Cool materials’ thermal emissivity

Comparison of experimental measurement methods

Abstract

Any surface hotter than its surroundings loses heat through radiative transfer, i.e., it emits long-wave infrared radiation which is represented by thermal emissivity coefficient. The aim of this article is to present a comparison between thermal emissivity measuring methods of elastomeric cool coatings by a non-contact thermometer indirect measurement method in accordance with ASTM E1933-14 using an infrared imaging radiometer and direct measurement with a portable emissometer. The values ​​measured with the infrared radiometer indicate that materials that absorb less heat, because they are more reflective, have lower emissivity and vice versa, while the values ​​measured with the emissometer are higher and present less variation among samples. The difference in the infrared wavelength detected by the two devices is the factor that considerably influences emissivity differences; therefore, it is important to indicate in which wavelength and temperature range the property value is valid.