Temporal variations in the apparent emissivity of various materials

1.0 ABSTRACT Spectral emissivity measurements gathered in the longwave infrared region of the spectrum during a recent airborne hyperspectral data collection experiment indicated that the spectral emissivity of certain organic polymers changed by as much as 10% throughout the day. Inorganic and many other organic materials that were measured at the same time during this experiment showed no change. As this was an unexpected event, a subsequent experiment was designed to make emissivity measurements of several organic and inorganic materials over a 24-hour period/diurnal cycle. The results from this experiment confirmed that certain materials showed a significant spectral emissivity variation over this period. This paper will discuss some possible explanations for this variation and emphasize the significance and implications of this fact on the integrity of spectral emissivity measurements and spectral libraries being constructed in this wavelength region. Spectral emissivity measurements in the longwave infrared portion of the spectrum were collected for several ground control panels deployed at Ellington Field in Houston, TX, USA during an aircraft collection campaign carried out using the SEBASS High-Altitude Research Project (SHARP) developed by the Aerospace Corporation. The collection plan called for the sensor to be flown both during the daytime and nighttime hours. The ground truth team was tasked to make measurements at coincident times to the aircraft overflight. Upon analysis of this data, unexpected differences in the measured spectral emissivity were identified. These differences, in some cases, exceeded 10% at certain spectral positions. In response to this unexpected finding, numerous additional spectral emissivity measurements were made of the materials in question around the clock over a several day period. A comparison of the results obtained from these varying conditions is discussed in the remainder of this paper along with possible explanations of the discrepancies. Some implications of these finding will be discussed as they relate to spectral emissivity libraries and the exploitation of remotely sensed imagery using data from these ground-based instruments.