Basics of Infrared Measurement

Why Infrared Measurements?

Infrared measuring instruments provide large advantages with regard to measuring tasks that cannot be solved with conventional contact thermometers. Examples:

  • Measurements of very high temperatures not allowing the use of thermocouples.
  • Measurements at surfaces with low thermal conduction and bodies with low thermal capacity.
  • Measurements at moving, inaccessible or live parts with a high rate of response (<1s).
  • Measurements at objects, which must not be influenced by contact measurements.

What is Infrared Radiation?

Every substance with a temperature above absolute zero emits an infrared radiation (spectral range of wavelengths from 0.7 to 1000µm) that corresponds to its temperature. This range is located below the longer red wavelength range and is not visible to the human eye. For measurements the most interesting range is located between 0.7 and 20µm.
The infrared radiation emitted by the test object follows the known optical rules and, therefore, can be deviated, bundled with lenses or reflected from catoptric elements.
The emissivity of a test object indicates how much infrared energy has been absorbed or released by radiation. The value can be between 0 and 1.0. The fact that the emissivity depends on the wavelength is relevant for measurements. With increasing object temperature the radiation maximum shifts to the short wave range. Therefore, IR thermometers are equipped with filters, which allow only one particular wavelength to pass through for the measurement. The spectral range for specific materials must be considered for the application.

How Infrared Thermometers Operate

The optical system of an infrared thermometer captures the energy emitted from a circular measuring spot and focuses it onto a detector. A material with a high transmission factor is used for the lenses. The energy captured by the detector is electronically amplified and converted into an electrical signal. The optical resolution results from the ratio of the measuring distance to the size of the measuring spot. The measuring spot must always be smaller than the test object or the measuring point of interest. The higher the optical resolution the smaller the measuring spots can be measured at further distances.

Special Infrared Pyrometers

Ratio Pyrometers determine the temperature from the ratio of the energy radiated in each of two wavelength ranges. This method allows for exact measuring results, even in case of a limited view to the test object due to vapour, steam, dust, dirty windows or lenses (up to 95% reduction of meas. signal). Furthermore, test objects, which are smaller than the measuring spot (e.g. measurement at wires), or low or varying emissivities at fast moving objects, do not affect the measuring result.
Line Scanners measure the object temperature along a line. Fixed installed line scanners provide coloured heat flow charts from a product passing under the measuring head (e.g. conveyors, rotary furnaces), but can also be moved to pass above objects (e.g. heat flow chart of a house wall). The infrared scanner measuring head AMiR 7880 scans up to 256 dots over an angle of 90°. 20 lines can be scanned within one second. One measuring tape can be divided into 3 sectors, side by side or overlapping.

What You Should Consider For Infrared Measurements

What to do in case of dust, vapour and aerosols at the measuring point?
If the atmosphere at the measuring point is contaminated with dust, vapour and aerosols, the radiation energy impinging on the sensor can be influenced by contaminated lenses. This can be avoided by using an air blow attachment that keeps the lens clean.

What to do in case of high ambient temperatures?
If the ambient temperature exceeds the temperature specified for the measuring head of the IR sensor, the measuring head must be protected by mounting an air or water cooling system along with an air blow attachment (to avoid water condensing on the lens). Furthermore, cables and cable routings with high temperature stability must be used.

What to do in case of heat sources located next to the measuring object?
If heat sources are located next to the test object, these can transmit or reflect additional energy. Such ambience radiations occur, for example, at measurements in industrial furnaces where the wall temperature is often higher than the temperature of the test object. Many infrared instruments allow for a compensation of the ambient temperature.

What to do in case of measurements in a vacuum?
In case of vacuum furnaces and similar applications it is necessary to mount the measuring head outside of the vacuum area and to perform the measurement through a window. When selecting the measuring window the transmission values of the window must match the spectral sensitivity of the sensor. Quartz glass or quartz are typically used for high temperatures. In case of low temperatures within the 8 to 14µm band the use of a special material, which is translucent for IR, is necessary, e.g. germanium, amtir, zinc selenide or sapphire. When selecting the window the temperature requirements, window thickness and pressure difference, as well as the possibility of keeping the window on both sides clean, must be considered. It might be advisable to consider an additional antireflective coating an the window on the window to increase the transmission capacity. Furthermore, it must be considered that not all window materials are translucent in the visible range.

