Material temperature and emissivity, ability to view an object by direct line-of-sight, and heat sources close to the sensor all affect infrared thermometer selection. How do you know which is best to use?

Figure 1. The fundamental issue in applying infrared thermometers is that real-world objects do not behave like a perfect radiator. An object can emit more radiation than a black body at the same temperature.

Temperature is fundamental to numerous manufacturing processes, but in many applications, it simply is not possible or desirable to contact the object or material being measured. Infrared thermometers provide straightforward temperature measurements in applications where contact probes such as thermocouples or RTDs are impractical or impossible. Such applications involve objects and materials that:

  • Are in motion.

  • Are inaccessible to contact probes.

  • Reside in process chambers or behind windows.

  • Are too hot for contact sensors.

  • Are in strong electromagnetic fields.

  • Experience rapid temperature changes.

  • Require only surface measurement.

  • Become damaged or contaminated if contacted.

  • Have wide surface temperature distributions.

  • Experience temperature changes if contacted.

  • Require frequent measurement.

Infrared temperature sensors provide fast response time, durability, noncontamination and ease-of-use to meet the requirements of demanding manufacturing environments.

Knowledge of thermal physics is not required to use infrared thermometers, but awareness of a few basic principles and concepts helps one better understand their operation and application.

Thermal or heat radiation is the rate at which an object emits energy. Except for gases, 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 o, characterizes an object's ability to emit thermal radiation. Emissivity (o) has a value between 0 and 1. A perfect radiator or black body has an emissivity reading equal to 1. By contrast, a perfect reflector has an emissivity reading equal to 0.

A real object cannot emit more radiation than a black body at the same temperature. A black body has an emissivity reading equal to 1; a gray body has constant emissivity (but an emissivity reading less than 1); and a nongray body has an emissivity that varies with wavelength but not temperature (figure 1). The fundamental issue in applying infrared thermometers is that real-world objects do not behave like a perfect radiator.

In applications where it may be impossible to view target objects by direct line-of-sight, an infrared sensor with a fiber-optic front end offers a solution.

Consider Field-Of-View

Viewing an object with an infrared thermometer is akin to viewing an object through a telescope with fixed magnification. Like a telescope, a given infrared thermometer has a particular field-of-view that defines the region within which it measures temperature. 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. Field-of-view is expressed as a viewing angle or as the ratio of target distance to target spot-size (D:S ratio). A D:S of 60 corresponds to a viewing angle of about 1°. 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 field-of-view. A critical consideration for single-wavelength (one-color) thermometers is whether the target object fills the instrument's field-of-view. Importantly, if the object does not fill the field-of-view, 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 field-of-view area by a factor of approximately 1.5 or greater. Ratio thermometers do not have this restriction.

Most infrared thermometers allow aiming 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 may include a built-in laser to pinpoint small target objects. The D:S ratio is 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 exceeding 300:1, allowing measurement of target spots smaller than 0.040" at a distance of 12".

In many applications, it is not possible or desirable to contact the object or material being measured.

Ratio Thermometer Sensitivity

Compared to one-color devices, dual-wavelength, two-color or ratio thermometers are less sensitive to emissivity variations and other environmental influences because they measure temperature from the ratio of two signals. Instead of measuring a signal in one wavelength region, ratio thermometers measure in two adjacent 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 absolute measurement signal level to temperature, ratio thermometers calibrate the signal ratio, S1 divided by S2, against temperature. In such a device, the temperature measurement depends on the ratio of the object's emissivity at the two wavelengths, E1 divided by E2. Because of this, ratio thermometers are less sensitive to emissivity variations and absolute emissivity values than one-color units. For this reason, ratio thermometers frequently are used in manufacturing applications with targets exhibiting low or variable emissivity. Ratio thermometers provide accurate temperature measurements given the proper value of o1 divided by E2. Gray- and nongray bodies will exhibit a constant emissivity ratio at two different wavelengths.

Figure 2. Several factors may influence the radiant energy received by an infrared thermometer. In this situation, the thermometer reading corresponds to the temperature of an equivalent black body.

Ratio thermometers also offer additional advantages. If an object between the sensor and the target partially obscures the field-of-view, both detector signals S1 and S2 are reduced the same fractional amount. Thus, the signal ratio 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 occupying less than the full field-of-view; atmospheric effects such as smoke, haze or dust, which obscure the object, and dirt contaminating the sensor's front lens influence S1 and S2 equally. Some ratio thermometers maintain accuracy despite signal-level reduction by as much as 95%. If object, size or other considerations such as physical or atmospheric obstructions preclude using a single-color device, ratio thermometers may be suitable.

It may be impossible to view some target objects by direct line-of-sight. For these applications, an infrared sensor with a fiber-optic front end may be the solution. A small lens assembly for focusing on target objects mounts at the tip of the fiber-optic cable. One can route the flexible fiber-optic cable around obstructions to view targets that are otherwise impossible to view directly. Fiber-optic sensors are available in both single-wavelength and ratio thermometer versions. Also, because the fiber-optic assembly contains no electronic components, it withstands higher ambient temperatures and operates in strong electromagnetic fields. As a result, fiber-optic thermometers often are used in environments too hot for nonfiber-optic units.

Response Time and Signal Processing

Infrared thermometers have good response time when compared to contact sensors. 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 can limit response time. Infrared thermometers have no such limitations. Because of their noncontacting nature, infrared temperature sensors respond almost instantaneously to temperature changes, permitting measurement of fast-moving objects or objects whose temperature changes rapidly. Some infrared thermometers respond to temperature changes in as little as a millisecond.

In many applications, knowing an object's temperature may not be sufficient. When heating a part, the peak temperature attained within a given time interval may be of greater importance than knowing the instantaneous temperature. Infrared thermometers frequently offer signal-processing choices, allowing the unit to indicate maximum temperature, minimum temperature or average temperature within a selected time interval. Response time-related readings coupled with appropriate signal processing is an important consideration in temperature measurement applications with rapidly changing conditions.

Line Scanners and Process Imagers

Unlike point infrared thermometers, line scanners or process imagers have, in effect, a point infrared thermometer viewing an object or web of material through a rotating mirror. This allows multiple temperature points (usually several hundred) to be measured along a scanned line. Each revolution of the mirror sweeps across an object. As material traverses or moves across the instrument's field-of-view, a different line is scanned on each subsequent rotation of the mirror. Acquiring and displaying temperature data from multiple scanned lines allows construction of a two-dimensional temperature map, or thermogram. Such thermograms have color-coded temperatures, allowing one to visualize the actual temperature distribution of the object or material scanned.

Putting Them to Use

Real objects seldom radiate like a black body. 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 may influence the radiant energy received by the infrared thermometer (figure 2):

  • 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.

This represents more of a real-world situation wherein the infrared thermometer reading corresponds to the temperature of an equivalent black body. For this equivalent temperature to represent the object's actual temperature, the user must have some knowledge of the measurement environment to select the sensor best suited to the application.

In most applications, objects freely radiate with known, constant emissivity. Other applications routinely employ ratio thermometers without any knowledge of target emissivity. Most applications do not have complex issues associated with atmospheric absorption, reflected radiation or emissivity variations, making radiation thermometers suitable for temperature measurement in many manufacturing applications. For most applications, using infrared thermometers is simply a matter of point and shoot. As long as one has awareness of what factors could influence accuracy, one will have greater confidence in selecting and using infrared thermometers.