Infrared Temperature Measurement Fundamentals
Contact temperature sensors are accurate and cost-effective but simply not practical for many industrial applications. Infrared temperature sensors routinely perform measurements that virtually are impossible for contact sensors. For example, measuring objects or materials that:
- Move, rotate or vibrate.
- Are in strong electromagnetic fields (e.g., induction heating).
- Undergo rapid thermal changes (faster than several hundred milliseconds).
- Are located in process chambers or behind windows.
- Require surface temperature measurement.
- Are physically inaccessible to contact thermometers.
- Are damaged or contaminated if contacted.
- Have varying surface temperature distributions.
- Temperature would change if contacted.
- Are made from materials with low heat capacity or low thermal conductivity.
- Appear transparent or are gaseous (combustion gases/flames).
- Require prompt or frequent temperature measurement.
Three important factors that influence infrared temperature sensor selection are:
- The object's temperature and ability to emit thermal radiation -- a property called emissivity.
- The ability to view the object by direct line of sight.
- Heat sources close to the infrared sensor or object and, in some cases, the intervening atmosphere.
Infrared temperature sensors provide superior response time, durability, noncontamination and ease-of-use to meet the strict requirements of demanding industrial environments. Use the following information to help you when it comes time to select a radiation thermometer for your application.
Thermal RadiationThe physics of thermal radiation helps one understand how infrared thermometers work, but knowledge of thermal physics is not required to use infrared thermometers. Thermal radiation is the rate at which an object emits energy because of its temperature. For most materials, the emission of thermal radiation is a surface phenomenon. An object's surface characteristics (resulting from its physical and chemical properties) strongly influence its ability to radiate thermal energy. The term emissivity, represented by the symbol E characterizes an object's ability to emit thermal radiation. Emissivity has a value between 0 and 1. A perfect radiator or blackbody has an emissivity equal to 1; by contrast, a perfect reflector has an emissivity equal to 0. A real object cannot emit more radiation than a blackbody at the same temperature.
The primary issue when using infrared temperature sensors is that a real object does not behave like a perfect radiator.
Infrared Thermometer Response CharacteristicsThe instrument manufacturer calibrates an infrared thermometer by aiming it at a blackbody source, which is designed specifically for testing and calibrating infrared thermometers. Blackbody sources resemble a small furnace with an opening to view a surface or cavity heated and controlled to a selected temperature. By design, blackbody sources have E approximately equal to 1.
By varying the source temperature, the manufacturer calibrates the sensor's internal measurement signal to known temperatures. The calibration relationship is stored in the instrument's electronics. Infrared thermometers process the internal measurement signal together with the stored calibration information to generate an output signal that is proportional to temperature. Typically, the output signal is a current (4 to 20 mA) or a voltage (0 to 5 V). Some infrared instruments also include a digital output and multipoint networking capabilities.
Having calibrated an infrared thermometer on a perfect radiator, the ability to measure temperature of a real object relates directly to how well the object's emissivity is known. An object with an emissivity of 0.7 emits only 70 percent of the energy of a blackbody, so, unless one accounts for an object's actual emissivity, the indicated temperature reading will be lower than the object's actual temperature. Most radiation thermometers provide for emissivity adjustment. Table 1 lists emissivity for several common materials.
Field of ViewFigure 1 depicts an infrared thermometer viewing an object. The instrument's detector receives thermal energy from the object through an optical system. The optical system and internal apertures determine the spot size viewed by the detector and define the instrument's field of view (FOV). FOV is expressed as a viewing angle or as the ratio of target-distance-to-target-spot-size (D:S ratio). For example, a D:S of 60 corresponds to a viewing angle of about 1o. If an instrument has a D:S of 30 and the distance to the object divided by the diameter of the object also is 30, the object completely occupies the instrument's FOV.
A critical consideration for signal wavelength (one-color) infrared thermometers is whether the target object fills the instrument's FOV. If the object does not fill the FOV, the indicated temperature will represent a mixture of object and background temperatures. For best performance, infrared thermometer suppliers typically recommend the target object area exceed the FOV area by a factor of about 1.5 (figure 2). Ratio thermometers (dual-wavelength) do not have this restriction.
Most infrared thermometers allow the user to aim and focus the sensor on the target by sighting through the instrument much like a telescope or camera. Infrared thermometers usually offer fixed- or variable-focus sighting and might include a built-in laser to pinpoint small target objects (figure 3). The D:S ratio (also called optical resolution) is extremely important when measuring small targets or small parts because it determines how much of an object the infrared thermometer views. Some infrared thermometers have a D:S that exceeds 300:1, allowing measurement of target spots smaller than 0.040" at a distance of 12".
