How do filters such as the Singh-Ray LB Polarizer transmit more light then other circular polarizers? I am particularly interested in this technology because they claim to only add 1-1/3 stop to the exposure, where as most CPL's that I am familiar with eat up somewhere around 2 stops. I would think that they would all transmit an equal amount of light if they are achieving an equal amount of reflection reduction and so on.
Hopefully someone else can give a more detailed answer, but a circular polarizing filter is a compound filter produced by several stacked elements. Additionally, there are multiple different techniques that can be used to make each of these layers.
Presumably, some light of the desired polarization is either absorbed or reflected during the process, however some processes and/or material qualities result in less absorption and/or reflection of the desired light.
I'm not sure what the most likely direct cause is though or if there is even a most likely one. It would probably take a detailed read of the specification for the filter to understand what is different about the optical properties. Also, I don't personally know which techniques would allow more or less overall light. I just know that there are multiple options for how to do it.