6th INTERNATIONAL CONFERENCE ON QUANTITATIVE INFRARED THERMOGRAPHY
QIRT 2002, DUBROVNIK, September 24-27,2002
submitted for poster presentation
Calibration and Characterization of Infrared Imaging Systems Traceable to the International Temperature Scale
Bernd Gutschwager, Jürgen Hartmann and Jörg Hollandt*
Physikalisch-Technische Bundesanstalt, Abbestraße 2-12, D-10587 Berlin, Germany
* Corresponding author: phone +49 30 3481 369, fax +49 30 3481 510,
email:
key words: Calibration, International Temperature Scale
IR imaging systems are to an ever increasing extent used in many fields, e.g. production control, technical maintenance, environmental analysis and medical diagnostics. The progressing development of high-quality imaging optics and novel IR detectors has substantially improved the thermal, spatial and temporal resolution of thermal imagers during the last years and the capability of thermal cameras to perform quantitative measurements with high resolution. The essential basis for accurate quantitative measurements, reliable long-term investigations and for world-wide comparability of data is traceability to the SI units, meaning that the result of a measurement can be related to a national or international standard, through an unbroken chain of comparisons all having stated uncertainties. In the case of temperature measurements, as a consequence of globalisation the trend towards generally acknowledged quality assurance in accordance with ISO/IEC 17025 and the word-wide equivalence of measurements require traceability to the SI unit of temperature, the Kelvin, according to the International Temperature Scale of 1990 (ITS-90).
The Physikalisch-Technische Bundesanstalt (PTB) is the national institute of natural and engineering sciences and the highest technical authority for metrology of the Federal Republic of Germany. In its section ‘Temperature Radiation’ PTB is operating a set of blackbody radiators which serve as national standards and provide temperature radiation of precisely known radiance temperature over the temperature range from –60 °C up to 3000 °C.
The Low and Medium Temperature Calibration Facility consists of four large aperture blackbody radiators (aperture diameters: 41 mm to 60 mm). The cavities of the blackbody radiators are formed by heat-pipes enabling homogenous temperature distribution. Coverage of the temperature range from –60 °C to 960 °C is ensured by the use of different working materials inside the heat pipes: NH3 (-60 °C to 50 °C), H2O (50°C to 270 °C), Cs (250 °C to 650 °C) and Na (500 °C to 960 °C). The cavity wall emissivities, combined with the cavity geometries, yield very high effective emissivities (=0.9996 to 0.9999). Due to the automated control of the temperature set-point and the high thermal mass of the heat-pipes, temperature stability of the radiators is better than 10mK. Traceability to the ITS-90 is achieved by measuring the heat-pipe temperature close to the bottom of the cavity using a standard platinum resistance thermometer, calibrated according to the ITS-90. A standard uncertainty of the radiance temperature of 35 mK (@ -50 °C) up to 100 mK (@ 960 °C) is achieved.
An automated translation stage allows precise three-dimensional positioning of the IR imaging system in front of the blackbodies.
The High Temperature Calibration Facility consists of a high-temperature blackbody radiator (HTBB) provided with a pyrolytic graphite cavity and an aperture of 27 mm in diameter. The isothermal emissivity of the cavity is 0.999. Radiance temperatures up to 3000°C can be achieved with the HTBB. According to ITS-90 the radiance temperature of the HTBB is determined by radiation thermometry relative to a gold fixed-point blackbody radiator, which is part of the High Temperature Calibration Facility. The standard uncertainty of the pyrometric temperature measurement is 200mK (@ 1000 °C) up to 700 mK (@ 3000 °C). However, this uncertainty is valid only if nothing but radiation from a well defined area of the cavity bottom is observed. In general, when the aperture of the HTBB is viewed with a thermal imaging system, also radiation from the cavity walls will be observed. The radiance temperature distribution along the cavity walls was carefully investigated for different operating temperatures and can be corrected. Again an automated translation table allows the imaging system and the pyrometer to be precisely positioned in front of the blackbodies.
Besides calibration of the signal transfer function against blackbody radiators of known radiance temperature, an IR collimator system allows the minimum detectable (MDTD) and minimum resolvable temperature difference (MRTD) of IR imaging systems to be investigated. Different targets (circular targets and four-bar targets) can be placed in the focal plane of an off-axis parabola mirror. The resolution of the system is about 0.1 mrad. To enable the investigating of an imaging system under different and well defined ambient conditions a climate box is available for temperature (0 °C to 80 °C) and humidity control.
The instrumentation of PTB for the calibration and characterization of IR imaging systems will be presented and model measurements on different types of thermal cameras described.