Method and apparatus for accurately and instantaneously determining the thermodynamic temperature of remote objects by continuous determination of the emissivity, the reflectivity, and optical constants, as well as the apparent or brightness temperature of the sample with a single instrument. The emissivity measurement is preferably made by a complex polarimeter including a laser that generates polarized light, which is reflected from the sample into a detector system. The detector system includes a beamsplitter, polarization analyzers, and four detectors to measure independently the four Stokes vectors of the reflected radiation. The same detectors, or a separate detector in the same instrument, is used to measure brightness temperature. Thus, the instrument is capable of measuring both the change in polarization upon reflection as well as the degree of depolarization and hence diffuseness. This enables correction for surface roughness of the sample and background radiation, which could otherwise introduce errors in temperature measurement.