Radiation Pyrometer

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About Pyrometry
In industrial and research applications, it is often necessary to measure the temperature of an object from a distance, without making contact; for example: when the object is moving, as on an assembly line; when it is very hot, as inside a furnace; or when it is inaccessible, as inside a high-vacuum chamber. The method used for making these non-contacting temperature measurements is known as radiation pyrometry. It is based upon the following principals:

All objects at temperatures above absolute zero emit electromagnetic radiation as a function of temperature precisely in accordance with the famous "Planck Equation", first formulated by the physicist Max Planck about 100 years ago. Based upon this relationship, the temperature of an object may be determined from any distance by measuring its emitted radiation. However, the spectral emissivity value of the surface, one of the parameters of the Planck Equation, must first be known, or a value must be assumed, before the actual temperature may be computed.

In practice, unfortunately, emissivity information is often in error since the actual surface conditions may not be known, or may be changing due to oxidation or other coatings, or the emissivity may be varying with the temperature of the object itself. This leads to error, often large, in traditional radiation pyrometry.

Quantum Logic Corporation, however, has developed, patented, and currently markets Laser/Microcomputer Pyrometers, which employ a new technology using small semiconductor lasers and microcomputers to determine from a distance the actual spectral emissivity of an object. With this information, and a simultaneous measurement of the emitted radiation, the microcomputer can calculate the true temperature of the object using the Planck Equation. The temperature measurement accuracy obtained with this method can often be more than an order of magnitude more accurate than is normally obtained with traditional radiation pyrometry.

Our eyes only see the tiny fraction of energy emitted by the sun in the form of visible light. However, if we could see the infrared rays emitted by all bodies—organic and inorganic--we could effectively see in the dark. Though invisible to the human eye, infrared radiation can be detected as a feeling of warmth on the skin, and even objects that are colder than ambient temperature radiate infrared energy. Some animals such as rattlesnakes, have small infrared temperature sensors located under each eye which can sense the amount of heat being given off by a body. These sensors help them to locate prey and protect themselves from predators.
Non-contact temperature sensors use the concept of infrared radiant energy to measure the temperature of objects from a distance. After determining the wavelength of the energy being emitted by an object, the sensor can use integrated equations that take into account the body's material and surface qualities to determine its temperature. In this chapter, we will focus on the history of radiation thermometry and the development of non-contact temperature sensors.

IR Through the Ages
Although not apparent, radiation thermometry has been practiced for thousands of years. The first practical infrared thermometer was the human eye (Figure 1-1). The human eye contains a lens which focuses emitted radiation onto the retina. The retina is stimulated by the radiation and sends a signal to the brain, which serves as the indicator of the radiation. If properly calibrated based on experience, the brain can convert this signal to a measure of temperature.

People have been using infrared heat to practical advantage for thousands of years. There is proof from clay tablets and pottery dating back thousands of years that the sun was used to increase the temperature of materials in order to produce molds for construction. Pyramids were built from approximately 2700-2200 B.C. of sun-dried bricks. The Egyptians also made metal tools such as saws, cutting tools, and wedges, which were crafted by the experienced craftsmen of their time. The craftsmen had to know how hot to make the metal before they could form it. This was most likely performed based on experience of the color of the iron.

Because fuel for firing was scarce, builders of Biblical times had to depend on the sun's infrared radiation to dry the bricks for their temples and pyramids. The Mesopotamian remains of the Tower of Babel indicate that it was made of sun-dried brick, faced with burnt brick and stone. In India, a sewer system dating back to 2500 B.C. carried wastewater through pottery pipes into covered brick drains along the street and discharged from these into brick culverts leading into a stream.

In ancient Greece, as far back as 2100 B.C., Minoan artisans produced things such as vases, statues, textiles. By using sight, they could approximate when a piece of material could be shaped. Terra-cotta pipes were built by heating them to a certain temperature and casting them into amold.

In more recent years, special craftsmen have relied on their own senses to visualize when a material is the correct temperature for molding or cutting. Sight has been used for steel working, glass working, wax molding, and pottery. From experience, skilled craftsmen learned to estimate the degree of heat required in the kiln, smelter, or glass furnace by the color of the interior of the heating chamber. Just as a classical blacksmith, for example, might judge the malleability of a horseshoe by itscherry-redcolor.

