Arrays of thermopile detectors arranged on flat panels were used to measure stray radiation from high‐power transmission lines at 140 GHz in the FTU tokamak electron cyclotron resonance heating experiment. These instruments are insensitive to polarization and to the direction of propagation within ±40° off normal incidence. They have a time constant of 120 ms, therefore amplification and synchronous detection are required for the measurement of short radiation pulses. No special measurement techniques are required with long pulses or cw radiation. The typical sensitivity is 2 mV cm2/mW in long pulse operation and 47 &mgr;V cm2/mW for 1 ms pulses. The detectors are sensitive to light and adequate filtering should be added to suppress it if necessary. These characteristics are adequate for the measurement of human exposure levels to electromagnetic radiation in the millimeter wave range. ©1996 American Institute of Physics.