Polarizing Filters
The ability of a polarizer to reduce or remove reflected glare from windows is dependent on not only the polarization efficiency of the filter but on the reflection angle of the light to the camera, and the rotational position of the polarizer. The UltraPol from Tiffen and the Schneider True-Pol, for instance, are both similarly efficient, and represent the best of what is available for cinematography. If you take two filters of the same type and rotate them one against the other, so that they achieve minimum light transmission, the darker they appear the more polarization efficient they are, and the better they will be at reducing glare. However, if you are trying to remove light from a glass surface, that light has to be reflecting at about a 33-degree angle from the glass for maximum effect. Any other angles will not be as effective in reducing glare. You can't shoot straight at a window from a 90 degree angle and get much effect. Of course, you also need to rotate the polarizer once everything is otherwise in position, until you get the best effect. Typically, a one-stop polarizer, which can be quite effective when you can't afford to lose the extra 2/3 stop the traditional polarizer absorb, is also not quite as polarization efficient, since that depends in part on the amount of light absorbed.