Polarized sunglasses: should the axes in both lenses be parallel? See the pictures below.  A pair of sunglasses I recently purchased has the polarization axis in one lens offset about 20 degrees (by eyeball estimation) from the other.  I don't have much experience with other polarized sunglasses, but this seems very obviously wrong to me.  And it's very noticeable while wearing the glasses.  When facing into glare from certain directions, one eye filters our considerably more glare than the other, which is very distracting and uncomfortable.
I can see arguments for making the polarization axis angle just about anything (though horizontal or vertical seem most logical), but what I cannot see is any argument for having it different in each lens on the same pair of glasses.
Thoughts?


 A: The main idea behind polaroid sunglasses is that reflexion from water, snow and other glary reflectors is mainly polarized in one direction. To understand this, witness the behaviour foretold by the Fresnel Equations (the graph below taken from the Wikipedia "Fresnel Equations" page):

so that you can see for a wide range of scattering angles from these surfaces, the reflected light reaching your eyes is mainly in the $s$-polarized direction (electric field vector orthogonal ("senkrecht" in German) with the plane of polarization), so if you quell this polarization, you get rid of most of the glare from these surfaces.
Why are your lenses twenty degrees off in their polarization axes? I'd say that this is a simple question of production economics. The power through a polaroid varies like $(\sin \theta)^2$, where $\theta$ is the angle between the actual polarizing axes and their ideal directions for quelling a given linear polarization. This functional dependence is very flat for a wide angle range around the null, so, if there is a twenty degree error, the attenuation ratio is still 0.1. So a polarizer that is twenty degrees off is still almost as good as an ideally aligned one for the lower-the-glare-in-human-sight application. Therefore, a manufacturer simply will not go to the extra cost of the quality control needed to align the polarisers more accurately: it really wouldn't make the product any better for the application at hand.
A: Optometrists say that the polarization axis tolerance between two lenses should be less than 2 degrees.  Check out http://www.2020mag.com/ce/TTViewTest.aspx?LessonId=10154 or http://www.optiboard.com