In countries around the world, the technique of sight is still being used. In Europe, glass molding craftsmen use sight to determine when glass is ready to be shaped (Figure 1-2). They put a large piece of glass in a heating furnace by use of a large metal rod. When the glass reaches the desired color and brightness, they pull it out of the oven and immediately form it into the shape they want. If the glass cools and loses the desired color or brightness, they put it back in the oven or dispose of it. The glass makers know when the glass is ready, by sight. If you have a chandelier made of glass, or hand-made glasses from Europe, most likely they were formed in this way.

Pyrometer is derived from the Greek root pyro, meaning fire. The term pyrometer was originally used to denote a device capable of measuring temperatures of objects above incandescence, objects bright to the human eye. The original pyrometers were non-contacting optical devices which intercepted and evaluated the visible radiation emitted by glowing objects. A modern and more correct definition would be any non-contacting device intercepting and measuring thermal radiation emitted from an object to determine surface temperature. Thermometer, also from a Greek root thermos, signifying hot, is used to describe a wide assortment of devices used to measure temperature. Thus a pyrometer is a type of thermometer. The designation radiation thermometer has evolved over the past decade as an alternative to pyrometer. Therefore the terms pyrometer and radiation thermometer are used interchangeably by many references.

The first patent for a total radiation thermometer was granted in 1901. The instrument used a thermoelectric sensor; it had an electrical output signal and was capable of unattended operation. In 1931, the first commercially-available total radiation thermometers were introduced.

These devices were widely used throughout industry to record and control industrial processes. They are still used today, but mainly used for low temperature applications.

The first modern radiation thermometers were not available until after the second World War. Originally developed for military use, lead sulfide photodetectors were the first infrared quantum detectors to be widely used in industrial radiation thermometry. Other types of quantum detectors also have been developed for military applications and are now widely applied in industrial radiation thermometry. Many infrared radiation thermometers use thermopile detectors sensitive to a broad radiation spectrum and are extensively used in process control instrumentation.

Infrared thermometers currently are being used in a wide range of industrial and laboratory temperature control applications. By using non-contact temperature sensors, objects that are difficult to reach due to extreme environmental conditions can be monitored. They can also be used for products that cannot be contaminated by a contact sensor, such as in the glass, chemical, pharmaceutical, and food industries. Non-contact sensors can be used when materials are hot, moving, or inaccessible, or when materials cannot be damaged, scratched, or torn by a contact thermometer.

Typical industries in which non-contact sensors are used include utilities, chemical processing, pharmaceutical, automotive, food processing, plastics, medical, glass, pulp and paper, construction materials, and metals. Industrially, they are used in manufacturing, quality control, and maintenance and have helped companies increase productivity, reduce energy consumption, and improve product quality.

Some applications of radiation thermometry include the heat treating, forming, tempering, and annealing of glass; the casting, rolling, forging, and heat treating of metals; quality control in the food and pulp and paper industry; the extrusion, lamination, and drying of plastics, paper, and rubber; and in the curing process of resins, adhesives, and paints.
Non-contact temperature sensors have been used and will continue to be valuable for research in military, medical, industrial, meteorological, ecological, forestry, agriculture, and chemical applications.
Weather satellites use infrared imaging devices to map cloud patterns and provide the imagery seen in many weather reports. Radiation thermometry can reveal the temperature of the earth's surface even through cloud cover.

Infrared imaging devices also are used for thermography, or thermal imaging. In the practice of medicine, for example, thermography has been used for the early detection of breast cancer and for the location of the cause of circulatory deficiencies. In most of these applications, the underlying principle is that pathology produces local heating and inflammation which can be found with an infrared imager. Other diagnostic applications of infrared thermography range from back problems to sinus obstructions.

Edge burning forest fires have been located using airborne infrared imagers. Typically, the longer wavelengths of the emitted infrared radiation penetrate the smoke better than the visible wavelengths, so the edges of the fire are better delineated.

Radiation Thermometer
A radiation thermometer, in very simple terms, consists of an optical system and detector. The optical system focuses the energy emitted by an object onto the detector, which is sensitive to the radiation. The output of the detector is proportional to the amount of energy radiated by the target object (less the amount absorbed by the optical system), and the response of the detector to the specific radiation wavelengths. This output can be used to infer the objects temperature. The emittivity, or emittance, of the object is an important variable in converting the detector output into an accurate temperature signal.