Why is the emissivity so important?
In case of ideal radiators the reflected and transmitted energy equals zero and the emitted energy corresponds 100% to the characteristic temperature. However, many bodies emit less radiation at the same temperature (non-selective radiator). The ratio of real radiation value and that of the ideal radiator is defined as the emissivity ε. For example, a mirror has an emissivity of 0.1 while a so-called ´black body´ has an emissivity of 1.0. Many nonmetals such as wood, rubber, stone, and organic materials have only low reflecting surfaces and, as a result, high emissivities between 0.8 and 0.95. However, metals, especially if they have glossy surfaces, can have ε = 0.1. Therefore, IR thermometers provide an option for setting the emissivity. The emissivity should be known as exact as possible. If a too high emissivity has been set, the indicated temperature is lower than the actual temperature, given that the temperature of the test object is higher than the ambient temperature. For example, if 0.95 has been set, while the emissivity is actually only 0.9, a temperature that is lower than the actual temperature will be indicated.

How can the emissivity be determined?
Several methods can be used to determine the emissivity. As a first starting point, the following emissivity table can be consulted. The table data only represents average values, as the emissivity of a material is influenced by various factors. These include: temperature, angle of measurement, surface geometry (plane, concave, convex), thickness, surface quality (polished, rough, oxidised, sand-blasted), spectral range of the measurement and transmission capacity (e.g. in case of thin plastic foils)

Application Examples for Infrared Thermometers

Temperature Range Spectral Sensitivity Application Examples
appr. 0 … 800°C

8 to 14 µm All non-metals, wood, paper, textiles, floor coverings, asphalt
3 to 5 µm lime floor, edibles, pharmaceuticals, as well as use with print
7 to 15 µm coating, laminating, drying/hardening, wave soldering and reflow soldering
7 to 18 µm for indoor installations, fire control, dust tips etc.
appr. 10 … 360°C nominal 7,9 µm Fabrication and processing of polyester foil, fluoroplastics, fluoropolymer, acrylate, nylon (polyamide), acetylene cellulose, polyamides, polyurethanes, PVC, polycarbonates.
appr. 260 … 1650°C nominal 5,0/5,2 µm Surface measurement on glass for heating up, forming, sealing, laminating
appr. 200 … 1200°C 3,9 µm Metal finishing, furnaces, melting furnaces, blast furnaces, measurements on thick glass. Measurements slightly influenced by CO2 atmosphere (combustion gases)
appr. 30 … 340°C nominal 3,43 µm Fabrication and processing of polyethylene, polypropylene, polystyrene and other foils
appr. 400 … 3000°C 2 to 2,7 µm Processing of ferrous and nonferrous metals, induction furnaces, glass production, melting furnaces, lab research
appr. 200 … 1800°C 1,6 µm Heat treatment of steel, bending, hardening, warming up
appr. 500 … 3000°C 1 µm Steel production, molten baths, for highest precision with shaping, casting and processing of metals, as well as the processing of glass, ceramics, semiconductors and chemicals

Compact Glossary of Important Terms

Atmospheric Windows: The wavelength ranges within the infrared spectrum, in which the atmospheric radiation energy is transmitted and the atmospheric absorption is minimal, approximately 3 … 5µm and 8 … 14µm.

Focal Point, Focal Distance: Measuring distance where the maximum optical resolution is reached.

Far Field: Measured distance, which is significantly larger than the focal length of a device, in most cases is larger than ten times the focal length.

Field of View: The test object area, which is measured by the infrared thermometer; the diameter of the measuring spot is proportioned to the distance from the test object; often also specified as an angular variable at the focal point. Also see optical resolution.

Non-Selective Radiator: Radiating body with an emissivity that, for all wavelengths, bears the same constant ratio to the emissivity of a full radiator at the same temperature, which is opaque to radiation of infrared energy.

Background Temperature: From the view of the measuring instrument the ambient temperature or the temperature behind the test object.

Measuring Spot: Diameter of the test object area, which is subject to a temperature measurement; the measuring spot is defined by the circular area, which typically allows to capture 90% of the infrared energy radiating from the test object to the optical receiving aperture of the measuring instrument.

Optical Resolution: Also called the distance ratio: The ´measuring distance/measuring spot size´ ratio (distance ratio E:M) of an IR measuring spot. The measuring distance is typically defined as the distance from the focal point and the measuring spot size as the diameter of the IR measuring spot measured at the focal point (typically the 90% energy measuring spot diameter). The optical resolution can be also defined for the far field, by using the values for the measuring distance and measuring spot size within the far field.

Degree of Reflection: Ratio of the radiation energy reflected from a surface to the incident radiation of the same surface; for a perfect mirror the value is approximately 1, for a full radiator the reflection is zero.

Full Radiator: Also: black body; ideal radiator. Body, which absorbs the whole impinging radiation energy of all wavelengths and which does not reflect nor transmit any radiation. The surface of a full radiator has a uniform emissivity of 1.

Spectral Sensitivity: Wavelength range for which an infrared thermometer is sensitive.