Quote Originally Posted by Pete H View Post
With a linear polariser a simple energy argument gives at least one stop of loss. These are circular polarisers though. What is the angular spectrum of the incident light?
Circular polarizers are linear polarizers. The effective bit is exactly the same.
The difference is in what happens behind the filter, i.e. after the linear polarizer has done what linear polarizers do.
Circular polarizers have an additional quarter wave retarder plate, which gives the polarisation vector a push, making it spin around the light's direction of travel.

So at least one stop for circular polarizers too.