Infrared radiation thermometers/ pyrometers, by specifically measuring the energy being radiated from an object in the 0.7 to 20 micron wavelength range, are a subset of radiation thermometers. These devices can measure this radiation from a distance. There is no need for direct contact between the radiation thermometer and the object, as there is with thermocouples and resistance temperature detectors (RTDs). Radiation thermometers are suited especially to the measurement of moving objects or any surfaces that can not be reached or can not be touched.

But the benefits of radiation thermometry have a price. Even the simplest of devices is more expensive than a standard thermocouple or resistance temperature detector (RTD) assembly, and installation cost can exceed that of a standard thermowell. The devices are rugged, but do require routine maintenance to keep the sighting path clear, and to keep the optical elements clean. Radiation thermometers used for more difficult applications may have more complicated optics, possibly rotating or moving parts, and microprocessor-based electronics. There are no industry accepted calibration curves for radiation thermometers, as there are for thermocouples and RTDs. In addition, the user may need to seriously investigate the application, to select the optimum technology, method of installation, and compensation needed for the measured signal, to achieve the performance desired.

Emittance, Emissivity, and the N Factor
Emittance was identified as a critical parameter in accurately converting the output of the detector used in a radiation thermometer into a value representing object temperature.

The terms emittance and emissivity are often used interchangeably. There is, however, a technical distinction. Emissivity refers to the properties of a material; emittance to the properties of a particular object. In this latter sense, emissivity is only one component in determining emittance. Other factors, including shape of the object, oxidation and surface finish must be taken into account.

Infrared Thermocouples
An infrared thermocouple is an unpowered, low-cost sensor that measures surface temperature of materials without contact. It can be directly installed on conventional thermocouple controllers, transmitters and digital readout devices as if it were a replacement thermocouple. An infrared thermocouple can be installed in a fixed, permanent location, or used with a hand-held probe.

Because it is self-powered, it relies on the incoming infrared radiation to produce a signal via thermoelectric effects. Therefore, its output follows the rules of radiation thermal physics, and is subject to nonlinearities. But over a given range of temperatures, the output is sufficiently linear that the signal can be interchanged with a conventional thermocouple.
Although each infrared thermocouple is designed to operate in a specific region, it can be used outside that region by calibrating the readout device accordingly.

Radiation Thermometers/Pyrometers
Radiation thermometers, or pyrometers, as they are sometimes called come in a variety of configurations. One option is a handheld display/control unit, plus an attached probe. The operator points the probe at the object being measured--sometimes getting within a fraction of an inch of the surface--and reads the temperature on the digital display. These devices are ideal for making point temperature measurements on circuit boards, bearings, motors, steam traps or any other device that can be reached with the probe. The inexpensive devices are self-contained and run off battery power.

Other radiation thermometers are hand-held or mounted devices that include a lens similar to a 35mm camera. They can be focused on any close or distant object, and will take an average temperature measurement of the "spot" on the target that fits into its field of view.

Handheld radiation thermometers are widely used for maintenance and troubleshooting, because a technician can carry one around easily, focus it on any object in the plant, and take instant temperature readings of anything from molten metals to frozen foods.

When mounted in a fixed position, radiation thermometers are often used to monitor the manufacturing of glass, textiles, thin-film plastic and similar products, or processes such as tempering, annealing, sealing, bending and laminating.

Fiber Optics Extensions
When the object to be measured is not in the line of sight of a radiation thermometer, a fiber optic sensor can be used. The sensor includes a tip, lens, fiber optic cable, and a remote monitor unit mounted up to 30 ft away. The sensor can be placed in high energy fields, ambient temperatures up to 800°F, vacuum, or in otherwise inaccessible locations inside closed areas.

Two-Color Systems
For use in applications where the target may be obscured by dust, smoke or similar contaminants, or changing emissions as in "pouring metals," a two-color or ratio radiation thermometer is ideal. It measures temperature independently of emissivity. Systems are available with fiber optic sensors, or can be based on a fixed or hand-held configurations.

Linescanners
A linescanner provides a "picture" of the surface temperatures across a moving product, such as metal slabs, glass, textiles, coiled metal or plastics. It includes a lens, a rotating mirror that scans across the lens' field of view, a detector that takes readings as the mirror rotates, and a computer system to process the data.

