A relatively cheap alternative is taking 2 polarized pieces of glass and making them rotate one on top of the other - that way the amount of light they let thro

A relatively cheap alternative is taking 2 polarized pieces of glass and making them rotate one on top of the other - that way the amount of light they let through is variable. The main drawback is that it requires circular windows with mechanical rotors. There is also a patent that allegedly doesn't require rotation, but I have never seen it realized: http://www.google.com/patents/US6414790 http://www.google.com/patents/US6414790