The infrared temperature sensor appears to be rather straightforward: point, press the button and read the temperature. However, measurement results will be quite disappointing without a thorough understanding of the instruments’ principle of operation and specifications.
Temperature measurement instruments can be divided into contact and non-contact types. An infrared temperature sensor used in contact-type instruments include thermocouples, resistance temperature detectors (RTDs), thermistors, and semiconductor temperature sensors. Since contact sensors measure their own temperature they require physical contact with the measured object to bring the sensor body to the object’s temperature.
In some applications, this contact creates problems: The measured object or media may be located at a distance or in a hazardous environment with no easy access. Measurements of moving objects are also difficult. A small object’s temperature may be altered when a relatively large sensor touches it and acts as a heat sink.
Noncontact infrared (IR) thermometers, if used properly, offer convenient solutions for these and many other measurement applications. However, you should select the measuring instrument and measurement techniques to be compatible with the application.
How IR Thermometry works
Heat is transferred from one body to another through conduction, convection, or radiation. Radiation is a process where heat energy in a form of electromagnetic waves is emitted by a hot object and absorbed by a colder object. Most of this radiation is in the infrared (IR) region of the electromagnetic spectrum, but some also spread into the visible light band. The IR wavelength band stretches from 0.7 to 1000 microns, however, practical IR measurement systems use only certain wavelength bands between 0.7 and 14 microns because the radiation is the strongest in this range.
If an object is exposed to IR energy radiated by a heat source, such as an electric heater, light bulb, sun, or other sources, the energy reaching the object is called incident energy. Part of this energy is reflected off the object surface. Theoretically, the object’s coefficient of reflectivity can vary from 0 (no reflection) to 1.0 (100% reflection). Rough, matt surfaces have low reflectivity. Polished and glossy surfaces, especially metals, have high reflectivity.
Depending on the object material, thickness, and the radiation wavelength, part of the radiation can go through the object or be transmitted. The coefficient of transmission can vary from 0 (no energy transmitted through object) to 1.0 (100% energy transmitted through object). High transmittance examples include glass, quartz, plastic film, and various gasses. Materials opaque in the IR spectrum have close to zero transmission coefficients.
The remaining energy is absorbed by the object and raises its temperature. A hypothetical body that has no reflection or transmission and absorbs all incident energy across the entire spectrum has a coefficient of absorption equal to 1.0 and is called a blackbody. Real-life objects, referred to as gray bodies, have coefficients of absorption that fall between 0 and 1.0.
Incident energy, WI, is defined as:
WI = WR + WT + WA
WI = incident energy received by the object, W
WR = energy reflected off the object’s surface, W
WT = energy transmitted by the object, W
WA = energy absorbed by the object, W
As the object absorbs energy and heats, it also emits energy. When an object is in a state of thermal equilibrium, the amount of energy it absorbs (WA) equals the amount of energy it emits (WE): WA = WE. When an object absorbs more energy and its temperature increases, the amount of radiation it emits also increases.
IR thermometry is based on the fact that anybody (solid, liquid, or gaseous) that has a temperature above absolute zero (0oK or -273oC) emits radiant energy. This energy is proportional to the forth power of the body temperature, and the body’s ability to absorb and emit IR energy is called emissivity. Energy radiated by a body can be expressed as follows:
W = E σ T4 A
W = energy, W
E = emissivity
σ = Stefan-Boltzmann Constant = 5.6703 10-8, W/m2K4
T = absolute temperature, oK
A = emitting area, m2
Emissivity can range from 0 to 1 for various bodies. A hypothetical blackbody emits and absorbs all energy and thus has an emissivity equal to 1. Real-life objects have an emissivity between 0 and 1.
When an infrared temperature sensor measures an object’s temperature, consider the energy that actually enters the lens. That is, in addition to emitting energy related to its own temperature, the object may reflect energy coming from another source, or transmit energy passing through it from a source behind it. For accurate measurements, survey the surrounding area for possible sources of extraneous IR radiation and choose the thermometer position and aiming angle to minimize the effects of those sources.
Infrared temperature sensor instrument design varies from simple hand-held thermometers that can be purchased for less than a hundred dollars to complex special-purpose instruments that cost hundreds and even thousands of dollars. However, some building blocks are common for most designs.
A typical infrared thermometer consists of optical components, IR detector, electronics, and a display or interface output stage. Optical parts focus radiation energy onto the IR detector and filter out radiation outside the desired wavelength band. These components include collecting optics, lenses, fiber optics, and spectral optical filters.
The majority of IR detectors are either single-wavelength (also called single-color), or dual-wavelength (also called two-color) type. The single-wavelength detectors measure IR energy within a certain wavelength band, and the instrument calculates object temperature based on the detector output and the preset emissivity. Some thermometers have adjustable emissivity, and most simple units have fixed emissivity.
Dual-wavelength detectors measure energy at two different wavelength bands, and the instrument calculates temperature based on the ratio of the two readings. If emissivity or the energy changes by the same amount at both bands, the measurement accuracy is not affected. Emissivity or the amount of radiated energy may change due to object change or movement, lens contamination or misalignment, or view obstruction. The dual-wavelength detector’s drawback is higher cost and lower accuracy under certain conditions.
The emissivity of many materials and surfaces remains relatively constant over the IR wavelength range, and measuring energy in any narrower band will be acceptable. Other materials have wavelength bands with higher and lower emissivity due to high reflectivity or transmission and require narrowband detectors tuned to high emissivity wavelengths.
Another factor is the atmosphere. Its transmission coefficient vs. wavelength curve has many peaks and valleys, which swing from almost 1.0 to near zero and block the IR energy transmission. Most general-purpose infrared temperature sensor use the largest high-transmission band between 7 and 14 microns to minimize atmospheric attenuation.
