Infrared and Microwave Effects on Skin Heating and Temperature Sensation

In order to study the physiological mechanisms of temperature sensation, the forehead area of seven subjects was exposed to controlled heating while the skin temperature was radiometrically measured and recorded. Continuous exposure to variations in far infrared radiation produced temperature sensations which could best be correlated with rates of change of skin temperature. Warmth threshold was accompanied by a rate of rise of skin temperature of 0.001°C/second; cool threshold was accompanied by a rate of fall of skin temperature of 0.005° to 0.006°C/second. Reports of temperature sensation continued to be given when no changes in skin temperature could be measured. Small, rapid fluctuations in skin temperature, exceeding the rates of rise and fall just given, evoked no reports of sensation. It was postulated that the temperature changes involved were confined to the most superficial layers of the skin, and therefore did not stimulate the cutaneous temperature receptors. The same sequence of sensation reports resulted from preliminarily heating up or cooling down the skin, and then allowing it to return spontaneously to its normal temperature level. Cool sensations accompanying rapidly rising skin temperature change rates were believed to be due to simultaneous inhibition of warmth receptors and excitation of cold receptors. Exposure of blackened and unblackened forehead skin to various pulse duration-intensity combinations of near infrared radiation sufficient to evoke threshold warmth sensation, permitted intracutaneous temperature changes to be calculated. All such changes producing a threshold warmth sensation caused a temperature rise of about 0.