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The New Generation of Noncontact IR thermometry becomes quite user friendly when you review Klaus-Dieter Gruner After time, temperature is the second most measured physical unit. In production as well as in quality control and maintenance, temperature represents an important indicator of product quality or equipment conditions.
Advantages of IR Thermometers
Factors to Consider Before Going IR
Basics of IR Theory Modern IR theory is based on the physical principle that all bodies having a temperature greater than absolute zero (0 K, –273.16°C, –459.69°F) radiate energy. The heat in such bodies causes molecular vibrations that induce electron vibrations which in turn provide the electromagnetic coupling to produce emission. The wavelengths of the emitted radiation are temperature dependent; the amplitude is also influenced by a surface characteristic--emissivity--of the radiating body. Emissivity is the ratio of the energy radiated by an object at a given temperature to that emitted by a perfect radiator (blackbody) at the same temperature. Two other properties, transparency and reflectivity, are important, but a thorough discussion is beyond the scope of this article. For a general idea, consider the blackbody.
A blackbody is an ideal object that emits all of its radiation. No energy is reflected from a nearby object, and none is transmitted through the blackbody from a source on the opposite side. All the radiation coming from the blackbody surface is therefore emitted energy, and this energy is a function of the object's temperature. Most nonmetallic objects have a low reflectivity, are not transmissive, and have an emissivity >0.9. Conversely, metals that are especially shiny or have polished surfaces can have a high reflectivity and therefore a low emissivity. In this case it is important to know the exact value of the emissivity, and there are several ways to measure it. A good estimate can be obtained from published emissivity tables. Once you have determined the emissivity of the surface of the object of interest, you enter this value into the IR thermometer and use it to calculate the temperature from the emitted energy. Nearly all IR thermometers on the market allow the user to adjust for emissivity.
It is important to understand also that the distribution of emitted radiation shifts to shorter wavelengths as the temperature rises (see Figure 1). Although it might seem that the best IR thermometer would be one that detects radiation from a wide range of wavelengths in order to detect the maximum amount of energy, this is not the case. For example, there is significantly more radiated energy at 1000°C than at 500°C at 1 micron while at 10 microns the difference in radiated energy between 1000°C and 500°C is very small. Larger energy steps result in a better temperature resolution and increased accuracy of the IR thermometer. IR thermometers are therefore made with different wavelength scales designed to offer the best performance over various temperature ranges. Another factor to consider is that materials such as glass or thin film plastics are highly transmissive and can be "seen" only at certain wavelengths. Typically, IR temperature devices measure in the 0.7–14 micron wavelength. IR Thermometer Trends Over the past three years the IR sensor market has seen two significant trends. First, IR thermometers cost significantly less--some types are only half the price they were five years ago. The most expensive parts are the lenses and detectors. Now new lens materials and technologies (e.g., plastic, Fresnel lenses) and efficient mass production techniques for IR detectors in consumer products such as toasters, hair dryers, and ear thermometers have resulted in lower prices for these two components.