To measure temperature of objects with emissivity that varies greatly over the IR wavelength spectrum and objects obscured by glass, smoke, steam, or other barriers, engineers need to use narrow band IR detectors. For example, short-wavelength detectors handle variable emissivity objects, lens contamination, and measurements through glass windows. Long-wavelength detectors are more prone to errors due to emissivity changes, but have a wide temperature range.
Special applications, such as measuring the temperature of glass, crystal, flame, gas, and thin film require detectors with specific narrow bands. For example, detectors with a narrow band centered on 5 microns give the best results when measuring glass temperature. Metals and metallic foils usually require 1 micron detectors where they have the highest level of radiation.
Based on the principle of operation, IR detectors fall into one of two categories: thermal detectors and photo detectors (photodiodes). Thermal IR detectors absorb the incident energy, raise the sensing element temperature, and change the detector’s electrical properties: thermopiles generate thermoelectric voltage, bolometers change resistance, and pyroelectric devices change their polarization. In general, they are slower than photo detectors.
A thermopile is made by connecting several thermocouples in series and placing their hot junctions in contact with a black body that absorbs the incident IR energy and heats the hot junctions. The cold junctions are placed in the area of the detector with adequate heat sinking. These detectors have fast response, broad band, large dynamic range, and are frequently used in general-purpose, automotive, air conditioning, and human-body thermometers.
Bolometers use a slab of material that changes its resistance in response to a change of temperature. The circuit converts resistance change to a voltage change, which is further processed by the instrument. Bolometers are frequently used for measuring low-level IR energy, often as an attachment to a telescope.
Pyroelectric devices become electrically charged when their body temperature changes. To produce a usable signal, the incident IR energy has to “pulse”. The output peak-to-peak AC signal is proportional to the pulse energy. Since energy emitted by measured objects is usually steady, thermometers that use pyroelectric detectors have a mechanical or optical chopper in front of the sensor. These sensors are used in many home security systems.
Photo detectors are built on a silicon substrate with an IR sensitive area that releases free electrons when impacted by the photons. The flow of electrons produces electrical signals proportional to the incident energy. These detectors are often used as arrays in thermal imaging systems.
A detector needs protection from the environment, and the selected window material must allow the correct wavelength band to pass through with minimum attenuation. A zinc sulfide or germanium window is best for the long-wavelength detectors, glass is suitable for short-wavelength detectors, and quartz for the mid-wavelength spectrum. Some instruments use a fiber-optic light guide to direct the radiation to the detector.
Since all types of IR detectors produce signals in the microvolt range, a high-gain amplifier should follow the detector. Detector output vs. temperature curves are not linear and fluctuate greatly with a change in ambient temperature. To remedy this a signal-conditioning circuit stabilizes the temperature and linearizes the signal. Many applications require an analog-to-digital converter (ADC) to convert the temperature reading to a digital format.
Hand-held and many other instrument types have a built-in display, while other devices connect to a computer, data acquisition system, or temperature control system via an RS232 or RS-485 cable. Some instruments simulate a thermocouple output, others have a 0 – 20 mA or 4 – 20 mA current loop, or voltage output.
The key specifications and considerations for any infrared temperature sensor application are field of view (FOV) and distance; spectral band; response time; accuracy and repeatability; emissivity of the object or media being measured; media between the object and infrared temperature sensor, such as vacuum, air, steam, gas, glass, or other; object temperature range; mounted or hand-held application; and type of output signal or display.
The FOV characterizes the diameter of a circle (target) that the IR detector will “see” at a certain distance from the measured surface. However, there is always a minimum target diameter that depends on the optical system and detector size. The detector measures and averages temperatures of all objects within the target area. FOV is commonly called distance-to-spot size ratio and is a ratio of the distance between the meter and the target to the diameter of the target.
For example, a 10:1 distance-to-spot size ratio means that if a measured surface is located 10 inches from the thermometer it will measure and average the temperature of a circle with 1 in.diameter. Move the thermometer away to 20 in. and the target will increase to 2 in., and so on. A thermometer with a 1:1 ratio will measure within a one-foot diameter circle when held one foot away from the target.
Thermometers designed for measurements of small areas have a very narrow FOV and measure temperatures of objects less than a tenth of an inch. For example, such a thermometer held near a component on a pc board will measure the temperature of just that component and ignore the
components around it.
Other optical systems allow accurate temperature measurement of a spot several inches in diameter at a distance of tens of feet. However, such measurements require accurate pointing. Though notches on top of the instrument provide some help, aiming lights and built-in laser pointers prove to be most helpful.
Unfortunately, a laser pointer may occasionally lead to erroneous measurements if the user is not familiar with the IR thermometer operation and the FOV concept. Some first-time users mistakenly think that the laser beam that they see has something to do with the process of measuring temperature. They presume that the instrument displays the temperature of the tiny spot where the laser beam meets the surface. Such measurements will not yield satisfactory results.
- Avoid degrading measurement accuracy by environmental elements, such as dirt, dust, smoke, steam, other vapors, extremely high or low ambient temperatures, and electromagnetic interference from other devices.
- Select an infrared temperature sensor with a wavelength band compatible with the measured object (especially high reflectivity objects) and with the media between the thermometer and measured object (especially glass, smoke, or steam).
- Select an instrument with a temperature range not much greater than the maximum application temperature. Wider than needed temperature ranges lead to lower accuracy or higher instrument cost.
- An infrared temperature sensor averages the temperature of all objects within its field of view: Select the instrument with an appropriate FOV, and calculate the proper distance so that only the desired area is measured.
- Avoid hot objects near the measured object. They radiate energy that can be reflected or transmitted by the measured object into the thermometer FOV.
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