2.5 THERMAL-RADIATION DETECTOR The thermal-radiation detector consisted of a phototube, amplifier, and high- and low-band-pass filters. The phototube output was produced by incident-thermal radia- tion from a nuclear deyice, lightning strokes, or other sources. This output was fed to a high band-pass filter that passed only signals with a rise time similar to those caused by nuclear detonations and to a low band~pass filter that passed only those sig- nals with a duration typical of nuclear detonations. Thus, an incident thermal-radiation signal had to have both a rise time and a duration typical of nuclear devices in order to activate the therma!-radiation detector. 27