BASIC PRINCIPLES of non-contact temperature measurement
By Optris 
1. Physical principles
2. Emissivity and temperature measurement
3. Optics, sighting techniques and electronics
4. Infrared thermometers and applications
5. Infrared cameras and applications
- Physical principles
With our eyes we see the world in visible light. Although visible light makes up only a small part of the radiation spectrum, the invisible light covers most of the remaining spectral range. The radiation of invisible light carries much more additional information.
Discovery of the infrared radiation
Searching for new optical material, William Herschel accidentally rediscovered the infrared radiation in 1800. He blackened the tip of a sensitive mercury thermometer and used it as measuring system to test the heating properties of different colors of the spectrum, which were directed to a tabletop by having beams of light shine through a glass prism. With this, he tested the heating of different colors of the spectrum. When he moved the thermometer in the dark area beyond the red end of the spectrum, Herschel noticed that the temperature continued to rise. The temperature rose even more in the area behind the red end of the spectrum. He ultimately found the point of maximum temperature far behind the red area. Today this area is called “infrared wavelength area”.
The infrared temperature measurement system
Each body with a temperature above absolute zero (–273,15 °C = 0 Kelvin) emits electromagnetic radiation from its surface, which is proportional to its intrinsic temperature. A part of this intrinsic radiation is infrared radiation, which can be used to measure a body’s temperature. This radiation
penetrates the atmosphere. With the help of a lens (input optics) the beams are focused on a detector element, which generates an electrical signal proportional to the radiation. The signal is amplified and, using successive digital signal processing, is transformed into an output signal proportional to the object temperature. The measuring value may be shown in a display or released as analog output signal, which supports an easy connection to control systems of the process management.
The advantages of non-contact temperature measurement are obvious – it supports:
• Temperature measurements of moving or overheated objects and of objects in hazardous surroundings
• Very fast response and exposure times
• Non-interactive measurement, no influence on the measuring object
• Non-destructive measurement
• Measurement point durability, no mechanical wear
The electromagnetic radiation spectrum
In a literal and physical sense, a spectrum is understood as the intensity of a mixture of electromagnetic waves that function as wavelength or frequency. The electromagnetic radiation spectrum covers a wavelength area of about 23 decimal powers and varies from sector to sector in origin, creation and application of the radiation. All types of electromagnetic radiation follow similar principles of diffraction, refraction, reflection and polarization. Their expansion speed corresponds to the light speed under normal conditions:
The result of multiplying wavelength with frequency is constant: λ · f = c
The infrared radiation covers a very limited part in the whole range of the electromagnetic spectrum: It starts at the visible range of about 0.78 μm and ends at wavelengths of approximately 1000 μm. Wavelengths ranging from 0.7 to 14 μm are important for infrared temperature measurement. Above these wavelengths the energy level is so low, that detectors are not sensitive enough to detect them.
In 1900 Max Planck, Josef Stefan, Ludwig Boltzmann, Wilhelm Wien and Gustav Kirchhoff precisely defined the electromagnetic spectrum and established qualitative and quantitative correlations for describing infrared energy.
The black body
A black body is an abstracted physical body, which absorbs all incoming radiation. It has neither refective nor transmissive properties.
= ε = 1 (α absorption, ε emissivity)
A black body radiates the maximum energy possible at each wavelength. The concentration of the radiation does not depend on angles. The black body is the basis for understanding the physical principles of non-contact temperature measurement and for calibrating infrared thermometers.
The construction of a black body is simple. A thermal hollow body has a small hole at one end. If the body is heated and reaches a certain temperature, and if temperature equilibrium is reached inside the hollow room, the hole ideally emits black radiation of the set temperature. For each temperature range and application purpose the construction of these black bodies depends on material and the geometric structure.
If the hole is very small compared to the surface as a whole, the interference of the ideal state is very small. If you point the measuring device on this hole, you can declare the temperature emitting from inside as black radiation which you can use for calibrating your measuring device. In reality, simple systems use surfaces, which are covered with pigmented paint and show absorption and emissivity values of 99 % within the required wavelength range. Usually, this is sufficient for calibrations of actual measurements.
Radiation principles of a black body
The radiation law by Planck shows the basic correlation for non-contact temperature measurements: It describes the spectral specific radiation Mλs of the black body into the half space depending on its temperature T and the wavelength λ.
