A conceptual model for effective directional emissivity from nonisothermalsurfaces

The conventional definition of emissivity requires the source of radiation to be isothermal in order to compare its thermal emission to that of a blac kbody at the same temperature. This requirement is not met for most land su rfaces considered in thermal infrared remote sensing, Thus, the effective o r equivalent emissivity of nonisothermal surfaces has been a poorly defined but widely used concept for years. Recently, several authors have attempte d to define this concept more clearly. Unfortunately, definitions such as e nsemble emissivity (e-emissivity) and emissivity derived from the surface b idirectional reflectance distribution function (r-emissivity) [27] do not f ully satisfy current needs for estimating true land surface temperature (LS T), We suggest the use of an additional term, the "apparent emissivity incr ement," which considers the effects of geometric optics to explain the dire ctional and spectral dependence in LST caused by the three-dimensional (3-D ) structure and subpixel temperature distribution of the surface, We define this quantity based upon the emissivity derived from the bidirectional ref lectance distribution function (epsilon(BRDF)) for isothermal surfaces and present a conceptual model of thermal emission from nonisothermal land surf aces. Our study also indicates that an average LST corresponding to the hem ispherical wideband epsilon(BRDF) will be useful in remote sensing-based LS T modeling and inversion.