Thermal Imager Selection: Buy Wisely
EP Editorial Staff | January 21, 2019
Make sure the chosen equipment is capable of providing clear images for your particular applications.
By R. James Seffrin, Infraspection Institute
Purchasing a thermal imager can be a daunting task for a seasoned thermographer, and especially difficult for the less experienced. Because the success of an infrared program is dependent upon the chosen test equipment, it’s imperative to select proper equipment. Knowing how to correctly specify and choose such equipment can help to avoid a costly purchasing mistake, as well as help ensure the success of your program.
As infrared thermography has gained wide acceptance, the task of selecting imagers has become more difficult. Thermographers now have access to a broad range of equipment from a record number of manufacturers. With the advent of lower-cost microbolometer imagers, there are more choices than ever before.
Procuring an imager is a challenge for many reasons:
• Initial purchase price can easily run into several thousand dollars.
• No imager is capable of performing all imaging applications.
• Imager performance varies widely.
• Performance specifications are not always available or comparable.
• Making an incorrect purchase can be costly.
An effective selection/purchasing process includes assessing your organization’s present and future needs, obtaining and comparing manufacturer specifications, and taking time to thoroughly evaluate the imager in the workplace. The following step-by-step approach is designed to guide you through the selection process, from initial consideration to final decision. In general, these steps are organized with the most important considerations listed first.
1. DETERMINE SPECTRAL RESPONSE.
Prior to selecting an imager, it’s crucial to determine the application(s) for which it will be used. Whenever possible, consider potential future applications. One of the most important performance criteria for infrared equipment is spectral response.
Manufacturers of modern thermal imagers generally select one of two infrared wavebands in which equipment will operate. Instruments that operate in the near infrared, also called shortwave, have spectral responses between 2 and 5.6 microns. Those that operate in the far infrared, also called longwave, have spectral responses between 8 and 14 microns.
Spectral response is a permanent characteristic of the equipment and cannot be changed. Selecting equipment with proper spectral response is extremely important given the fact that many applications are wavelength-specific. Choosing equipment with an incorrect spectral response may preclude the ability to collect accurate data. Table I shows recommended spectral responses for several common applications.
2. EVALUATE OBJECTIVE SPECIFICATIONS.
Objective specifications describe performance characteristics for a specific model line. These specifications are not changeable and will, in many cases, determine whether an imager can be used to successfully accomplish an inspection. Objective specifications are usually available from the manufacturer’s product sheets.
To best compare the objective specs of thermal imagers, refer to the manufacturer’s published data for the subject imager(s) and develop a spreadsheet noting as many specification values as possible. Upon completion, this spreadsheet will allow relevant comparisons between/among any imagers under consideration. Some of the most important objective specifications are listed in Table II. You may wish to add others to this list.
3 . DETERMINE PERFORMANCE SPECS.
Performance specifications refer to how an imager operates in the field, as well as the history of the subject model line. While historical information is usually available from the manufacturer, performance history is best obtained from references provided by others who own the subject equipment. Your manufacturer’s representative should be willing to provide the names of other users to contact for equipment reference. Some criteria to consider, along with relevant notes, include:
• length of time the subject imager has been in production (It may be wise to delay purchasing a recently introduced model until after it has been proven to be reliable in similar installations.)
• references from actual users of the subject imager
• insight from physically trying an imager before purchase (This can be accomplished by obtaining a loaner unit or renting the desired model. Manufacturers may credit short-term rental fees toward purchase price. Be certain to thoroughly try the
unit under the exact conditions you will encounter in your job.)
• software options available for the camera (Be certain that selected software is capable of performing the desired analysis.)
4. OBTAIN SERVICE/WARRANTY INFORMATION
In general, service and parts can only be obtained from the manufacturer of the subject equipment. Because calibration procedures are proprietary, calibration adjustments can only be performed by the manufacturer. Since service is not available from third parties, the success of your infrared program can be greatly affected by the ability of a manufacturer to service and support your infrared equipment. Prior to purchase, consider the following:
• manufacturer’s experience in building and servicing infrared equipment and capability to provide future service
• recommended service or calibration frequency and anticipated costs
• expected delivery time for any required repairs
• length of warranty and covered parts
• location of equipment service centers
• loaner/rental availability during repair.
5. EVALUATE FOR SUBJECTIVE CHARACTERISTICS.
Subjective characteristics describe how the imager feels to the operator. Since you may spend considerable time with your chosen imager, comfort will become increasingly important. When evaluating equipment, consider the following:
• Are imager controls easy to use and understand?
• Is the equipment designed to be rugged and durable?
• Is the imager ergonomically comfortable?
• Will imager size or weight present problems for long-term usage?
• Is the display clear and free of noise and distortion? Although this is one of the most important considerations when selecting an imager, there’s no methodology for assigning an objective value to image quality.
• Is the imager display adequate in size and compatible with operator safety glasses or other personal-protective equipment, including hard hats, face shields, hoods, and respirators.
• Is the imager display viewable in direct sunlight?
6. UNDERSTAND EQUIPMENT COST.
From a performance standpoint, cost should be the least of one’s considerations when purchasing equipment. Equipment that is incapable of accomplishing a task is no bargain at any price. For some infrared cameras, cost may be negotiable, as may be items such as extended warranty and service contracts. The good news is that prices for some thermal imagers now begin at less than $1,000. Although prices increase with added features and resolution, there are many fully featured, high-quality thermal imagers currently available for less than $10,000.
Once you’ve made your final selection and completed the purchase, be sure to obtain quality certification training for the thermographers who will be using the imager. For new users, training should include infrared theory and heat-transfer concepts, equipment operation, image capture and analysis, standards compliance, applications-specific inspection techniques, documentation of findings, and temperature-measurement techniques. EP
Imager Resolution and the Great Pixel Debate
Resolution is one of the most important objective specifications for any thermal-imaging system. Although pixel count is frequently offered as a measure of image quality, it’s only one of many factors that affect imager resolution.
The focal-plane-array (FPA) detector assemblies used in modern infrared imagers are made up of several tiny, picture elements or pixels. Each pixel is a discrete infrared detector that collects thermal data. Individual pixels are arranged to form an array that ultimately allows the imager to produce a thermal image and associated temperature values.
FPA detectors are commonly specified according to pixel count and ratio. Typical detector sizes for industrial imagers range from 160 x120 to 640 x 480 pixels; some detectors may have more or less. To determine the total pixel count for a detector, the horizontal and vertical values are multiplied together.
Imager manufacturers often cite pixel count as a measure of imager resolution. Imager sales are won and lost as entire ad campaigns focus heavily on this single objective specification. Actually determining resolution is not that simple. Although resolution generally increases with the number of pixels, there are several other factors that influence image clarity or resolution. These include, but are not limited to: pixel viewing angle, imager optics, signal-to-noise ratio and the imager’s display screen.
When evaluating an imager for resolution, physically try the imager under actual working conditions. Imagers that produce clear images should be sufficient for the task, regardless of pixel count.
Frequent contributor Jim Seffrin is the Director of Infraspection Institute, Burlington, NJ, and a practicing thermographer with more than 30 years of experience in the field. For more information on a range of thermal-imaging topics, along with training and certification options, email firstname.lastname@example.org or visit infraspection.com.