18-20 June, 2003, Budapest, Hungary OSSKI Center (Törley Palace)

with Exhibition and Pre-Session on Thermal Energy in Hungarian
"THERMO-BRIDGE"
between East and West for technology transfer and information exchange

Portable infrared thermometers integrate all of the important functional elements in one housing. The form factor of the instrument is determined by the infrared optical design as well as by the optical aiming system. Designs can be differentiated between units with visual sighting and those with laser sighting. The units with visual sighting tend to have form factors similar to the design of binoculars or video cameras whilst appliances with laser sighting are more likely to resemble handguns.

Figure 1. Portable IR thermometer with laser sighting

The signal of the infrared detector is initially preamplified in the unit and is converted into a digital signal, which is computed into a value of object temperature by a microprocessor. A LCD or LED displays the reading. This results in a self-contained single piece unit, battery powered and easily transported and used. Because of this, portable infrared thermometers are preferred in many applications including maintenance and quality control. There are three major design trends for portable infrared thermometers: Firstly, reduction of manufacturing costs, secondly, availability of new laser sighting systems for indicating the size of the measuring spot and thirdly, certifiable calibration for lower object temperatures      

The significant reduction of manufacturing costs of portable infrared thermometers in recent years is a result of important technological progress. The very expensive previously used germanium lenses have often been replaced by polymer materials, which can be produced at a low price by injection moulding. The use of Fresnel lenses made of polyethylene and of mirrors made of polycarbonate has become very typical. With very few exceptions, modern portable infrared thermometers use thermopiles as infrared detectors.  During the 1990’s, the price of these detectors has dropped to a tenth of the former price because of new manufacturing methods based on micro systems technology. Components for processing and converting the signals of infrared detectors have benefited from the general trend towards more cost effective electronics components. A trend towards integrated devices such as processors or preamplifiers with integrated analogue-digital converters is very important in this context. Application specific integrated circuits are increasingly used and allow for significant reduction of manufacturing costs.

The general initial disadvantage of laser sighting for portable infrared thermometers compared to through-the-lens sighting was that they were only able to point at the centre of the measuring spot using one laser dot. About ten years ago the first portable radiation thermometers using two laser beams appeared on the market, those two beams being produced by a traditional beam splitter.

Figure 6. Laser circle sighting in diagonal use  

In the mid-90’s, units were put on the market that used one single laser beam being circularly deflected by a motorised system. Similar to devices with diffractive optics that were introduced recently those units only indicate the correct spot size at one specific distance typically at the focal distance. Measuring spots that are shown in shorter distances are too small; spots that are indicated in distances behind the focal point appear to be significantly bigger in size than the measured area. This can lead to inaccurate measurement, especially if the user has a lack of familiarity ad expertise.
  The latest laser sighting systems use combined diffraction and refraction optics to create a laser illumination mark that shows the correct size of the measuring area for any measuring distance. A single laser beam is diffracted such that the circular marking at the exit of the unit is approximately the size of the diameter of the infrared optical path aperture. A refraction optic component surrounds the IR aperture and this aligns the diffracted laser light according to the path of the measuring point. An important advantage of the circular laser-sighting units is that they give a realistic impression of the size and shape of the measuring spot even if measurement is taken at an angle to surfaces being measured.

An increasing number of applications demand a good degree of absolute precision. One example for this kind of use is in the Food industry. Recent food hygiene regulations caused a strong growth in the market segment of food thermometry. The method of „Hazard Analysis Critical Control Point" (HACCP) that is used as a basis for this regulation demands frequent measurement and documentation of temperature. Some manufacturers have developed infrared thermometers that have been adapted for the special demands of food measurement. Calibration that can be traced to National Standards is normally part of these special demands. Whereas National Calibration Laboratories have for years been able to work with transfer radiation thermometers and transfer radiation sources (e.g. tungsten band lamps), this has not been possible at low temperatures  (below 500° C) until recently.
  The European joint project “Traceability in Infrared Radiation Thermometry” (TRIRAT) was initiated to fight this problem. Now European National Institutes are able to calibrate industrial transfer radiation thermometers with a high accuracy even down to minus 50°C. Manufacturers producing radiation thermometers use these transfer radiation thermometers as a basis for the traceability of calibration schemes. In this way they allow the traceability of calibration of portable IR thermometers for food control and other critical areas.

Table 7. Calibration table of a transfer radiation thermometer	Figure 8. Calibration of a radiator with a transfer

Industrial process control and automation uses stationary infrared sensors. In the same way as for portable infrared thermometers, but not to the same extent, manufacturing costs have dropped considerably. The costs of recently introduced comparable model types have been reduced by 50% compared to 5 years ago. Reasonably priced optics and infrared detectors played a big part in reducing the costs for the production of such sensors.

The aperture of the optical path basically determines the size of the outer housing. In the past it was only possible to use lenses with a large diameter to increase the transmission of IR energy, because of the lower radiation sensitivity of the detectors.

Figure 9. Infrared miniature measuring head 14x28 mm

Progress in detector technology, the use of micro systems technology as well as new low-noise analogue amplifiers, contributed to the production of infrared measuring heads that are considerably smaller in size. The sensor head of the infrared thermometer shown above achieves 0,1 K temperature resolution with a fast response time. These sensors are widely used in industrial machinery as they take up little space.

