Over the years, there have been different ways to reduce glare from mirrors for drivers - dimmable electrochromic mirrors being the latest option. These mirrors often have a limited lifespan when exposed to the environment, and are often costly to replace.
It is now feasible to have a rear-view mirror assembly whose reflectance is mechanically adjustable. This is accomplished by using polarized light and a pair of specially constructed polarizers. These polarizers attenuate light by sliding past one another in a linear fashion, as opposed to rotating with respect to one another to cross their transmission axes. This is ideal for rear-view mirrors whose shapes are often oblong and rotation of a polarizer is awkward.
In the event of a power failure, these polarizing mirrors are still capable of dimming. They can be made to function well in both hot and cold environments, have a long lifespan, and are inexpensive when compared to electrochromic mirrors.