MODIS LST Group
Laboratory Measurements

To determine land surface temperature, one has to take into account the reflectivity and emissivity of the surface. An object has its optical characteristics, called the bidirectional reflectance distribution function (BRDF), that describes the spectral reflectance at different incident and reflected angles. Because there are few recorded values for this property, we designed and constructed a system to make BRDF measurements. The instrument measures spectral infrared bidirectional reflectance and emissivity (thus, the name SIBRE) and works by scanning one mirror in a Michelson interferometer (a device that measures optical interference phenomena) and recording the resulting interferogram. By Fourier transforming the interferogram, one can determine the spectral radiance received by the detector. A shutter cancels the effects of background irradiance and a model of surface heating eliminates the effect introduced by heating of the sample. Several hundred scans are averaged to produce the BRDF, which can then be used to calculate emissivity using Kirchoff's law (emissivity = 1 - reflectivity).

The SIBRE system consists of a Fourier transform infrared (FTIR) spectrometer, a thermal infrared (TIR) source, a solenoid-driven shutter system, a reference plate, and a hemispherical pointing system. The spectrometer, a FTIR spectrometer unit (MIDAC, model M2405) with a two-color detector, closed-cycle cryogenic cooler, and a powerful mirror drive system, measures the reflected radiance inside a solid angle of approximately 1 msr. The active TIR source is a 12cm x 12cm ceramic plate that produces wideband infrared radiance on the surface with a solid angle of 43 msr. The reference readings are produced by a 25cm x 25cm diffuse gold plate that has a known BRDF. The pointing system consists of aluminum tubes rolled into a radius of 1.5m and welded into concentric hoop assemblies. A carriage rolls along a double hoop and positions the spectrometer, while the spectrometer's angle is set manually.

By computing the source radiation from the kinetic temperature and measured emissivity of the heated ceramic plate, calculating the cosine of the source declination and the solid angle of the source, and measuring the reflected radiance with the spectrometer, we can get the BRDF to within a absolute accuracy of ~5%, or an emissivity to within ~0.5%.

We also use a TIR spectrometer/integrating-sphere system to measure BRDF in the lab.