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Noncontact Temperature


Online infrared thermography training by distance learning.

The uses of noncontact temperature sensors are many; the understanding of their use is, in general, relatively poor. Part of that complication is often the need to deal with emissivity, or more precisely with spectral emissivity.

In many industrial plants noncontact sensors are not yet standardized to the extent that thermocouples and RTDs are. In spite of this, there are numerous showcase uses of them and they more than pay their way in process plants such as steel, glass, ceramics, forging, heat treating, plastics, baby diapers and semiconductor operations, to name just a few.

More recently the medical world has adopted the IR ear thermometer (it has its own set of standards) that is basically a single waveband radiation thermometer.

However, we believe that limited standardization is hampering wider use in process and related areas. Standards have been developed that aid the user in specifying, buying and maintaining such devices, but they are not widely used. More training and education of the user community is an obvious need that until now has been provided mostly by equipment vendors.

The advent of the Focal Plane Array, a significant improvement in Thermal Imaging, is drawing the formerly seperate areas of Thermal Imaging and noncontact spot temperature measurement closer together. It is likely that the active training community developed to support Thermal Imagers will begin to provide more organized, in depth training for all infrared temperature sensors, in addition to imagers for Thermographers.

Just to be sure we are addressing the subject you are seeking, please be aware that these devices are called by a bewildering variety of names. They all work, or are based on the same law of physics, Planck's Law of the thermal emission of radiation.

Here's just a few of the names used in current technical and popular literature (never mind the unprintable names these devices are often called when the temperatures they report defy all logic-that happens a lot-see our E-missivity Trail section for a partial understanding of this latter phenomenon): ir thermometer, radiation thermometer, ir pyrometer, infrared thermometer, spot thermometer, spot radiometer (our favorite technical misnomer), line scanner, radiation pyrometer, single waveband pyrometer, dual waveband pyrometer, ratio pyrometer, 2 color thermometer, 2 colour thermometer, two color thermometer, two colour pyrometer, radiometer, spectral radiometer, IR thermocouple, total radiation pyrometer, fiber optic pyrometer, disappearing filament pyrometer, quantitative thermal imager, dfp, optical pyro, multiwavelength pyrometer, and on and on.

It seems that whenever a new technical or marketing person comes into the "business" a new product name is coined either out of ignorance of the device history or as an effort to be technically "pure" (whatever that is) or as a way to differentiate their product from others. The names used here, as far as we know, do not include the trademarked names or commonly used product line names. There isn't enough room on this page for all of them! We shall try to follow the most often used terminology, that fostered in the excellent work of DeWitt & Nutter in their 1988 book "Theory & Practice of Radiation Thermometery". The complete citation can be found on the references page.

Good luck and best wishes. If you have some interesting success, let us know and we'll help you share that with others who visit these pages.

Index of Sensors

  1. Radiation Thermometers

    Includes Pyrometers, Infrared Thermal Imaging Cameras (with temperature measurement capability), line-measuring thermometers (most of the time they're called line scanners-but all don't scan) and infrared radiation thermometers, or, perhaps the most-misused term, spot radiometers (Note: radiometers are calibrated in units of power, such as microwatts, watts, kilowatts, temperature measurement devices are calibrated in units of temperature).
    The noncontact temperature sensors with many names and many shapes, sizes, prices and capabilities are well and flourishing. Based on Planck's Law of the thermal emission of electromagnetic radiation; many industries could not produce goods as efficiently or quickly were it not for them.

    More recently the medical world has adopted the IR ear thermometer (it has its own set of standards) that is at heart a single waveband radiation thermometer

    The majority of devices in use are single waveband thermometers (they measure a portion of the received thermal radiation in a single waveband, or portion of the infrared part of the electromagnetic spectrum). However, the number of ratio thermometers (two color pyrometers) on the market has grown considerably in the past ten years, or so.

    Single waveband radiation thermometers are usually designed to measure the true temperature when they receive all the radiation from an object that has an emissivity effectively of 1.0, or under blackbody conditions. This occurs most often when the devices are being calibrated, since they are calibrated under simulated blackbody conditions. The accuracy of the simulation bears much on the uncertainty of the calibration of the device.

    When these devices are used under effectively blackbody conditions, and their emissivity correction is set at 1.0, they can measure very accurately, indeed. Few people seem to appreciate that blackbody conditions occur regularly in many process applications, such as in portions of furnaces that are close to thermal equilibrum, such as glass melters & forehearths, steel mill soaking furnace zones or when a radiation thermometer is correctly sighted into a closed isothermal cavity, such as a miniature cavity on the end of a sapphire light pipe or quartz fiber optic.

  2. Thermal Imagers

    Quantitative thermal imagers are a special sub-class of these thermal imaging devices, they measure radiation temperature distributions as well as shown a false color thermal image. They are basically single waveband radiation thermometers that measure a two dimensional space instead of just radiation from a single spot. These are used so widely that they are described in more detail in a seperate section of this site that is all about thermography or thermal imaging.
  3. Emissivity

    The topic of emissivity is also a broad and complex one. One cannot mention radiation thermometry without mentioning emissivity.

    Some fundamental understanding of it is essential to successful use and application of any temperature measuring radiation thermometer. It might be limited to just the details of one specific application; that's enough in many cases. It is not magic, it is not unknowable, otherwise all advanced thermal processes in the world would be running at lower efficiencies than they are.

