thermography allows us, with true Buck Rogers finesse, to practically see through walls.
While thermography is still primarily used for crew and auditor training, a second wave of use is finally starting to reach weatherization crews. The investment required for equipment is significant, but returns from a well-designed training program can be worth many times that in increased productivity and effectiveness.
Thermography is especially powerful when it's used on every single building in a program. Energy auditors can work more quickly to locate and prioritize air leakage, and to provide a framing map for sidewall crews. The amount of wall space filled by sidewall insulation can approach 90% on the first pass, with 100% coverage guaranteed by a final follow-up inspection. With thermography, crew efficiency increases and job time decreases.
The cost of an imaging system (upwards of $15-25,000), when shared by several agencies or crews, looks more reasonable on a per inspection basis. One option is for state entities to purchase the cameras, and then loan or lease them out. However, careful attention should be given to the logistical details of sharing, or the savings can evaporate.
Programs Come Before Equipment
Before imaging equipment is purchased, there's a vital first step: One must set up a program where the equipment will be used effectively. Too often programs neglect this important first step with costly consequences. Personnel training and support should be emphasized as much as hardware. Thermographers need to be trained, the scope of services provided must be defined, and a framework with which to integrate them into the program has to be established. A system must be developed for reporting the thermal findings.
Then comes the fun stuff--selecting and buying an imaging system. A whole, integrated system includes an electronic infrared imaging camera, a power source, and a recording device. Image processing or report generation software is another option, but may be beyond one's everyday needs. There's good news and bad news: The selection process can be confusing, but there are some excellent choices out there. Many people now have experience using the equipment and can share it.
Based on years of teaching and consulting, I've found the following to be the most important and frequently asked questions about selecting imaging equipment:
Will any imaging system do the job, or am I limited in my choice? Yes, they all work but some do the job better than others--for various reasons. Currently the market seems to be driven by the manufacturers and they are providing many bells and whistles in an attempt to get a sale. Bells and whistles (color imaging, for instance) are OK, but a simple, solid system will get the job done.
Do I even need an imaging system, or can I buy an inexpensive radiation thermometer? Radiation thermometers sense over a very small area and convert the radiant energy into an apparent temperature value. Using a typical instrument with a 30:1 field of view and standing 15 ft. from a wall, one will see a 6 in. circle. That is too large to monitor insulation quality or easily detect voids. Unfortunately one also cannot actually see the thermal pattern, so it is often point and guess. To inspect an entire house with such a device is impossible.
OK, if I'm going to buy an imaging system, should I buy a scanner or a pyroelectric vidicon (PEV) tube camera? (And by the way, what's the difference?) Both work well for weatherization applications. Each type of system detects invisible infrared radiation and visually portrays thermal differences as small a 0.5deg.F. Scanners employ a high-speed (15,000 rpm) rotating mirror to distribute the incoming radiation beam over a super-cooled detector set. Scanners used to be cooled with cumbersome argon gas or liquid nitrogen but are now cooled electrically using batteries.
A PEV camera produces a high resolution image using a radiation-sensitive target crystal mounted in an electronic tube. Tube technology, despite what the scanner salesperson may say, is rugged. Many 15-year-old systems are still working well. PEVs are also less expensive than scanners. They must, however, be panned (moved slightly) to sustain an image. Because this is perceived by some as a problem (and occasionally it is) PEV manufacturers now offer various forms of low speed synchronous choppers that perform this function in the camera. This allows them, like scanners, to be used as staring systems.
Do I need a staring camera, or can I get good results with a panning system? For 99% of routine inspection work, a panning camera will suffice. It may take a little longer to become skilled with a panning camera, and the audience may wonder why the operator can't hold it still, but PEVs work well for these applications, as demonstrated by the fact that over 95% of the systems used in the Weatherization Assistance Program are panning cameras. If the primary application is for auditor or crew training, some extra consideration should be given to a staring system, either PEV or scanning, because a non-panned image may be easier for novices to understand.
Are some systems more thermally sensitive than others? Yes, but in the real world it's usually an insignificant difference. The standard tests for measuring sensitivity are not particularly helpful because they do not apply equally to scanners and PEVs. Also individual imaging systems can vary from the specifications. More importantly, other variables--such as field conditions or the thermographer's temperament and eyesight--are too subjective to measure. It is misleading to test two or more imaging systems under the same conditions unless they're actually sitting side by side. Even then, small differences in sensitivity are not as important as other considerations.
What does long wave or short wave mean, and is it important? Imaging systems are designed to see thermal radiation. Like visible light, this is a form of electromagnetic radiation with a wavelength of 2-15 microns or millionths of a meter. Two primary wave bands are detected using imaging systems: long wave (8-15 microns) or short wave (2-6 microns). Both work well, although objects at ambient earth temperatures tend to radiate more energy in the long wave bands. When working outside, images from short wave systems are more adversely affected by the sun. For these two reasons, long wave systems may present a slight advantage, but like sensitivity, this factor alone is usually not persuasive for weatherization applications.
Is there any imaging system I can use year round for weatherization work? Not really, but with experience and timing one can work much (if not most) of the year depending on the location. As a rule of thumb, with roughly a 20deg.F indoor to outdoor temperature difference in either direction, one will see studs and missing insulation. These conduction patterns can be enhanced with a blower door or by changing the indoor temperature. With a temperature difference of just a few degrees and a blower door, one can see the results of air leakage. Daytime imaging from the outside on sunny days is generally not possible, but most work will be from inside anyway. The key here is to schedule in some flexibility to accommodate weather conditions during the marginal parts of the inspection season.
Some imaging systems can measure a temperature value and display it on the screen. Is this feature necessary? No, a high quality thermal image that displays relative temperature differences in black, white, and shades of gray is what counts.
