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In present day tokamaks runaway electrons can be confined long enough to gain energies in the order of several tens of megaelectron volts. At these energies synchrotron radiation is emitted in the infrared wavelength range which can easily be detected by thermographic cameras. The spectral features of this synchrotron radiation are reviewed. On TEXTOR-94 a diagnostic exploiting this synchrotron radiation has been developed and is presented here. It is shown how to deduce the runaway parameters like runaway energy, pitch angle, runaway current and beam radius from the measurements. Based on the experience at TEXTOR-94 the feasibility of a similar synchrotron diagnostic on the International Thermonuclear Experimental Reactor is discussed. The maximum emission is expected in the wavelength range from 1-5 mum. A beam of 10 MeV runaway electrons with a current of about 15 kA will already be detectable. (C) 2001 American Institute of Physics.