MλS = –––––– –––––––– = ––– ––––––––
2πhc2 1 C1 1
λ5 ehc/λkT –1 λ5 eC2/λT –1
c speed of light
C1 3.74 * 10 –16 W m2
C2 1.44 * 10 –2 K m
h Planck‘s constant
k Boltzmann constant
The following illustration shows the graphic description of the formula depending on λ with different temperatures as parameters.
With rising temperatures the maximum of the spectral specific radiation shifts to shorter wavelengths. As the formula is very abstract it cannot be used for many practical applications. But, you may derive various correlations from it. By integrating the spectral radiation intensity for all wavelengths from 0 to infinite you can obtain the emitted radiation value of the body as a whole. This correlation is called Stefan Boltzmann law.
MλS = σ · T4 [W · m-2] σ = 5.67 · 10–8 Wm–2 K–4
The entire emitted radiation of a black body within the overall wavelength range increases proportional to the fourth power of its absolute temperature. The graphic illustration of Planck’s law also shows that the wavelength, which is used to generate the maximum of the emitted radiation of a black body, shifts when temperatures change. Wien’s displacement law can be derived from Planck’s formula by differentiation.
λ max · T = 2898 μm · K
The wavelength, showing the maximum radiation, shifts with increasing temperature towards the range of short wavelengths.
The gray body
Only few bodies meet the ideal of the black body. Many bodies emit far less radiation at the same temperature. The emissivity ε defines the relation of the actual radiation value and that of the black body. It is between zero and one.
The infrared sensor receives the emitted radiation from the object surface, but also reflected radiation from the surroundings and potentially infrared radiation that has been transmitted through the black body.
ε + ρ + τ = 1
ost bodies do not show transmissivity in infrared. Therefore the following applies:
ε + ρ = 1
Construction and operation of infrared thermometers
The illustration shows the basic construction of an infrared thermometer. Using input optics, the emitted infrared radiation is focused onto an infrared detector. The detector generates an electrical signal that corresponds to the radiation, which is subsequently amplified and can be used for further processing. Digital signal processing transforms the signal into an output value proportional to the object temperature, which is then either shown on a display or provided as an analog signal.
To compensate environmental temperature influences, a second detector records the temperature of the measuring device or its optical channel. The calculation of the temperature of the measuring object is done in three basic steps:
1. Transformation of the received infrared radiation into an electrical signal
2. Compensation of background radiation from device and object
3. Linearization and output of temperature information
In addition to the displayed temperature value, the thermometers also support linear outputs such as 0/4 – 20 mA, 0 – 10 V and thermocouple elements, which allow easy connection to process management control systems. Furthermore, due to internal digital measurement processing, most of the currently used infrared thermometers also feature digital interfaces (e.g. USB, RS485, Ethernet) for data output and to enable access to device parameters.
The most important element in each infrared thermometer is the radiation receiver, also called detector. There are two main groups of infrared detectors.
1. Thermal Detectors
Bolometer FPA (for IR cameras)
2. Quantum Detectors
With these detectors, the temperature of the sensitive element changes due to the absorption of electromagnetic radiation. The temperature chance causes a modification of the temperature-dependent property of the detector, which is electrically analyzed and serves as a measure for the absorbed energy.
Radiation thermocouple elements (thermopiles)
If the connection point between two different metallic materials is heated, the thermoelectrical effect results in an electrical voltage. The contact temperature measurement has been using this effect for a long time with the help of thermocouple elements. If the connection is warm because of absorbed radiation, this component is called radiation thermocouple. The illustration shows thermocouples made of bismuth / antimony which are arranged on a chip round an absorbing element. In case the temperature of the detector increases, this results in a proportional voltage, which can be caught at the end of the bond isles.
The illustration shows the basic construction of a pyroelectric detector. This sensitive element consists of pyroelectric material with two electrodes. As a result of the temperature change of the sensitive detector element, caused by the absorption of infrared radiation, the surface loading changes due to the pyroelectric effect. The so created electric output signal is processed by a preamplifier. Due to the nature of how the loading is generated in the pyroelectric element, the radiation flow has to be continuously and alternately interrupted. The advantage of the frequence selective preamplifying is a better signal-to-noise ratio.
