Lambertian surfaces represent an important assumption when constructing thermal radiance transfer equations for remote sensing observations of ground surface temperatures. We identify the properties of ground surfaces in thermal infrared regions as Lambertian surfaces via field experiments. Because Lambertian surfaces present homogeneous thermal emissions levels in hemispheric directions for a specific ground surface under specific kinetic temperatures and emissions, we conducted a series of field experiments to illustrate the properties of such ground surfaces. Four typical ground surfaces were selected for the experiments to observe thermal emissions: bare soil, grass, water, and concrete. Radiance thermometers were used to observe ground emissions from seven directions: 30°, 45°, 60°, 90°, 120°, 135°, and 150°. Solar zenith angles were considered for the observation of ground emissions. Experiments were conducted in five different regions of China (Beijing, Nanjing, Xilinguole, Yongzhou, and Jiangmen) during both daytime and nighttime. To determine whether different observation angles have significantly different effects on radiance, statistical analyses (ANOVA and Friedman test) were conducted. Post hoc multiple comparison tests and pairwise multiple comparisons were also conducted to examine the various pairings of observation angles and to measure the radiance differences. Roughly half of the radiance groups of all observed sites were tested via an ANOVA, and the remaining groups with unequal variances were subjected to the Friedman test. The results indicate that statistically significant differences in the radiance levels occurred among the seven angles for almost all of the sites (39 of the 40 groups). The results of our experiments indicate that the selected ground surfaces, especially the grass and the bare soil, may not behave with Lambertian properties in the thermal infrared region. This is probably attributed to the roughness of the selected surface, because we found that roughness is an important factor affecting the observed magnitude of thermal emission from different directions of the ground surface under study. Therefore, whether or not a terrestrial surface can be assumed to be a Lambertian surface should be based on their geometric structure. When the surface is relatively smooth, we can say that it is close to the Lambertian property in thermal emission.

The Lambertian surface has been a very important assumption in constructing the thermal radiance transfer equation for remote sensing of ground surface temperature. Crops are often planted in a row structure, and complex row architectures may lead to differences in terms of sunlight. Study of the radiation difference caused by observation directions for cropland is important for crop research by remote sensing technology. In this study, we studied the Lambertian property of cropland surfaces in the thermal infrared region through rigorous experiments in the field. Because a Lambertian surface has a homogeneous intensity of thermal emissions in hemispherical directions for a specific ground surface under specific kinetic temperature and emission conditions, we set up a series of field experiments to demonstrate the properties of the cropland. Three typical ground surfaces were selected for the experiments to observe thermal emissions: paddy field, bare soil and grass. Radiance thermometry was used to observe the ground emissions from 7 directions: 30°, 45°, 60°, 90°, 120°, 135° and 150°. The solar zenith angle was considered for the observation of ground emissions. Experiments were conducted at three different sites, both in daytime and nighttime, in Nanjing, China. Down-welling atmospheric emissions were considered for correction of the observed radiance to compute the ground surface radiance in the experiments. To determine whether different observation angles have significantly different effects on radiance, statistical analyses (ANOVA and Friedman test) were carried out. Post hoc multiple comparison tests and pairwise multiple comparisons were also conducted to inspect the various pairings of observation angles and to discover the radiance difference between them. According to the experiments and analysis of results, it can be concluded that the cropland does not have Lambertian characteristics in the thermal infrared region for thermal remote sensing. This means that both the thick vegetative canopy and the smooth bare soil have non-Lambertian properties. The results also indicated that 1) the radiance difference among the seven observation angles were larger in the daytime than at nighttime, and 2) the paddy field, bare soil and grass had their own characteristic differences in radiance among the different observation angles.