Comparison of the Stimulation of the Warmth Sense Organ by Microwave and Infrared

¹ From the Department of Medical Physics of the University of Nijmegen, Nijmegen, and Department of Medical Physics of the University of Utrecht, Utrecht, Holland

Irradiation experiments on the forearm with infrared and microwave are described. The time between the moment that the irradiation of a part of the skin of the forearm starts and the moment that a warmth sensation is obtained (latency time) was measured as a function of the intensity of the irradiation. The experimentally determined latency time-energy relations are analyzed with the aid of a theoretical calculation of the temperature distribution in the irradiated skin. Microwave radiation can be very well used for investigation of the warmth sense organ. An advantage is that the temperature of the whole skin as a function of time increases linearly during short exposures with constant intensity. The comparison of the experimental data obtained with infrared and microwave yields some interesting results. It is shown that it is unlikely that a change of the spatial temperature gradient can be considered as the adequate stimulus of the warmth sense organ. The experimental facts support the theory that a threshold sensation is obtained when the temperature of the warmth receptors is increased by a certain amount. Irradiating an area of the inner forearm of 13 cm² with microwave resulted in a threshold temperature rise of 0.4–1.0°C, depending on the subject, which is in agreement with the threshold temperature rise obtained with infrared at a depth of about 0.3– 0.5 mm, taking into account the experimental error. For stimulus duration longer than 3–5 seconds a pronounced influence of the rate of change of temperature is found.