Bolometers exploit the temperature dependency of electric resistance. The sensitive element consists of a resistor, which changes when it absorbs heat. The change in resistance leads to a changed signal voltage. The material should have a high temperature factor of the electrical resistance in order to achieve high sensitivity and high specific detectivity. Bolometers that operate at room temperature use the temperature coefficient of metallic resistors (e. g. black layer and thin layer bolometer) as well as of semiconductor resistors (e. g. thermistor bolometers).
Nowadays, infrared imagers are based on the following technological developments:
The semiconductor technology replaces mechanical scanners. FPAs (Focal Plane Arrays) are produced on the basis of thin layer bolometers. Consequently VOX (Vanadium oxide) or amorphous silicon are used as alternative technologies. These technologies significantly improve the price-performance ratio. Today, common detector sizes are 160 x 120, 320 x 240 and 640 x 480 pixels.
The decisive difference between quantum detectors and thermal detectors is their faster reaction on the absorbed radiation. The mode of operation of quantum detectors is based on the photo effect. The visible photons of the infrared radiation lead to an increase of the electrons into a higher energy level inside the semiconductor material. When the electrons fall back, an electric signal (voltage or power) is generated. Also, a change of the electric resistance is possible. These signals can be precisely evaluated. Quantum detectors are very fast (ns to μs).
The temperature of the sensitive element of a thermal detector changes relatively slowly. Time constants of thermal detectors are usually bigger than time constants of quantum detectors. Roughly approximated, one can say that time constants of thermal detectors can be measured in milliseconds whereas time constants of quantum detectors can be measured in nanoseconds or even microseconds. Despite the fast development in the field of quantum detectors, there are many applications where thermal detectors are more suitable. That is why they share an equal status with quantum detectors.
Transformation of infrared radiation into an electrical signal and calculation of the object temperature
Since per the Stefan Boltzmann law, the electric signal of the detector is as follows:
U ~ εTobj4
As the reflected ambient radiation and the self-radiation of the infrared thermometer must also be considered, the formula is as follows:
U = C · [εTobj4 +(1 – ε) · Tamb4 – Tpyr4]
U Detector signal
Tobj Object temperature
Tamb Temperature of backround radiation
Tpyr Temperature of the device
C Device-specific constant
ρ = 1 – ε Reflection of object
Since infrared thermometers do not cover the total wavelength
range, the exponent n depends on the wavelength λ.
At wavelengths ranging from 1 to 14 μm.
n is between 17 and 2 (at long wavelengths between 2 and 3
and at short wavelengths between 15 and 17).
U = C · [εTobjn +(1 – ε) · Tambn – Tpyrn]
Thus the object temperature is determined as follows:
Tobj = n √–––––––––––––––––––––––––––––––
The results of these calculations for all temperatures are stored as curve band in the EEPROM of the infrared thermometer. This guarantees quick access to the data and fast calculation of the temperature.
The formula shows that the emissivity ε is essential, if you want to determine the temperature with radiation measurement. The emissivity measures the ratio of thermal radiation, which is generated by a gray and a black body of equal temperature. The maximum emissivity for the black body is 1.
A gray body is an object, that has the same emissivity at all wavelengths and emits less infrared radiation than a black radiator (ε <1). Bodies with emissivities, which depend on the temperature as well as on the wavelength, are called nongray or selective bodies (e.g. metals).
Calibration of infrared thermometers
Infrared thermometers are calibrated with the help of reference radiation sources, so called black bodies. These radiation sources are able to produce different temperatures with a high stability (see also page 5, The black body). Knowing the exact value of the radiation temperature is essential for the calibration process. It can be measured by either using a contact thermometer (in combination with the determination of the emissivity) or a transfer standard infrared thermometer. This value can then be used to determine the device constant for an initial calibration of the infrared sensors. In order to conduct post-calibration by customers or local calibration facilities, the calibration temperature should be close to the temperatures which occur during the respective applications.
Optris uses the transfer standard radiation thermometer LSPTB (see image) to measure the radiation temperature of a reference source. The LS-PTB is based on the portable IR thermometer optris® LS. The LS-PTB must be traceable to the International Temperature Scale from 1990 (ITS-90). It is calibrated by the PTB (German National Metrological Institute) in regular intervals.
ITS-90 is a very good approximation of thermodynamic temperature. It is based on 17 well-reproducible fixed values such as melting points of highly purity metals. Within the scope of ITS-90, the LS-PTB is compared to PTB national temperature standards within a closed chain of comparative measurements with known uncertainty.
