Not long ago, infrared cameras were 30-pound, cart-mounted affairs that required hourly infusions of liquid nitrogen to keep them running and a good imagination to interpret the resulting images. Prices exceeded $60,000. How quickly all that has changed!
In recent years, infrared has become widely used as a fast, reliable, nondestructive testing technique for diagnostic and quality-control inspections, especially moisture detection. Unlike other moisture-detection techniques, 100 percent of a roof typically can be inspected with infrared devices.
Since the introduction of infrared cameras, prices have dropped significantly; image quality has improved; and cameras—now weighing about 2 pounds—finally are being designed with end users in mind. As a result, infrared thermography has become a popular tool for roof system maintenance and building diagnostics.
However, the confusion many inevitably will face when trying to make an intelligent decision about investments in infrared technology still can pose a problem. With current prices ranging from $6,000 to $60,000, it is important to carefully evaluate the investment and the return you want to make. Fortunately, you do not necessarily need a top-of-the-line system to perform great work.
Important features
Examining specifications for infrared cameras can be confusing, especially when some salespeople may use specifications to promote their products. Although some specifications—such as thermal sensitivity and detector size—may be useful when evaluating performance, in the end, how a camera functions and feels while you perform your work is the most important determining factor. Therefore, it is essential to test cameras with field conditions before you buy. Many manufacturers will allow you to borrow or rent a camera to try before purchase.
When you begin your search for a camera, keep the following information in mind.
Thermal features
Thermal sensitivity of cameras no longer is the big issue it once was because current detectors typically can resolve temperature differences of less than half a degree Fahrenheit; this means they can see the difference between two objects that are half a degree different, such as 70 F and 70.5 F. For roofing work, sensitivity is important; more sensitivity generally is better, but any camera on the current market should be sufficient. Several suppliers are selecting their most sensitive detectors—as low as 30 millikelvins (mK)—for cameras they're marketing for roofing and building professionals. For building work, in particular, this extra sensitivity means you can work more often and see more thermal detail.
A camera's thermal detector, which produces an electrical response when infrared radiation is focused on it, generally is one of two types: amorphous silicon (a-Si) or vanadium oxide (VOx). Although there are differences in performance, either can perform well for roof system diagnostic work.
Coolants are not necessary with either, which is a big relief for anyone who used such systems in the past. Both types of detectors sense longwave infrared radiation—as is the case with nearly all systems on the market. Unfortunately, there may be some limitations for using longwave cameras when inspecting some unballasted single-ply roof systems. These membranes tend to be slightly reflective, which can make underlying thermal signatures related to wet insulation more difficult to observe compared with results from older and now unavailable shortwave technology.
Lenses
An infrared camera lens will have a fixed field of view (FOV) instead of a zoom lens. A wide-angle lens is most useful on a roof, but a normal lens also can work well.
The FOV for a lens is specified in degrees for horizontal and vertical views. Typical for roofing work is a normal lens with an FOV of about 20 degrees vertical by 20 degrees horizontal or wide-angle lens with an FOV of 40 degrees vertical by 40 degrees horizontal. Either will perform well.
Infrared systems currently come with three choices for focusing: manual, motor assisted or fixed focus. Although new users often develop a preference for one focus type, any of them can work well and produce images in sharp focus. Interestingly, each type also has its own set of limitations though none are particularly problematic.
The depth of field for new lenses typically is narrow. Regardless of the type of focus mechanism, focus is critical to getting good images and analyses. Therefore, care must be taken to learn to adjust and use a camera properly.
Detector arrays
Currently, the single greatest differentiator among cameras probably is the size of the detector array. All systems, as is the case with a digital visual camera, now use a matrix of small detectors that comprise the focal plane array assembly (for this reason, they are termed focal plane array systems or FPAs).
Two array sizes commonly are used—160 x 120 detector elements (19,200 detectors) and 320 x 240 detector elements (76,800 detectors). Larger arrays typically cost slightly more. The array size and lens used also determine the spatial resolution—or detail seen—at any given distance. Obviously, infrared camera resolution is not what we are used to seeing in visual digital cameras, but it is adequate for roof diagnostic work.
Early 160 x 120 systems often were quite pixelated compared with 320 x 240 arrays. However, better array designs and image processing now result in good image quality from either array size. Recently, the price differences between these two array sizes became much less than in the past. Arrays smaller than 160 x 120, though good for many predictive maintenance applications, typically are not useful on a roof and should be avoided.
Even more important than array size is image quality. This is somewhat subjective and depends on the array size, lens, processing electronics and display. My advice is to try a camera on a roof before you buy it.