    There are many people who underated the subject and can explain it. This is our part in that educational direction. We started a section on this site devoted to helping people better understand some of the basics of the subject from an applications perspective. Pardon our cynicism, but the section was initiated after this site author attended a "Seminar" on Infrared Thermometry a few years ago. The topic of emissivity came up many times and it was clear that the company representative giving the presentation had little to no understanding of the subject, unless the purpose of the talk was to confuse matters. Most people came away, we believe, with a poorer understanding of the subject at the end than at the beginning. It's sad when those apparently helping do not do their job competently.

  4. Ratio Thermometers

    The ratio pyrometer, ratio thermometer or two color pyrometers (or two colour thermometers, if you prefer) are unique devices, touted imprecisely by all too many vendor marketing people as being emissivity independent when they are nothing of the sort. They measure in two separate wavebands and internally create the ratio of signals (usually that of the shorter waveband in the numerator to avoid the complication of dividing by zero-because usually the shorter waveband signal drops out as a function of received radiation, before the longer waveband signal).

    The ratio of radiances in two wavebands has been shown to be a function of temperature and a function of the ratio of the spectral emissivity in the two wavebands as well (So much for the emissivity independence, guys!) When measuring objects that have an emissivity ratio of 1.0, they can have their emissivity ratio correction set to 1.0, just like a single waveband thermometer does when measuring under blackbody conditions; in this latter case one is said to be measuring under graybody (greybody) conditions.

  5. Optical Pyrometers

    The old and trusty Optical Pyrometer not only refuses to go away, there's even a new version on the market. Check out our page and learn about the two USA companies that still make these devices.(Just between us: These things are really just another variation of the Planck's Law-based Radiation Thermometers described above, albeit one of the tried and accepted versions..But these darn things garnered so much fame and fans over the years that some people just won't settle for anything else. No matter that the technology can and does produce better devices, but snake-oil salesmen who can't produce better results with their new devices foster this sort of conservatism on the part of an undereducated user community.)

  6. Fiber Optic Temperature Sensors

    There's enough uses and varieties of fiber optic-related temperature sensors these days to require a separate hyper-link category for them, To complicate matters a little more, there really are two groups of them contact and noncontact fiber optic thermometers. They're all covered on this one page. One of the fabulous uses for these thermometers is to actually provide a temperature limit signal for operating jet engines in flying aircraft. It's not all that new, either. Rolls-Royce engines in some European military planes have been flying for about 20 years using this technology.

  7. Other Temperature Sensors

    Temperature measurement occasions often seem to stretch the capabilities of existing sensors and inventive minds continue to create new and/or better ways to measure those temperatures. There's quite a list of them, the "Other" devices, beginning with line scanners, two wavelength radiation thermometers, hybrid systems and multiwavelength pyrometers, already and it's sure to grow.

Buyer Beware!

Some vendors are more capable and/or honest than others, just like in every business. The watch words are, as always: "Buyer beware", until you build up confidence in their business ethics and technical abilities (and that of the organization they represent).

There are no standards for the names of the spot-measuring infrared devices. Most metrologists that work in this field tend to call them "Radiation Thermometers". Could that imply, from the enormous variation in names of these devices, that standardization is non existent or not mature in the noncontact temperature sensor field? Yes, YEs, YES-there is room for a great deal of work in this area!!

Don't get caught in the spec trap, thinking that you know what the specification on a data sheet means. Fact is there are not even any standards for the nomenclature used to describe technical features of these devices, starting with calibration uncertainty! Manufacturers know (we hope) but don't always tell all. Few agree in any aspect of their specifications beyond the temperature range of a given unit. The ASTM in the USA has brought some order to this with Standard E 1256, but it is voluntary and not required by many buyers, as of 2003.

The lack of standards and precise nomenclature in terms of quantitative temperature measurement (called "radiometrically calibrated devices" by most of the suppliers) is an obstacle thermal imaging technology needs to overcome to master many of the more demanding uses such as human body temperature screening and process monitoring and control. The "Buyer Beware" caution applies doubly or triply with these devices simply because they are more complex than radiation thermometers, there is much technical confusion and not a widespread understanding about temperature calibration and measurement with many of the traditional imager manufacturers. Most of their attention over the past decades has been in imaging characterization in terms of contrast.

The entry into the thermal imager market in the past few years by the top-tier radiation thermometer companies, who live and thrive on precision and accuracy of IR temperature measurement, may create some changes and improve this aspect of the business. In addition, one organization that has attempted to bring some order to the temperature measurement issue is the Infraspection Institute, one of the oldest and largest thermography training companies. They have had information on their website and that of their Symposium Group, IR/Info, on this issue for the last year, at least. More recently, The Standards organization of Singapore, known by the acronym SPRING, has been developing a Technical Standard for basic evaluation of thermal imaging cameras used in screening humans for possible elevated body temperature as a result of the SARS scare in early 2003.

Finally,if you have a requirement spelled out and are sure it is covered by current standards or by a set of detailed specifications you have developed, are you going to be using the sensor in a ISO900x production or a process or test that is critical? Then you will most certainly need a traceable calibration for the devices you seek. Without it any measurements will have errors that can only be guessed, not verified, nor ever verified or repeated except by happenstance. Often the last item on a purchase checklist is the most important, as is this one: Traceable Calibration. In their 2001 book, "Traceable Temperatures" (see our References page) Nicholas and White observe that measurements without traceability, are not measurements at all but effectively some vague effort that, in a critical analysis, is seen not worth the time and money.

That all implies, among other things that there is a test or demonstration of capability that a unit must pass to be accepted. Even devices that will be used to only compare two temperatures (measure a temperature difference) need to have some reference to which they can be re-tested to verify that they are meeting the requirement. Often simulating the need under true application conditions of measurement can be challenging and difficult to do and maintain a traceable calibration to within the desired norms.

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