The measurement of radiant temperature in neonatal thermal environments

Heat exchange through radiation is recognized as the dominant mode of heat transfer for an infant nursed in an incubator or warmer. A radiometer was constructed to measure the planar radiant temperature experienced by the infant. Two heat-flow sensors of different emissivities were mounted onto a heat sink such that one measured principally convective heat exchange while the other measured convective and radiant heat exchange. The radiant heat exchange was obtained from the difference between these values, from which the planar radiant temperature could be calculated. The spatial variation in planar radiant temperatures within an incubator and warmer was determined by orienting the instrument towards the five orthogonal hemispheres sensed by the infant abdomen, sides, head, and feet. In the incubator, the spatial variation in radiant temperatures exceeded 2 degrees C, or four times the spatial variation in air temperatures (0.5 degrees C). The radiant warmer had a spatial variation of 18 degrees C in radiant temperature at three-fourths of maximum power, corresponding to a variation in heat flux over the infant's surface of 100 W/m2. This thermal asymmetry would be expected to influence the variation in surface skin temperature, and thus the thermal stimuli to the infant thermoregulatory system. Further research is needed to understand the clinical significance of this asymmetry. Furthermore, the precise control over air temperature in modern incubators provides a false sense of thermal control and stability. Radiant temperature needs to be measured in addition to air temperature if the thermal performances of incubators and warmers are to be fully understood.