In addition, spatial resolution usually is specified as an angle (in milliradians [mRad]) called instantaneous field of view (IFOV). As the IFOV specification increases, spatial resolution decreases. Industry standards suggest an IFOV of 3.8 mRad or less is needed for rooftop inspections.
Images
Images can be frozen and stored by almost all cameras either on removable media, such as a PC card, or in flash memory. When considering a camera, think realistically about how many images you need to store before downloading to a computer; typically, several hundred or fewer will be sufficient. Downloads through a card reader or a USB or FireWire port are, for the most part, fast and painless once the procedure is established.
Most, but not all, infrared cameras can output a live analog video signal that can be played on a monitor or recorded on a VCR or camcorder if it has video input. This can provide useful means of documenting a roof moisture inspection.
Basic adjustments on all infrared systems are similar; the two most important are thermal level and span—essentially brightness and contrast. Controls for these and other functions may be buttons, pull-down menus or a combination of the two. Some have automatic image adjustment modes that can work well though it also may be useful to be able to adjust the image manually when needed. Although the basic functionality of some camera designs is better than others, it should be possible to quickly become proficient at adjusting any camera to get great images.
The "dynamic range" of a stored image is an important specification to consider. A large dynamic range (12 bit or 14 bit) means image thermal level and span can be manipulated after capture. Some cameras store data in an 8-bit format that cannot be adjusted after it is stored. Although such post-processing is not necessary for all images, it can be useful—or even essential—at times. All else being equal, choose a 12- or 14-bit system or greater.
Many cameras on the market display the image on generously sized LCD screens. These allow you to see the image easily and, when you want to, quickly show it to others who may be looking over your shoulder. However, some displays are fixed in their viewing angle and cannot be tilted. Because this can be a significant limitation, I strongly recommend testing the camera on a roof before purchase.
At least one camera supplier sells a fairly high-priced, optional "heads-up display" that, though it looks like a solution, for reasons of cost, safety and complexity should be considered only after careful trial and evaluation. This display also can be purchased directly from the display manufacturer at considerable savings.
Many who are considering purchasing an infrared system will have a preference as to whether the image is color or grayscale. The truth is, depending on a number of complex, interactive factors and subjective response of the end user, some systems work better in one or the other. Generally, thermographers will work either in grayscale or a monochromatic color palette. Reports may look more impressive in a full rainbow of colors, but it is extremely difficult to work with that palette. Most software programs allow for the image palette to be changed as the report is being written. As you evaluate various systems, try them in all palettes to see which provides the best results.
Other features
The frame rate of most cameras either is 30 frames or 60 frames per second, but several cameras have slower frame rates. Despite the fact that some salespeople may try to convince you otherwise, any of these rates will perform well for roof applications with little, if any, perceivable loss of quality.
In addition, batteries have improved. Proprietary batteries now tend to be Ni metal hydride or lithium ion; both will perform well even if they may be somewhat costly. Other systems use batteries that are AA, regular or rechargeable.
Expect that any system may require repairs or service at some point. An annual budget of 3 percent to 5 percent of the purchase price should be more than enough. Suppliers' recommended annual maintenance typically is not necessary, so save your money. Most systems are quite rugged but should be treated with the same respect due any costly piece of electronics. Regardless of the system, you'll want to protect it from dust and dampness. A thin-film plastic bag works well because it is quite transparent to infrared radiation.
Many infrared cameras are designed to allow a user to make radiometric temperature measurements. This add-on feature can be useful for many thermography applications, but it is not necessary for roofing work.
Zoom lenses are not available though many infrared cameras have an electronic zoom capability that simply magnifies the image. This can be a useful feature but does not result in any improvement of resolution and typically would not be a compelling reason to buy one camera instead of another.
Several infrared systems also incorporate digital visual image cameras. The primary benefit of this feature is the infrared and visual image files are mated electronically. Unfortunately, the image quality of these integrated visual cameras often is poor, especially on a roof at night.
With some systems, it is possible to make in-camera voice or text annotation electronically tied to the image file. This data then automatically can be inserted into the report document. Both features can be useful, but the lack of either should not be a deal breaker.
Basic reporting software packages typically are included to allow for image downloading and copying into various nonproprietary formats for use in a standard word-processing document. But beware: Some companies sell report software that can add considerably to the purchase price while providing few features roofing contractors need.
Affordability
With all the features available, it is easy to become infatuated with new infrared systems and forget your budget. Some manufacturers cleverly have priced their systems to make it easy to add on a long list of features—some unnecessary and all at a price. Make sure you look at the complete package price, including lenses, reporting and/or analysis software, data storage cards, LCD view screens, batteries and training.