Based on the LS-PTB, Optris produces the LS-DCI as a high-precision reference IR thermometer for its customers. The DCI units are produced with pre-selected components chich ensure high measurement stability. In combination with dedicated calibration at three calibration points, the LS-DCI achieves higher accuracy at these reference points.
The optics of an IR thermometer is described by the distanceto-spot-ratio (D:S). Depending on the quality of the optics, a certain amount of radiation is also received from sources outside the specified measurement spot. The maximum value here equals the radiation emitted by a hemispheric radiant
source. The respective signal change in correlation with resizing the radiation source is described as the size-of-source effect (SSE).
As a result of this correlation, all manufacturers of IR thermometers use accurately defined geometries for the calibration of their units; meaning depending on the aperture of the radiation source (A) a distance (a) between the IR thermometer and the reference source is defined. Thus, the value specified in datasheets and technical documentation as measurement field is generally a specific defined percentage of this radiation maximum – values of 90 % or 95 % are common.
3. Optics, sighting techniques and electronics
Construction of the infrared thermometers
Infrared thermometers have various configurations and designs, which differ in optics, electronics, technology, size and housing. Despite these differences, the signal-processing chain is always the
same: It starts with an infrared signal and ends with an electronic temperature output signal.
Lenses and windows
The measuring chain begins with an optical system – usually consisting of lens optics. The lens receives the emitted infrared energy from a measuring spot and focuses it onto a detector. Measurements based on this technology can only be correct, if the measuring object is bigger in size than the detector spot. The distance ratio describes the size of the measuring spot at a specific distance. It is defined as D:Sratio: relation of measuring distance to spot diameter. The optical resolution improves with increasing values of the D:S ratio.
Depending on their material, infrared lenses can only be used for a certain wavelength range. The following chart presents typical lenses and window materials for infrared thermometers with their corresponding wavelength.
Some measurements make it necessary to take the temperature through an appropriate measuring window, as in closed reaction containers, ovens or vacuum chambers. The transmissivity of the measuring window should match the spectral sensitivity of the sensor. Quartz glass is suitable for
high measuring temperatures, while special materials like germanium, AMTIR or zink selenide should be used for low temperatures in the spectral range between 8 – 14 μm.
The following parameters should also be considered when selecting a window: diameter of the window, temperature conditions and maximum pressure difference. A window of 25 mm in diameter, which has to resist a pressure difference of 1 unit of atmosphere, should be 1.7 mm thick. To focus the sensor on the measuring object for measurements in, for example, vacuum container, it makes sense to use window material, that is also transparent in the visible range.
Windows with anti-reflection coating have significantly higher transmissivity (up to 95 %). The transmission loss can be corrected with transmissivity adjustment on the window, providing that the manufacturer has specified transmissivity for the corresponding wavelength range. Otherwise, it must
be experimentally determined with an infrared thermometer and a reference source.
Latest trends in sighting techniques
New measuring principles and sighting techniques enable greater accuracy in the use of infrared measuring devices. Innovations in the field of solid state lasers are adapted by using multiple laser systems to mark spot sizes. As a result, actual spot sizes inside the object field are displayed using laser crosshair technology. In other devices, video camera chips replace optical sighting systems.
Development of high-performance optics in combination with crosshair laser sighting technologies
Simple, cost-effective portable infrared thermometers use single spot laser pointers in order to mark the center of the spot with a parallax default. Applying this technique, the user has to estimate the spot size with the help of the spot size diagram and the likewise estimated measuring distance.
If the measuring object covers only a part of the measuring spot, temperature rises are only displayed as an average value of hot area and ambient cold area. If, for example, the higher resistance of an electric connection due to a corroded contact results in an overheating, this rise in temperature will only be shown as mnor heating for smaller objects and oversized spot dimensons, and the potential danger of the situation will not be recognized.
In order to correctly display spot size, optical sighting systems were developed with size marking in the crosshairs, which enable precise targeting. Since laser pyrometers are significantly easier and safer than contact thermometers, engineers have tried to mark the spot size with laser sighting techniques independently from the distance – according to the distance-spot-size ratio in the diagram.
