The effects of radiation from the surroundings on the impressions of freshness of 106 men and 39 women members of the staff of the Building Research Station were investigated during the first 6 months of 1948. Subjects were exposed to three types of environment: (1) where the walls were cooler than the air, (2) where the walls and air were at the same temperature, and (3) where the walls were warmer than the air. The tests were carried out in calm air.It was found that the difference between the mean radiant temperature and the air temperature affected freshness impressions, but the effect was relatively slight. Environments which felt cool tended to feel fresh, yet a rise in the mean radiant temperature—which would increase the warmth of the environment—tended to produce an impression of greater freshness. At a given equivalent temperature, environments with the surroundings warmer than the air were found to be definitely fresher than cold- and neutral-wall environments. This was thought to be mainly due to the fact that the humidity of the air in the warm-wall environment was lower than that in the other two environments. Changes in humidity insufficient to affect sensations of warmth affect impressions of freshness. Under the conditions of these experiments, and to keep freshness impressions constant, a rise of 1° F. in the temperature of calm air must be compensated by a fall of about 5% in the relative humidity.The subjects had a distinct preference for the warm-wall environment; 73% of them found it pleasanter than either the cold- or the neutral-wall environment.Freshness impressions are considered to be related to transient fluctuations in the rate of heat loss from the head.

When human skin is stimulated with thermal radiation so that sensations of warmth are evoked, it is most sensitive to radiation with a peak wave-length of 35,u, according to . However, during an investigation of the effects of infra-red radiation on thermal comfort (Chrenko, 1953), discomfort was experienced when the temperature of the source of radiation was only 650 C, so that the skin was more sensitive to radiation with a peak wave-length longer than 3x5 It.Further experiments have been made to determine quantitative relations between the peak wave-length, i.e. the wave-length of maximum energy of thermal radiation, skin temperature, and the sensory threshold for warmth. METHODS ApparatusThe thermal stimulus was radiation emitted from a black body or full radiator. The inner core of the black body was a 30 cm diameter hollow sphere of ceramic material, with an aperture circular in shape and 10 cm in diameter, fitted to a hollow cylinder of the same diameter and length. Bricks of refractory material were used as thermal lagging, which covered the sphere so that the furnace assumed the shape of a cube with sides 60 cm in length.The furnace was mounted so that the longitudinal axis of the cylinder leading to the hollow sphere was horizontal. The furnace was electrically heated and its temperature was altered by varying the voltage of the electrical input. The maximum electrical loading of the furnace was 3 kW. The arrangements are shown in Fig. 1. Details of the thermocouple used to measure the furnace temperature are shown in Fig. 2. From the furnace temperature, the peak wave-length was calculated from Wien's equation, tf = (2892/A,, .) -273, where Amax. is the peak wave-length (,u), and tf is the temperature of the source of radiation (O C). The intensity of the radiation emitted by the furnace was measured with a calibrated thermopile with an output of 64-5 microvolts/milliwatt/square centimetre of incident radiant energy.A large water-cooled screen, with a circular aperture and shutter, was mounted vertically in front of the mouth of the furnace. The shutter was controlled by an electronic timing 1 Physiol. 173

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