Transmission geometry neutron optical polarizer devices offer very substantial advantages compared to conventional bender systems by low inherent indensity losses and by conserving the angular distribution of the beam. In order to achieve high polarizations they require very high reflectivity polarizing supermirrors. A technique of producing this kind of mirrors with a reflectivity >= 97% has now been established at BENSC. Both the calculations based on measured supermirror characteristics and the first partial experimental results confirm the expected high performance of so-called 'polarizing cavity' type transmission devices. Another result of polarizing neutron mirror studies at BENSC is the development of single layer totally reflecting coating with high magnetic remenance which is adequate for use in neutron polarizers placed in zero or even oppositely oriented magnetic fields.