Two warped laser beams approximately show the narrowing of the measuring beam and its broadening in longer distances. However, the diameter of the spot size is only indicated by two spots on the outer circumference. Due to the design, the angle position of these laser points on the measuring circuit moves, which makes aiming difficult.
One advancement are video pyrometers, which enable precise measuring field marking with help of a simultaneous use of a video module and a crosshair laser sighting technology.
5. Thermographic cameras and applications
What web cams and IR cameras have in common The ability to see local warming and consequently detect weak points in our environment has always been a fascinating aspect within modern thermal imaging technology. Increasingly efficient manufacturing technologies for IR optical image sensors have not only resulted in drastically improved price-performance ratio.
The devices have become smaller, more durable and more economic in their power consumption. Thermographic measuring systems have been available for quite some time, which – similar to a traditional web-cam – are operational with only a USB port.
Thermographic cameras function like normal digital cameras:
They have a sighting area, the so called field of view (FOV), which can vary between 6° (telescopic optic) and 90° (wide angle optic). Most standard optics work with a 26° FOV. The more distant the object, the larger the observed area will be, and with it also the part of the image is, which represents
a single pixel. The advantage of this circumstance is that the radiation density is independent from the distance for sufficiently large measuring surfaces. Temperature measurements are therefore largely uninfluenced by the distance to a measuring object.
In the long-wave infrared range, thermal radiation can only be focused with lenses made of germanium, germanium alloys, zinc salts or with surface mirrors. When compared to conventional, mass-serial produced lenses in the visible spectral range, such hardened and tempered lenses still represent a significant cost factor for thermal imagers. They are designed as spherical three lens or aspheric two lens versions and, especially for cameras with exchangeable lenses, each lens must be calibrated for every single pixel in order to obtain correct measurements.
Thermal imager with power supply via USB of a tablet PC Measurement field of the thermal imager optris® PI with the standard 23° x 17°
The core of almost all globally used thermographic systems is a focal plane array (FPA), an integrated image sensor with sizes of 20,000 to 1 million pixels. Each individual pixel is a microbolometer with the dimensions 17 x 17 μm2 to 35 x 35 μm2. Thermal radiation heats such 150 nanometer thin thermal
detectors within 10 ms to about a fifth of the temperature difference between object and chip temperature. This extremely high sensitivity is achieved by a very low thermal capacity in connection with a superb insulation to the evacuated environment. The absorption of the semitransparent receiver area is improved by the interference of the transmitted and on the surface of the read out circuit reflected light wave with the succeeding light wave.
To exploit this effect of self interference, the etched vanadium oxide or amorphous siliconbolometer surface must be positioned about 2 μm distance from the read-out circuit. The surface and bandwidth specific detectivity of the described FPAs achieve values of 109 cm Hz1/2 / W. It is therefore one magnitude better than other thermal detectors, which are used for example in pyrometers.
Changes to the bolometer’s intrinsic temperature changes its resistance, which is transformed into an electrical voltage signal. Fast 14 bit A/D converters digitize the previously amplified and serialized video signal. A digital signal processor calculates a temperature value for each pixel and generates the known false color images in real time.
Thermal imaging cameras require a rather complex calibration, in which a number of sensitivity values are allocated to each pixel at different chip and black body temperatures. To increase measuring accuracy, bolometer FPAs are often stabilized at defined chip temperatures with high control accuracy.
Due to the development of better performing, smaller and at the same time less expensive laptops, UMPCs, netbooks and tablet PCs, it is currently possible to use their
• big displays for attractive thermal image presentations,
• optimized Li-Ion rechargeable batteries as power supply,
• computation capacity for flexible and high-quality real time signal display,
• large memories for practically unlimited infrared video records and
• Ethernet, Bluetooth, WLAN and software interfaces for the integration of the thermographic system into their application environment.
The standardized USB 2.0 interface is available everywhere and assures data transmission rates of
• 30 Hz with 320 x 340 pixel image resolution and • 120 Hz with image sizes of 20.000 pixel.
The USB 3.0 technology is even suitable for XGA thermal image resolutions up to 100 Hz video frequency. The use of the webcam principle in the field of thermography enables totally new product features with significantly improved price performance ratio. The infrared camera is connected via a 480 MegaBaud interface in real time with a Windows© based computer, which simultaneously supplies the required power.
