Thermal Sensation: Infrarej and Microwaves

• .>*'•The number of studies of how microwaves arouse warmth sensations can be counted on the fingers of one hand. They leave many, many unanswered questions, but the nature of the questions to be asked will in general take shape in the light of the much larger amount of information thati we have about the perception of infrared radiation. So ^he main substance of this paper is to use present knowledge to help formulate a hypothetical program of psychophysical experiments with microwaves that will take account of the main variables that determine the subjective response. ^ One important question concerns what levels of power density are needed to trigger a just-detectable sensation, i.e., what is the absolute threshold of warmth sensation for microwaves versus infrared radiation. We shall see below that this phrasing of the question is deceptively oversimplified. Nevertheless, we address first the question of how infrared and microwave thresholds compare when measured in the same subjects under comparable test procedures. Two studies give first-order answers to the question. Hendler, Hardy, and Murgatroyd (1963) compared thresholds of infrared and 3-cm microwaves for a 37 cm, circular area of the forehead. Four subjects were tested by a variety of standard psychophysical procedures, including the classical methods of constant stimuli and limits. The threshold depended on the duration of the stimulation (the shorter the stimulation, the higher was the threshold). At all durations, however, the microwave thresholds were about twice as high as the infrared thresholds. The second study (Justesen, Adair, Stevens, and BruceWolfe, 1982) employed microwaves of 12.4 cm wavelength,