An active method for measuring thermal infrared effective emissivities: Implications and perspectives for remote sensing

Abstract Remote sensing from spacecraft in the thermal infrared bands deals mainly with the radiance of heterogeneous surfaces, recorded as a whole. It is shown that these radiances can be interpreted quantitatively as if they were coming from equivalent homogeneous surfaces with well defined effective emissivity and temperature, provided that the heterogeneity of these surfaces measured with a well defined heterogeneity factor is less than 2‰, which is generally the case. An active method using a pulsed external source is then shown to be very efficient to measure these effective emissivities with good accuracy. This method avoids any surface temperature measurement and eliminates completely the effects of the surroundings. A first realization of the required instrumentation is presented along with its characteristics. Preliminary results obtained with this apparatus on water, sand and grass are shown. Possible implications for remote sensing are then discussed.