As the mirror rotates, the line scanner takes multiple measurements across the entire surface, obtaining a full-width temperature profile of the product. As the product moves forward under the sensor, successive scans provide a profile of the entire product, from edge to edge and from beginning to end.

The computer converts the profile into a thermographic image of the product, using various colors to represent temperatures, or it can produce a "map" of the product. The 50 or so measurement points across the width can be arranged in zones, averaged, and used to control upstream devices, such as webs, cooling systems, injectors or coating systems.
Linescanners can be extremely expensive, but they offer one of the only solutions for obtaining a complete temperature profile or image of a moving product.

Portable vs. Mounted
Non-contact temperature measurement devices also can be classified as portable or permanently mounted. Fixed mount thermometers are generally installed in a location to continuously monitor a process. They often operate on AC line power, and are aimed at a single point. Measured data can be viewed on a local or remote display, and an output signal (analog or digital) can be provided for use elsewhere in the control loop. Fixed mount systems generally consist of a housing containing the optics system and detector, connected by cable to a remote mounted electronics/display unit. In some loop-powered designs, all the thermometer components and electronics are contained in a single housing; the same two wires used to power the thermometer also carry the 4 to 20 mA output signal.

Battery powered, hand-held "pistol" radiation thermometers typically have the same features as permanently mounted devices, but without the output signal capability. Portable units are typically used in maintenance, diagnostics, quality control, and spot measurements of critical processes.

Portable devices include pyrometers, thermometers and two-color systems. Their only practical application limit is the same as a human operator; i.e., the sensors will function in any ambient temperature or environmental condition where a human can work, typically 32-120°F (0-50°C).

At temperature extremes, where an operator wears protective clothing, it may be wise to similarly protect the instrument. In shirt-sleeve manufacturing or process control applications, hand-held instruments can be used without worrying about the temperature and humidity, but care should be taken to avoid sources of high electrical noise. Induction furnaces, motor starters, large relays and similar devices that generate EMI can affect the readings of a portable sensor.

Portable non-contact sensors are widely used for maintenance and troubleshooting. Applications vary from up-close testing of printed circuit boards, motors, bearings, steam traps and injection molding machines, to checking temperatures remotely in building insulation, piping, electrical panels, transformers, furnace tubes and manufacturing and process control plants.

Because an infrared device measures temperature in a "spot" defined by its field of view, proper aiming can become critical. Low-end pyrometers have optional LED aiming beams, and higher end thermometers have optional laser pointing devices to help properly position the sensor.

Permanently mounted devices are generally installed on a manufacturing or process control line, and output their temperature signals to a control or data acquisition system. Radiation thermometers, two-color sensors, fiber optics, infrared thermocouples, and linescanners can all be permanently mounted.

In a permanent installation, an instrument can be very carefully aimed at the target, adjusted for the exact emissivity, tuned for response time and span, connected to a remote device such as an indicator, controller, recorder or computer, and protected from the environment. Once installed and checked out, such an instrument can run indefinitely, requiring only periodic maintenance to clean its lenses.

Instruments designed for permanent installation are generally more rugged than lab or portable instruments, and have completely different outputs. In general, systems that operate near a process are ruggedized, have NEMA and ISO industrial-rated enclosures, and output standard process control signals such as 4-20 mA dc, thermocouple mV signals, 0-5 Vdc, or serial RS232C.

For very hot or dirty environments, instruments can be equipped with water or thermoelectric cooling to keep the electronics cool, and nitrogen or shop air purging systems to keep lenses clean.

Accessories, Features & Options
Radiation thermometers and thermocouples are available with a host of features to solve a wide range of application conditions. All infrared sensors are available in a wide range of wavelengths, temperature ranges and optical systems. Portable units almost always are available with carrying kits, and permanently mounted units are ruggedized. Listed below are other options, features and accessories that make these sensors more useful for certain types of applications.

Backlit LCD displays, integrally attached or remotely mounted from the thermometer, are available. Multiple variables can be viewed simultaneously on these displays. These data can include current temperature, minimum measured temperature (time based), maximum measured temperature (time based), average temperature measured (time based), and differential temperature (for example, between the target and the surroundings).

Microprocessor-based radiation thermometers have input options to allow data to be integrated into the measurement from other sensors or thermometers in the loop. For example, a separate thermocouple or RTD input to the thermometer can be used to compensate the measured target temperature for changing ambient temperature conditions.

