We all know about Polaroid glass – it’s been around for ages. But this is a new one for me at least and has greater potential because the level of transparency/opacity is controlled by the user and not by ambient light.
SPD smart window is constructed by using two panes of glass separated by a conductive film with suspended, light absorbing, microscopic particles. Microscopic light-absorbing particles are dispersed within a thin film. When no electrical voltage is applied to the film, these particles absorb light, making the glass dark. When voltage is applied, the particles align and allow light to pass through. By simply adjusting the electrical voltage manually or automatically, the amount of light passing through the SPD-glass product can be controlled quickly and precisely.
A competing technology with SPD smart windows is the electrochromic smart window. Electrochromic windows consist of two glass panes with several layers sandwiched in between. It works by passing low-voltage electrical charges across a microscopically-thin coating on the glass surface, activating an electrochromic layer which changes color from clear to dark. The electric current can be activated manually or by sensors which react to light intensity. One advantage of the electrochromic smart window is that it only requires electricity to change its opacity, but not to maintain a particular shade.
Link: Smart Glass