Even though useable cameras now are available for far less than in the past, you still should ask, "Can I really afford an infrared camera?"
Consider the fact that a new infrared system, if well-cared for, should last 10 years. Even with a modest budget for maintenance, the cost of a $10,000 system will be about $1,500 per year. If the system is used on one roof each week all year, the cost for the infrared camera only is $30 per inspection.
Given the entire annual investment could be recouped if the information provided helps you more accurately bid a job, get a job or avoid a callback, the payback is as good as it gets.
Training
Current infrared systems are easy to use. Anyone who can use a conventional video camcorder or digital still camera quickly can learn to get good thermal images. However, accurate interpretation of the images requires not only good camera skills but also an in-depth understanding of heat transfer, radiation physics, conditions for inspections, roof construction and basic roof diagnostics.
Roofs are complex structures. And inspections often are performed when conditions are less than ideal. You must understand how that affects what you are seeing—and not seeing—in the thermal image. Without a solid foundation, you can expect to make mistakes—some of which could be costly.
Good training options are available, but make sure those you choose specifically address the camera you have, as well as your specific application needs. For groups of five people to 10 people, consider training at your site focused on your needs and equipment.
Be wary, too, of training companies that purport to "certify" those who purchase their training products. Unfortunately, much of this is just marketing hype. For better or worse, there are no meaningful industry requirements for roof thermography professionals. The true keys to proving you are qualified are getting appropriate training and having documented experience.
View of the future
The current choices for roofing contractors wanting an infrared system costing less than $15,000 are astounding. It even is possible to purchase a used system online (though be extremely cautious if you go this route). Although prices may continue to drop somewhat, waiting another year may only be false economy when the potential for immediate payback is considered.
Although there are many confusing and complex choices, the good news is it is hard to go wrong with most available systems. Still, do your homework; try several systems before buying an infrared camera.
John Snell is president of Snell Infrared, Montpelier, Vt.
Roof moisture survey
During the 1970s, thermographers learned when conditions were right it was possible to detect wet insulation under a roof membrane—a task previously accomplished mainly with the "educated toe" technique. Once water is trapped in insulation, most roof systems never dry out, and this contributes significantly to their degradation.
Conditions for inspections are critical to success, especially those related to weather. A roof surface, including any ballast or flood coat, must be dry. Two good options exist for inspections—summer and winter surveys.
A summer survey depends on the difference in thermal capacitance of wet and dry insulations. The sun heats a roof, and then the roof cools because of rapid radiational loss to a clear night sky. At some point during the evening, the dry insulation cools below the temperature of the wet insulation and remains detectable for a time period that varies with conditions. When there is strong sun, good cooling and little wind, wet absorbent insulation easily can remain detectable long into the night.
A winter survey—which can be conducted anytime during the year—depends on the difference in conductivities of wet and dry insulations. When the inside of a building is warmer than the air outside, wet insulation transfers more heat out of the building than dry insulation. As a result, any area of wet insulation appears warmer on the roof. As with the summer survey, a dry roof is essential, and the same ideal weather conditions again serve to enhance the thermal signatures that often can be detected even in the early morning.
Circumstances can make infrared technology difficult to use effectively. First and foremost, inspections highly depend on the weather. Lack of sun, cloudy night skies, winds or heavy dew can render wet insulation undetectable. Although any quantity of trapped moisture in any roof system can be damaging, the technique certainly works best for insulations that are more absorbent, such as wood fiber, mineral board or fiberglass. Foam board insulations can be inspected but typically only after they age a few years.
Inspections also can be conducted effectively from the air using either a helicopter or fixed-wing craft. The same weather conditions apply, but a higher-resolution camera typically is used. For large or complex roofs or multiple buildings in a location, aerial inspections can be cost-effective, even when a follow-up rooftop inspection may be required.
Additionally, heavily ballasted single-ply roofs and systems that have been reroofed with a second layer of insulation over the first can be challenging because of their complex thermal relationships. Inverted membrane roofs typically are not inspected at all using thermography. Perhaps the most insidious limitation is the fact that some single-ply membranes, when viewed directly, are quite reflective of the longwave radiation the cameras detect. This makes underlying thermal signatures related to wet insulation more difficult to observe. However, all these limitations pale in comparison to being able to locate wet insulation on many other types of roof systems to within an inch or two.
As with any technology, it is vital to understand how to use an infrared camera; how conditions can affect the survey; and how to interpret the images. A few days of formal training combined with support and experience make all the difference between success and failure. One standard for conducting roof moisture inspections exists: ASTM C1153-97(2003)e1, "Standard Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging." It addresses summer and winter surveys, as well as rooftop and aerial inspections, and is available at www.astm.org.
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