Stationary infrared thermometers have increasingly short response times, caused by the demand for more frequent measurements on moving equipment. Nowadays, response times even of lower cost sensors can be around 100ms or less. There are also infrared thermometers available with time constants of 1ms or smaller.

Demands for optical resolution have increased because of the general trend towards miniaturisation during recent years. Consequently, units with small spot sizes are offered to a greater extent especially for the measurement of low temperatures.

Measurement at short wavelengths (e.g. of 150°C at 1,6 µm) demands detectors with extreme sensitivity due to the low radiation energy at the low temperatures. Options include Indium-Gallium-Arsenide detectors that were initially developed for optical data transfer via glass fibre. These detectors have been increasingly used in telecommunications, so that the production costs have reduced while performance has increased. Due to this, new application fields in the measurement of gas concentration and medicine have opened up. InGaAs-diodes are available for the following wavelengths: 1,6µm; 1,9µm; 2,2µm and recently also for 2,6µm. They provide detectivities that were not possible at these wavelengths by previous uncooled sensors.

The technology of digital electronics allows fast processing of measurement results, high accuracy and enables the functionality of digital interfaces. Infrared thermometers with digital circuits are able to supply the user with much more information than analogue sensors. Various new stationary infrared radiation thermometers can be adjusted or calibrated on site by using new software. By doing this, the installed life time of the measuring devices can be increased and costs can be saved.

Figure 11. Block diagram of digital circuitry in a modern infrared thermometer

Sensors with digital interfaces enable a set up program to be used before they are installed, which ranges from remote setting of sensor parameters to the use of several sensors onto a bus system. Figure number 13 shows an example of a set up display of an infrared thermometer.

Figure 13. Example of a set up program to install specific sensor parameters using a computer

Most of the bus protocols used nowadays (e.g. Profibus, CANBUS, DeviceNet, ControlNet and Fieldbus) use the RS485 interface as hardware basis. With this interface, up to 32 sensors can be installed in parallel. Each sensor is equipped with its own address. With a two-wire electrical installation the sensor acts as a receiver or transmitter, being guided from the master (PLC or PC). The four-wire electrical installation allows a simultaneous transmitting and receiving of information. As each bus standard is equipped with its own protocol, sensors that are used for (e.g.) Profibus cannot work together with CANBUS-sensors. For this so-called Gateways are necessary. Consequently, the challenge to develop and commercially introduce a bus protocol for the use worldwide remains. The Ethernet is also the focus of being a possible candidate because of its wide application in the field of office communication. In addition, the components can be bought at correspondingly reasonable prices.

In industrial processes thermal imaging was traditionally less used because of the high costs. New Focal Plane Array-technologies promise extraordinary cost reductions of sensor chips in the automotive and defence industry because of the development of mass markets. This development results in new market-place opportunities for reasonably priced portable imaging devices especially in the field of maintenance and quality control. Such devices will be used additionally to and partially instead of portable spot measurement thermometers. Measurement demands can be characterized by the following qualities:

-   High measuring speed concerning the frequency of lines and images
-   High optical resolution
-   High dynamic temperature range
-   Spectral variability (the best possible measuring wavelength) as well as
-   Good pixel related signal uniformity to detect even smallest inhomogenieties of materials (low fixed pattern noise).

Figure number 15 shows one of the systems that are on the market:

This scanner is used to scan objects at room temperatures with thermoelectrically cooled MCT detectors. Standard PC RS232 interfaces can even be used at line frequencies of 50 Hz and pixel detection rates of 20 microseconds by integrating processing electronics with the necessary data compression.

Figure 16. Use of linescanner in hardening systems for automotive glass

Temperature critical glass processing is a very important field for linescanner use. Figure 16 shows the thermographic online measurement of automotive glass in glass bending and hardening processes. The homogeneity of heating up and the subsequent rapid cooling down of the glass is in direct relation to the fracturing of the glass on impact. Modern laminating processes increasingly gain importance by allowing a 100% quality control of real time monitoring of splits and holes along with the monitoring of the material homogeneity.

[1.] Volker Schmidt, Hans-Jürgen Rostalski, Ulrich Kienitz, New Laser Sighting Systems for Radiation Thermometers, In: Proceedings II – Tempmeko 99, Vol. 2, Delft 1999, pp. 714 – 719.
[2.] Volker Schmidt, Non-Contact Calibration Scheme for Radiation Thermometers, In: Proceedings IRS², Wunstorf 2000, pp. 61-64.
[3.] Ulrich Kienitz et. al.: Measuring device, European Patent 0617263.
[4.] Volker Schmidt et. al.: Vorrichtung zur Temperaturmessung, German Patent 195 28 590.
[5.] Ulrich Kienitz et. al.: Vorrichtung zur berührungslosen Temperaturmessung, German Patent Application 196 54 276 A.

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OSSKI Center (Törley Palace)
"Fodor József" National Center of Public Heath
"Frédéric Joliot-Curie" National Research Institute for
Radiobiology and Radiohygiene. (OKK-OSSKI)
www.osski.hu