"The Aqueous Blackbody System is a broadband blackbody calibration
target for radiometers operating in the low THz range. By absorbing
99.6% of radiation incident on the aperture, the physical temperature
of the water is equal to the apparent radiometric temperature to
within 0.4%. This level of accuracy is achieved due to an 'optical
trap' design, which ensures no less than four water-surface
reflections (two in each polarization) for any radiation that enters
the aperture."