What about color imaging systems? Again, not necessary. While colorized images can be useful, persuasive, and sexy, the cameras are more complicated to operate, the imagery is occasionally confusing, and the systems are always more expensive than required for basic weatherization work. Thermographers usually wind up using black and white anyway because it's simpler.
Is more expensive better? Not necessarily. In fact, some of the low end systems offer incredible value for weatherization applications. Many of the features found on high end systems are not necessary. A system is not necessarily a dog just because it wasn't high priced!
How can I ever justify this kind of expense? First, it is not an expense--it's an investment. A program's plan should include a timeline for getting a return on that investment. Although it may be difficult to quantify immediately, there will be a return in many areas including reduction of fraud, increased productivity, improved work performance, and better staff training. It's one of the best investments that can be made in a program.
What features are important to consider in a system? Total weight of the system in the field is important. (None of them are light enough!) Cart mounted systems can be extremely difficult to get in and around a house. Those that are harness mounted or balance on a person's shoulder can be difficult or even hazardous to use in tight areas like an attic. For maximum comfort one should choose a light-weight, portable system offering ergonomic flexibility with features such as a shoulder strap, rotating viewfinder, or functional use in several carrying positions. It's best to try the equipment before you buy it--in various real life situations for more than a five minute sales demo!
Is the lens length important? Generally it will be much easier to work in buildings with a wide angle lens (approximately 25 mm focal length). A lens up to 50 mm can be used although it is really too telephoto for comfortable work inside a house. On some systems lenses can be quickly changed. Beware of single element lenses. The price may be attractive, but the imagery is generally not good.
How can I document the thermal image? The most cost effective documentation method--and the most useful--is videotape. Most imaging systems connect directly to a conventional portable video recorder, either VHS or 8 mm format. Some systems are elegantly simple, while others are a jumble of cables and buttons. A nice option is a black and white video printer (such as the Sony UP-860) which delivers a high quality thermal print in a few seconds for about a dime. This is great for permanent documentation. Some more expensive imaging systems are capable of storing the image on an on-board computer disc. This is a nice feature, but most of us will never need it.
What about the systems that display both the thermal image and a corresponding visual image? Several companies make such systems. They are definitely worth looking at, particularly if they will primary be used for auditor or crew training. Those who aren't familiar with interpreting a thermal image find the visual correlation helpful. However, not all systems offer the same features. Some simply alternate between visual and thermal, while others have a picture within a picture. The most elegant systems offer full fade from one to the other.
How reliable is imaging equipment? Like most electronic equipment, imagers are generally reliable. However, all equipment needs service or repair at some point. A better question then is: How good is service after the sale? Fellow energy conservation professionals may be able to offer advice here. Many will have horror stories. It's a good idea to ask the sales person for the names of conservation professionals who have purchased the equipment and then call them to see what the turnaround time on repairs is. Also one should read the fine print on the warranty. The whole system may be warranted, or one may have to pay extra for an extended warranty that others include in the base price. Finally, if flexibility is important, it may be possible to add new features or increased capabilities later.
What kind of training do I need to use an imaging system? A little training goes a long way. Typically three days of professional training can replace six months of experimenting. The equipment is not difficult to use, but the interpretation and gaining operational finesse can be. Honing thermal thinking skills comes with training and experience. Companies often offer hands on training. Follow-up support can quickly help iron out the inevitable problems that crop up in a developing program.
What about certification? No one needs to be certified to use this equipment. Although several private companies certify thermographers, their programs are little more than a marketing ploy. There are currently no nationally recognized programs to certify that thermographers are actually qualified. The American Society for Nondestructive Testing (ASNT) has recently embarked upon a program to do this, but even when it is fully operational in fall 1993, it may be more complicated than required for weatherization (see New Guidelines for Thermograpers, HE Sept/Oct '92, p.11). If formal qualification is important, an in-house program based on specialized needs and personnel may be appropriate.
Are my first impressions about an imaging system important? Yes and no. Certain systems seem to appeal to certain folks immediately. However, people should make certain they can use any available system because all will do the job. If after that exercise they still have a favorite, so be it. At least they know their preference is just that--a subjective opinion.
What's the next step?
The next step is to sit down and sketch out a simple plan of action. More and more conservation programs will be using thermography in the next few years. Others in the business are already using thermography or are just getting started. It may be a good idea to visit folks who already use the equipment for weatherization. It's wise to take the time to research available equipment. And again, try it before you buy it! Most of all, a program structure must allow full and effective use of the equipment. n
What is Infrared Thermography?
Buildings have thermal signatures that, if we learn how to interpret them, can help us better understand how a building is performing. Signatures can be seen in the snow and ice melt pattern on a roof or, when conditions are right, a condensation pattern on a sidewall. Unfortunately signatures are usually not visible. Infrared radiation is similar to visible light, but is invisible to human eyes until the radiant object reaches incandescence at about 1,400deg.F. Infrared sensitive 35 mm film can detect radiation at temperatures cooler than incandescence, somewhere around 480deg.F, still inappropriate for building energy analysis.
Only electronic infrared detectors, based either in a non-contact radiation thermometer or an imaging system, give us the thermal sensitivity needed for building analysis. Using such systems we can see thermal signatures with as little as 0.5deg.F temperature difference by detecting the invisible infrared radiation, or heat energy, they give off. All objects above absolute zero radiate thermal energy. Generally, the warmer an object, the more radiation it gives off. Thus, from the outside of a heated building a stud will typically appear warmer than an insulated cavity and a void will appear even warmer than the stud. From inside a heated building patterns will reverse. Training and experience are needed to accurately interpret thermal images.
Primary vendors of thermal imaging equipment:
Agema Infrared Systems
Insight Vision Systems
ISI Group, Inc.
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