Second, IR sensors are getting smaller. In the past, capturing enough radiated energy to make low-temperature measurements necessitated the use of low f-number lenses with relatively large diameters. Progress in detector technology and the use of microsystem techniques, together with improved low-noise analog preamplifiers have helped to greatly reduce the dimensions of the IR sensing head. Because the newer sensors have greater response characteristics, they require less received radiation to achieve a usable signal.The miniature IR sensor in Photo 1, with a lens diameter of only 14 mm, has a temperature resolution of 0.1°C and can measure down to 0°C. These sensors are used in the plastic, food, and textile industries, especially in small machines where space is limited. Detector and Optics IR thermometers incorporate either a thermal or a photoelectric, or photon, detector. Thermal detectors must first change their own temperature in response to the sensed radiated energy. This temperature change, similar to that of a thermocouple, produces a certain voltage level that corresponds to a detected temperature. Photoelectric detectors (e.g., photodiodes) react to the photons emitted by the object of interest, resulting in an increase of electron/hole pairs and a concomitant increase in the current signal. Thermal detectors respond more slowly than photoelectric devices because they first must "feel" a temperature change. The difference in response time is not large, but it can be significant in certain applications. In the case of thermal detectors, "slow" means time constants in the millisecond range as opposed to the nano- or microseconds required by photoelectric types. The IR detectors in common use today are:
New InGaAs Detectors Indium-gallium-arsenide (InGaAs) detectors were first used in data communications (fiber optics) and to measure gas concentrations (pollution monitoring) and humidity. As with most technologies, there has been a demand to reduce costs, improve sensor features, and identify new markets and applications. For instance, the demand for inexpensive gas concentration and blood pressure measurement systems designed for consumer use has created a huge technology push for InGaAs sensors. This demand has benefited the manufacturers and the medical field and the consumer as well. InGaAs detectors are now available with wavelengths at 1.6 microns, 1.9 microns, 2.2 microns, and most recently, at 2.6 microns at a reasonable cost (see Figure 2). These devices have nearly replaced the conventional germanium detectors because they are more powerful.
What are the advantages of these new detectors? Referring again to Figure 1, we know that if we choose a lower wavelength IR thermometer and we want to measure at lower temperatures, we can get only a small signal. Sensors with better detectivity or sensitivity make it possible to get a larger signal for the same radiation level. We can even expect a reasonable signal when there is much lower energy or temperature. It is best to use the shortest possible wavelength, especially when measuring metal objects whose emissivity is typically higher at shorter wavelengths. Assuming a 10% error in emissivity, the error with a shorter wavelength will be less than the error with a longer wavelength (see Figure 3). If we have an object at 400°C, for instance, the error in reading with a 2.2 micron spectral wavelength will be 2%; at the 8–14 micron range, the error will be >7%. And there is also another effect to consider: emissivity usually changes with temperature. This dependency is much less at shorter wavelengths and makes IR measurement much easier when dealing with nonferrous metals. In the automotive industry, for example, steel is being replaced with nonferrous metals, allowing the production process (forming, tempering, hardening) to run at much lower temperatures. There are IR thermometers capable of performing well at these temperatures. Electronic Design Trends As part of a general effort to cut costs, maximize production, and monitor quality, the process industries are stepping up their demand for sensors with higher accuracy and repeatability. Ease of installation, operation, and maintenance continue to be important, as does the sensors' ability to withstand harsh environmental conditions. Field checking and field calibration features are becoming more and more desirable as companies look to optimize their personnel and equipment resources. Furthermore, manufacturers want IR sensors with a wide operating temperature range that can address a greater number of applications by eliminating the need for costly and cumbersome water/air-cooled housings.
Advances in other industries have served to make a variety of IR products readily available and affordable that up until about five years ago were restricted by their high cost to specific military applications. The PC industry, for example, continues to make strides with multimedia applications, high-resolution A/D and D/A converters, and communication interfaces such as USB, firewire, and programmable logic. Other industries that are influencing IR thermometer developments include the security market with high-precision detector amplifiers, AV consumer products, and cellular phones. Low-power, low-voltage circuitries, low-cost LCDs, and miniaturized electronic components are all being used in today's IR thermometers. The digital "2-color" IR thermometer in Figure 4 has a digital design that allows fast response time and high accuracy to be combined with the full feature set of digital communications capabilities. A sensor based on a fully digital design is able to deliver considerably more information than a unit using purely analog signal processing techniques. Advances in IR thermometer detectors and optics are helping manufacturers and their customers expand the types of applications where this technology can enhance speed and accuracy and improve the ROI picture. Part 2 of this article will address the developments and trends in digital communications and networking for sensors.
Klaus-Dieter Gruner, Ph.D., is VP for Marketing and Alan Young, Ph.D., is Product Manager, Raytek Corp., 1201 Shaffer Rd., PO Box 1820, Santa Cruz, CA 95081-1820; 831-458-1110, fax 831-458-1239, klaus@raytek .com or alany@raytek.com
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