Chapter 6 Blackbody and other Calibration Sources

The chapter presents a discussion on the history, theory, design and application of blackbody radiators for radiation thermometry applications. The calibration of optical instruments for measuring thermal radiation can be performed in one of the two ways: either the absolute spectral responsivity of a detector with respect to a primary detector standard is determined, or the detectors (or more generally detector-based systems) are calibrated through using known sources of spectral radiation. In practice, because of the major investment required to acquire a synchrotron source, only the application of blackbody radiation offers a practical solution for instrument calibration at nearly all National Metrology Institutes (NMIs) and calibration laboratories. Sources of thermal radiation are required to investigate and calibrate radiation thermometric devices for temperatures from about 170°C up to over 3,000°C. To cover this large temperature range a variety of types and concepts of thermal radiation sources is used. The most sophisticated one- the concept of blackbody radiators- has already been introduced in the late 19th century. From the beginning a wide survey from the most sophisticate blackbody radiation sources used in top-of-the-art laboratories of the National Metrology laboratories to the flat plate radiators and lamps used in industrial level calibrations laboratories is presented in the chapter. The chapter also presents an outline of the important features and characterization of such thermal radiation sources. The chapter is completed with a description of the methods of calculating the emissivity of blackbody sources and the influence of the index of refraction on the thermal radiation emitted by blackbodies.