Polarizers with transmittances of 35 to 45% are common (Nicol prisms, dichroic polarizers); with non-reflecting coatings, such polarizers can be made with transmittances approaching 50%. Beam-splitting polarizers can be made with transmittances approaching 100%, but these polarizers are not “spathic”: The geometry of the emergent beam is quite different from the geometry of the entering beam.
In this paper it is shown that a spathic polarizer with a transmittance of 100% is impossible; such a polarizer would violate the laws of thermodynamics. It is shown how one can construct in principle a spathic polarizer with a transmittance of about 89%, the exact figure depending on the brightness and wavelength of the light. It is further shown that as far as thermodynamic limitations alone are concerned, a spathic polarizer having a transmittance of about 99% is possible.
The quantitative results are based on Planck’s thermodynamic theory of heat radiation.
R. CLARK JONES, "Ultimate Performance of Polarizers for Visible Light," J. Opt. Soc. Am. 52, 747-750 (1962)