Ratio ThermometersAnother type of infrared thermometer is the ratio thermometer, which also is called dual-wavelength or two-color. These devices are less sensitive to emissivity variations and other environmental influences than one-color devices because they measure temperature from the ratio of two signals. Instead of measuring in one wavelength region, ratio thermometers measure in two wavelength regions using two detectors (with suitable filters) and determine temperature from the ratio of the two detector signals S1 and S2. While single-wavelength devices calibrate their internal measurement signal level to temperature, ratio thermometers calibrate the signal ratio, S1/S2, against temperature. The temperature measurement also depends on the ratio of the object's emissivity at the two wavelengths, E1/E2. Because of this, ratio thermometers are less sensitive than one-color infrared thermometers to emissivity variations (and absolute emissivity values). For this reason, ratio thermometers frequently are used in industrial applications with targets that exhibit low or changing emissivity. Ratio thermometers provide a highly accurate temperature measurement given the proper value of E1/E2. Most commercially available ratio thermometers allow for emissivity-ratio adjustment.
In addition to reduced sensitivity to emissivity, ratio thermometers offer other advantages. If some physical structure partially obscures the sensor's field of view by some fraction, both detector signals (S1 and S2) are reduced the same fractional amount. Thus, the resulting signal ratio S1/S2, remains unchanged by the obstruction and, because temperature depends on signal ratio, the corresponding temperature remains unaffected by the obstruction.
Similar reasoning applies to an object smaller than the full field of view; atmospheric effects such as smoke, haze or dust obscuring the object, or dirt contaminating the sensor's front lens (assuming such factors influence S1 and S2 equally). Some ratio thermometers maintain accuracy despite signal-level reduction as high as 95 percent. Ratio thermometers cost a little more than single-wavelength units. However, if the object size or other considerations such as physical or atmospheric obstructions preclude using a single-color device, ratio thermometers might provide an answer.
Fiber-Optic ThermometersIt is impossible to view some target objects by direct line of sight. For these applications, an infrared thermometer with a fiber-optic front end might help (figure 4). A small lens assembly for focusing on target objects mounts at the tip of the fiber-optic cable. One can "snake" the flexible fiber-optic cable around obstructions to view targets that are otherwise impossible to view directly. Fiber-optic sensors are available in single-wavelength and ratio-thermometer versions. Also, because the fiber optic assembly contains no electronic components, it withstands higher ambient temperatures (typically 400oF [204oC]) and operates in strong electromagnetic fields. As a result, fiber-optic thermometers find frequent usage in industrial applications where the fiber-optic assembly is located in high-ambient-temperature environments too hot for non-fiber-optic units. Fiber-optic cable lengths to 30' are common.
Response Time, Peak Temperature ReadingsWhen compared to contact sensors, an advantage of infrared thermometers is their response time. The thermal mass of a contact sensor, the process of conducting heat from the object into the sensor, and the associated thermal resistance at the point of contact dramatically limit a contact sensor's response time. By contrast, infrared thermometers have no such limitations. Because of their noncontacting nature, infrared thermometers respond almost instantaneously to temperature changes, permitting measurement of fast-moving objects or objects whose temperature changes rapidly. A few commercially available infrared thermometers respond to temperature changes as quickly as 1 ms.
As already noted, few real objects radiate like a blackbody. The measurement environment consists of structures close to the object and the atmosphere between the sensor and object. These factors are not the same nor are they as well controlled as during calibration. Several factors influence the radiant energy received by the infrared thermometer (figure 5):
- Thermal emission from the object, reflection of emitted radiation.
- Reflection of radiation from other sources of heat nearby.
- Atmospheric absorption and emission of radiation in the viewing path of the instrument.
However, in most applications, objects freely radiate into their surroundings with known, constant emissivity. Other applications routinely employ ratio thermometers without any knowledge of emissivity. Most applications do not have complex issues associated with atmospheric absorption, reflected radiation or emissivity variations, making radiation thermometers well suited for temperature measurement in many industrial applications. For the vast majority of industrial applications, using infrared thermometers is simply a matter of "set it and forget it." Awareness of the factors that influence measurement accuracy allows one to have greater confidence in selecting and using infrared thermometers.
Infrared thermometers increasingly are used in applications that require data acquisition and analysis, temperature documentation, manufacturing process documentation, process optimization and control. In addition to their performance advantages, infrared thermometers offer economic benefits resulting from improved product quality, increased productivity, reduced maintenance and downtime.