Protection from high electromagnetic and radio frequency interference (EMI/RFI) is available if the thermometer must be installed in a difficult environments.

Most infrared thermometers can be supplied with an emissivity adjustment. In addition, some devices can be supplied with an adjustable field of view. This is accomplished by installing an iris in the optical system that can be opened or closed to provide wide or narrow angle field of views.

Handheld IR Thermometers
Handheld instruments are generally completely self-contained, battery-powered units, with manual controls and adjustments and some form of digital readout. Units can be mounted on tripods. Other accessories include:

Laser sights, which paint a visible spot on the target, making it easier to determine where the instrument is pointed. This option is available both integrally attached or detachable from the thermometer. Hand-held devices used for up-close spot temperature measurement (for example, to measure component temperature on printed circuit boards) can have audible focusing guides instead of light markers.

Dataloggers, for acquiring data from thermometers and recording it for future use;

Digital printers Electrical system scanners, designed specifically for finding hot spots in electrical panels, switchgear, fuse panels, transformers, etc.

Handheld, shirt-pocket-size scanner for general surface temperature measurement.

Infrared Thermocouples
These self-powered devices generate a thermocouple signal output using radiated energy, but usually have no signal processing or display systems. An infrared thermocouple is a sensor only, but it does have a few options and accessories.

Two-color pyrometry unit that uses short-wave and long-wave infrared thermocouples.

Fiber Optic Sensors
Probes are available in lens cells of various sizes, with replaceable glass or quartz tips. Options include a ceramic/metal tip for high temperatures, a polymer bolt for extrusion applications, ejector pin probe for injection molding, and right angle prisms. Sensor probes also are available as optical rods up to 60 cm long.

Cables can be supplied in single, bifurcated or trifurcated fiber optic bundles, and enclosed in jackets made of flexible stainless steel (standard), ceramic, heavy duty wire braid for abrasion resistance, or Teflon for high radio frequency fields. Cables typically are up to 30 ft long.

Indicators and Controllers
Display units and controllers are available in models ranging from a simple digital panel meter that displays the signal as a temperature in °F or °C, to complex multi-channel processors that perform signal conditioning, linearization, peak-picking, alarm monitoring, saving min/max values, signal averaging, data logging and a host of other signal processing and manipulation functions.

Mounted IR Thermometers
The same basic features, options and accessories are available for radiation thermometers, two-color systems, and line scanners. Ruggedized for use on the plant floor, all these devices have several accessories to help them survive in hostile environments.

Air purge--Attaches to front end of sensor housing and provides positive air pressure in front of the lens, preventing dust, smoke, moisture and other contamination from reaching lens. In two-color systems, it can attach to front of re-imaging lens.

Air or water cooling jackets--Available for warm (35°F above ambient) and hot (up to 400 °F) environments, cooling jackets keep sensor temperature at normal levels inside the enclosure.

Peltier effect cooling--Some line scanners have electronic cooling systems, using Peltier effect devices.

Sighting accessories, including sight tubes, laser pointing devices, and scopes.

Onboard data logging functions are available, as well as options for thermal printers to retrieve stored data. Data can also be remotely transmitted digitally.

Transmitters--Ruggedized NEMA 4 housing with 4-20 mAdc and/or RS232C/RS485 outputs.

The modern radiation thermometer is still based on this concept. However the technology has become more sophisticated to widen the scope of the applications that can be handled. For example, the number of available detectors has greatly increased, and, thanks to selective filtering capabilities, these detectors can more efficiently be matched to specific applications, improving measurement performance. Microprocessor-based electronics can use complex algorithms to provide real time linearization and compensation of the detector output for higher precision of measured target temperature. Microprocessors can display instantaneous measurements of several variables (such as current temperature, minimum temperature measured, maximum temperature measured, average temperature or temperature differences) on integral LCD display screens.

These classifications are not rigid. For example, optical pyrometers can be considered a subset of narrow band devices. Fiber optic radiation thermometers, to be discussed in detail in another section, can be classified as wide band, narrow band, or ratio devices. Likewise, infrared radiation thermometers can be considered subsets of several of these classes.

