Experimental system for the study of the directional thermal emission of natural surfaces

A new automatic experimental system was designed to improve the accuracy of multidirectional thermal infrared measurements. This experimental system mainly consists of two identical thermal cameras operating at 8-13 mu m, one metal ring to keep the constant view area for different view angles and a goniometer, which is composed of: (1) a semicircular roadway of 2 m diameter to change the observation angle in the azimuth direction; (2) an elevator of 1 m height to adjust the measuring level to the target level; (3) a rotating arm equipped with one thermal camera for changing the observation angle in the zenith direction; and (4) a fixed arm equipped with another thermal camera to record at nadir the target temperature variation with time during the measurements. The system can be disassembled for easy transport and all of the data acquisition procedures are automatically monitored. For a given azimuth angle, the system needs about 2 minutes to make the directional measurements from about -70degrees to 70degrees, and for completing one hemispheric measurement it needs about 20 minutes if the multidirectional measurements are conducted by a step of 30degrees in the azimuth direction. The preliminary data acquired using our new system on bare soil and winter wheat are displayed and analysed. The results show that the angular variation of surface brightness temperature is measurable and presents some regular directional distribution and can be used quantitatively to study the directional thermal emission of the natural surfaces.