Thermal analysis software guarantees flexibility
No driver installation is required, since USB IR cameras use the standard USB video class and HID driver already integrated in Windows XP and higher. The single pixel related real-time correction of the video data and temperature calculation are done on a PC. The impressive image quality at only 20,000 sensor pixels is achieved by a complex software-based rendering algorithm, which calculates temperature fields in VGA format.
The application software is characterized by a high flexibility and portability. In addition to standard functions, the software also has the following advanced features:
• Numerous data and thermal imaging export functions to support reports and offline analyses
• Mixed scalable color pallets with isotherms
• Freely positionable profile display
• Unlimited number of measuring areas with separate alarm options
• Comparative video displays based on reference images, temperature/ time diagrams for different regions of interest
The software also provides a layout mode, which saves and stores different modes of presentation. An integrated video software enables radiometric AVI (.ravi) file editing. These file types can also be analyzed offline using software with multiple parallel usability. The video acquisition modes also allow the intermittent recording of slow thermal processes and their fast display.
The transfer of real time data to other programs is done using a comprehensively documented DLL as part of a software development kit. All other camera functions can be controlled via this DLL interface. Alternatively, the software can communicate with a serial port and, for example, directly connected to an RS422 interface.
In the next chapter, five typical applications are discussed, which describe the wide range of USB infrared camera applications.
1. Manufacturing process optimization
The production of plastic parts like PET bottles requires a defined temperature increase of the so-called preforms in order to guarantee a homogeneous material thickness during blow molding. Test runs are done with only a few of the 20 mm thick blanks at full working speed of about 1 m/s.
In order to measure the temperature profile of a preform, a video sequence with 120 Hz must be recorded since the moment when the blanks are in the field of view can vary. The camera is positioned in such a way that it follows the motion of the material in an oblique angle – similar to the view of the last wagon of a moving train. The IR video sequence ultimately delivers the temperature profile, which is essential for the adjustment of heating parameters.
During vacuum forming of large scale plastic parts for refrigerators, video recordings enable the exact measurement of the cooling behavior at various areas of form pieces. Different cooling speeds may result in material warping. In addition, cooling speed optimization may prevent memory effects in he plastic. Those effects basically represent form changes after a certain time e.g. on dash-boards. Similar to an oscilloscope for the analysis of electric signal behaviors, the IR video camera is an important tool to qualify dynamic thermal processes.
2. H1N1 fever inspection of travelers
Ebola and the swine flu virus epidemic created a global demand for suitable screening techniques enabling fast non-contact detection of travelers possibly having fever, to prevent them from spreading the disease by air travel. It is based on the measurement of face temperature in the eye cavity area to determine the core body temperature.
Although this method does not represent an absolutely accurate fever temperature measurement, it is still suitable for screening larger groups of travelers with sufficiently high detection reliability. Normal IR cameras are only +/–2 °C accurate due to the limited
• stability of the sensing system and the
• imaging quality of the widely opened optics.
For measurements in the medical area, this uncertainty is insufficient. Therefore, reference radiators have been developed that provide a measuring accuracy of 0.2 °C at 34 °C radiation temperature. Those emitters are positioned on the border of the IR image at the same distance as the skin surface. Core of the measurement system is a certified IR thermometer with 25 mK thermal resolution. This device integrated in the reference radiator measures thermal radiation and transmits the actual temperature values via 4 – 20 mA interface to the analog port of the IR camera. The software calculates a correction value in the corresponding image area, which is also used for all other pixels of the measuring image. An alarm is automatically triggered if the present fever temperature is measured and a radiometric image is stored for documentation. For the affected persons, a contact fever measurement must subsequently be taken e.g. using an ear thermometer.
3. Line scanning cameras in glass tempering facilities
After construction glass has been cut to its final form, its surface must usually be toughened. This is done in glass tempering facilities, where the glass is heated in furnaces to about 600 °C. After this heating process, rollers transport the material from the oven into a cooling section, where the surface is cooled down quickly and evenly. This creates a fine crystalline hardened structure, which is essential for safety glass. The fine structure and especially the breaking resistance of the glass depends on uniform heating and cooling of all surface parts.
Since furnace housing and cooling section are located close to each other, it is only possible to monitor the glass surfaces leaving the oven through a small slot. As a result, the infrared image of the material is only shown a few lines. The software displays the glass surface as an image generated out of a specified amount of lines.