Broadband Radiation
Broadband radiation thermometers typically are the simplest devices, cost the least, and can have a response from 0.3 microns wavelength to an upper limit of 2.5 to 20 microns. The low and high cut-offs of the broadband thermometer are a function of the specific optical system being used. They are termed broadband because they measure a significant fraction of the thermal radiation emitted by the object, in the temperature ranges of normal use.

Broadband thermometers are dependent on the total emittance of the surface being measured. Figure 3-2 shows the error in reading for various emissivities and temperatures when a broadband device is calibrated for a blackbody. An emissivity control allows the user to compensate for these errors, so long as the emittance does not change.

The path to the target must be unobstructed. Water vapor, dust, smoke, steam and radiation absorptive gases present in the atmosphere can attenuate emitted radiation from the target and cause the thermometer to read low.

The optical system must be kept clean, and the sighting window protected against any corrosives in the environment.

Standard ranges include 32 to 1832°F (0 to 1000°C), and 932 to 1652°F (500 to 900°C). Typical accuracy is 0.5 to 1% full scale.

Narrow Band Radiation
As the name indicates, narrow band radiation thermometers operate over a narrow range of wavelengths. Narrow band devices can also be referred to as single color thermometers/pyrometers (see Optical Pyrometers). The specific detector used determines the spectral response of the particular device. For example, a thermometer using a silicon cell detector will have a response that peaks at approximately 0.9 microns, with the upper limit of usefulness being about 1.1 microns. Such a device is useful for measuring temperatures above 1102°F (600°C). Narrow band thermometers routinely have a spectral response of less than 1 micron.
Narrow band thermometers use filters to restrict response to a selected wavelength. Probably the most important advance in radiation thermometry has been the introduction of selective filtering of the incoming radiation, which allows an instrument to be matched to a particular application to achieve higher measurement accuracy. This was made possible by the availability of more sensitive detectors and advances in signal amplifiers.

Common examples of selective spectral responses are 8 to 14 microns, which avoids interference from atmospheric moisture over long paths; 7.9 microns, used for the measurement of some thin film plastics; 5 microns, used for the measurement of glass surfaces; and 3.86 microns, which avoids interference from carbon dioxide and water vapor in flames and combustion gases.

The choice of shorter or longer wavelength response is also dictated by the temperature range. The peaks of radiation intensity curves move towards shorter wavelengths as temperature increases, as shown in Figure 3-3. Applications that don't involve such considerations may still benefit from a narrow spectral response around 0.7 microns. While emissivity doesn't vary as much as you decrease the wavelength, the thermometer will lose sensitivity because of the reduced energy available.
Narrow band thermometers with short wavelengths are used to measure high temperatures, greater than 932°F (500°C), because radiation energy content increases as wavelengths get shorter. Long wavelengths are used for low temperatures -50°F (-45.5°C).

Narrow band thermometers range from simple hand-held devices, to sophisticated portables with simultaneous viewing of target and temperature, memory and printout capability, to on-line, fixed mounted sensors with remote electronics having PID control.

Standard temperature ranges vary from one manufacturer to the next, but some examples include: -36 to 1112°F (-37.78 to 600°C), 32 to 1832°F (0 to 1000°C), 1112 to 5432°F (600 to 3000°C) and 932 to 3632°F (500 to 2000°C). Typical accuracy is 0.25% to 2% of full scale.

Ratio Radiation
Also called two-color radiation thermometers, these devices measure the radiated energy of an object between two narrow wavelength bands, and calculates the ratio of the two energies, which is a function of the temperature of the object. Originally, these were called two color pyrometers, because the two wavelengths corresponded to different colors in the visible spectrum (for example, red and green). Many people still use the term two-color pyrometers today, broadening the term to include wavelengths in the infrared. The temperature measurement is dependent only on the ratio of the two energies measured, and not their absolute values as shown in Figure 3-4. Any parameter, such as target size, which affects the amount of energy in each band by an equal percentage, has no effect on the temperature indication. This makes a ratio thermometer inherently more accurate. (However, some accuracy is lost when you're measuring small differences in large signals). The ratio technique may eliminate, or reduce, errors in temperature measurement caused by changes in emissivity, surface finish, and energy absorbing materials, such as water vapor, between the thermometer and the target. These dynamic changes must be seen identically by the detector at the two wavelengths being used.

