Radiation thermometers can be used for non-contact temperature measurements, when physical contact with a target (measured object) is unwanted or even impossible, but in the last decade the use of a thermographic (thermal) imagers is becoming more and more frequent. A thermographic imager has an array of sensing elements or FPA – focal plane array, which can measure temperature distribution on a target. Unlike radiation thermometers that have a circular field of view, the thermographic imagers have a rectangular field of view. The thermographic imager Flir T650sc, that we used to perform measurements with, has a resolution of 640×480 pixels. It is necessary to provide sufficient number of pixels for more accurate average temperature reading of a measured object.
In the chapter Introduction is an overview of discoveries, which set the foundation for non-contact temperature measurements. The accuracy of measured temperature with a thermographic imager is affected by many variables that have to be considered when performing a measurement. The effect, which is probably the least researched, is the size of source effect.
The chapter Non-contact temperature measurement describes the basic concept of electromagnetic specter, infrared specter, Stefan-Boltzmann law, Planck law and emissivity. It also specifies parts of a thermographic imager and its functionality. Thermographic imager Flir T650sc, which we used for measurements, is described in detail. The calibration procedure and the size of source effect is also defined.
In the chapter Measuring the size of source effect of a thermographic imager planning and making of the aperture tiles is described. The tiles were made with rectangular and square apertures. There were different number of apertures of different sizes and different mutual distance between them. Measurements at different temperatures and with different black bodies were planned. For better comprehension of the aperture tiles, we listed labels for each individual aperture tile. We also labeled different temperatures at different black bodies. The preparation, equipment placement and measuring procedure is described. The measurements were performed with the thermographic imager, which was aligned with an aperture tile and a black body. We performed measurements at eight different temperatures of different black bodies and at five different distances with one rectangular aperture tiles. As expected, the size of source effect was showed as decreasing of the measured temperature when reducing the width of apertures and with increasing the distance between the thermographic imager and an aperture tile. The uncertainty increases with decreasing pixel number for average temperature calculations. The average temperature measurement is impossible with very low number of pixels (less than 3×3 as specified by the manufacturer). We compared measured average temperatures of rectangular and squared apertures of the same size. The results consistently showed higher measured average temperature at rectangular aperture tiles than at square aperture tiles. This chapter also includes all graphic interpretations and examples of the thermographic images.
Finally, in chapter Conclusions the findings from processing and interpretation of the thermal images and the graphic interpretation are listed.
The thesis also includes the list of references.