The camera measures the slot in a diagonal mode, enabling an overall field of view of 60° with a 48° lens. Glass has different emissivities depending on its coating layers. An IR thermometer measures the exact temperature on the noncoated lower side at the optimal wave-length of 5 μm for those surfaces. These temperatures, measured along a column of the measuring image, are transmitted to the analog input of the camera and compared with the corresponding camera measuring values. The result is a corrected emissivity, calculated for the overall measuring image. Ultimately, the measuring images allow exact adjustment of all heating sections in the furnace, assuring good thermal homogeneity.
4. Infrared cameras for aerial thermography
The application of thermal imaging cameras on drones and other airborne objects is growing. The scope of application is hereby quite broad: from the control and thermal analysis of major industrial facilities and buildings, follow-up fire monitoring to find smoldering nests and missing persons, and even for taking population census in the field. Airborne operated thermography for quality assurance and maintenance of photovoltaic systems is of particular significance. The systems must operate efficiently to quickly redeem the high acquisition costs. To ensure reliable operation, faulty solar modules must be promptly repaired.
5. Inline temperature measurement technology for food production plant management
The challenge in producing ready-made meals, is to produce a finished product containing different ingredients that tastes good despite industrial preparation. For reasons of food safety, all ingredients are required to be heated to 95 °C. If a steam cooker was used for processing, the vegetables would turn to mush before the meat was cooked. [Soome] processing technology uses microwave technology for heating. It takes advantage of the effect that each food has its own individual frequency for particularly fast heating. In compliance with the HACCP concept (Hazard Analysis and Critical Control Points), both the monitoring of pasteurization temperatures for the ready-made meals, which are sealed in PE foil-covered tubs, and the management of the system are conducted using an optris® PI 160 infrared camera.
In regard to flexibility and broad scope of applications, the new camera technology is an innovation on the infrared market. In addition to sophisticated temperature analysis, when connected to tablet PCs the device can also be used to solve simple maintenance tasks. With the exception of the hardware of the USB IR camera measuring heads, both of the other key components of the thermographic system described here – Windows software and PC hardware – can also be updated later. On the one hand, this is done by simply downloading software updates and upgrades. On the other hand, the standard USB interface makes it possible to supplement the measuring system at any time with innovative technological and functional PC hardware.
Appendix: Selection criteria for innovative infrared technology
IR temperature measurement devices
Selection criteria for infrared thermometers
A wide selection of infrared sensors is available for non-contact temperature measurement. The following criteria will help to find the optimal measuring device for your application:
• Initial question
• Temperature range
• Environmental conditions
• Spot size
• Material and surface of the measuring object
• Response time of infrared thermometers
The basic question is: point measurement or surface measurement? Based on the aim of application, the use of either an infrared thermometer or an infrared camera is possible. Once this is established, the product must be specified.
Choose the temperature range of the sensor as optimal as possible in order to reach a high resolution of the object temperature. The measuring ranges for IR-cameras can be adjusted to the measuring task manually or via digital interface.
The maximum acceptable ambient temperature of the sensors is very important. The optris® CT line operates in up to 250 °C without any cooling. By using water and air cooling the measuring devices operate in even higher ambient temperatures. Air purge systems help keep the lenses clean from additional dust in the atmosphere.
The size of the measuring object has to be equal to or bigger than the viewing field of the sensor in order to reach accurate results. The spot diameter (S) changes accordingly to the distance of the sensor (D). The brochures specify the D:S relation for the different optics.
Material and surface of the measuring object
The emissivity depends on material, surface and other factors. The common rule reads as follows: The higher the emissivity, the easier the measurement generates a precise result. Many infrared sensors offer adjustment of the emissivity. The appropriate values can be taken from the tables in the appendix.
Response time of infrared thermometers
The response time of infrared sensors is very fast as compared to contact thermometers. They range between 1 ms to 250 ms, strongly depending on the detector of the device. Due to the detector, response time is limited in the lower range. The electronics help to correct and adjust the response time according to the application (e.g. average or maximum hold).
Signal output interfaces
The interface supports the analysis of the measuring results.
The following interfaces are available:
• Output / alarm: 0/4 – 20 mA
• Output / analog: 0 – 10 V
• Thermocouple: Type J, Type K
• Interfaces: CAN, Profibus-DP, RS232, RS485, USB, Relais, Ethernet