Emissivity of all materials does not change equally at different wavelengths. Materials for which emissivity does change equally at different wavelengths are called gray bodies. Materials for which this is not true are called non-gray bodies. In addition, not all forms of sight path obstruction attenuate the ratio wavelengths equally. For example, if there are particles in the sight path that have the same size as one of the wavelengths, the ratio can become unbalanced.

Phenomena which are non-dynamic in nature, such as the non-gray bodiness of materials, can be dealt with by biasing the ratio of the wavelengths accordingly. This adjustment is called slope. The appropriate slope setting must be determined experimentally.

Figure 3-5 shows a schematic diagram of a simple ratio radiation thermometer. Figure 3-6 shows a ratio thermometer where the wavelengths are alternately selected by a rotating filter wheel.

Some ratio thermometers use more than two wavelengths. A multi-wavelength device is schematically represented in Figure 3-7. These devices employ a detailed analysis of the target's surface characteristics regarding emissivity with regard to wavelength, temperature, and surface chemistry. With such data, a computer can use complex algorithms to relate and compensate for emissivity changes at various conditions. The system described in Figure 3-7 makes parallel measurement possible in four spectral channels in the range from 1 to 25 microns. The detector in this device consists of an optical system with a beam splitter, and interference filters for the spectral dispersion of the incident radiation. This uncooled thermometer was developed for gas analysis. Another experimental system, using seven different wavelengths demonstrated a resolution of +/-1°C measuring a blackbody source in the range from 600 to 900°C. The same system demonstrated a resolution of +/-4* C measuring an object with varying emittance over the temperature range from 500 to 950°C.

Two color or multi-wavelength thermometers should be seriously considered for applications where accuracy, and not just repeatability, is critical, or if the target object is undergoing a physical or chemical change.

Ratio thermometers cover wide temperature ranges. Typical commercially available ranges are 1652 to 5432* F (900 to 3000°C) and 120 to 6692°F (50 to 3700°C). Typical accuracy is 0.5% of reading on narrow spans, to 2% of full scale.

Optical Pyrometers
Optical pyrometers measure the radiation from the target in a narrow band of wavelengths of the thermal spectrum. The oldest devices use the principle of optical brightness in the visible red spectrum around 0.65 microns. These instruments are also called single color pyrometers. Optical pyrometers are now available for measuring energy wavelengths that extend into the infrared region. The term single color pyrometers has been broadened by some authors to include narrow band radiation thermometers as well.

Some optical designs are manually operated as shown in Figure 3-8. The operator sights the pyrometer on target. At the same time he/she can see the image of an internal lamp filament in the eyepiece. In one design, the operator adjusts the power to the filament, changing its color, until it matches the color of the target. The temperature of the target is measured based upon power being used by the internal filament. Another design maintains a constant current to the filament and changes the brightness of the target by means of a rotatable energy-absorbing optical wedge. The object temperature is related to the amount of energy absorbed by the wedge, which is a function of its annular position.

Automatic optical pyrometers, sensitized to measure in the infrared region, also are available. These instruments use an electrical radiation detector, rather than the human eye. This device operates by comparing the amount of radiation emitted by the target with that emitted by an internally controlled reference source. The instrument output is proportional to the difference in radiation between the target and the reference. A chopper, driven by a motor, is used to alternately expose the detector to incoming radiation and reference radiation. In some models, the human eye is used to adjust the focus. Figure 3-9 is a schematic of an automatic optical pyrometer with a dichroic mirror. Radiant energy passes through the lens into the mirror, which reflects infrared radiation to the detector, but allows visible light to pass through to an adjustable eyepiece. The calibrate flap is solenoid-operated from the amplifier, and when actuated, cuts off the radiation coming through the lens, and focuses the calibrate lamp on to the detector. The instrument may have a wide or narrow field of view. All the components can be packaged into a gun-shaped, hand-held instrument. Activating the trigger energizes the reference standard and read-out indicator.

Fiber Optic Radiation
Although not strictly a class unto themselves, these devices use a light guide, such as a flexible transparent fiber, to direct radiation to the detector, and are covered in more detail in the chapter beginning on p. 43. The spectral response of these fibers extends to about 2 microns, and can be useful in measuring object temperatures to as low as 210°F (100°C). Obviously, these devices are particularly useful when it is difficult or impossible to obtain a clear sighting path to the target, as in a pressure chamber.