Commonly called smart glass or switchable glass, variable transmittance glass can change from light to dark or from opaque to transparent and back again when exposed to voltage, light or heat. The catalyst triggers ions within the glass to align and reduce the amount of light the glass will transmit – the light transmittance of the glass is variable, depending on the situation, thus the name variable transmittance glass.
The most widely recognized use of this kind of glass is transition lenses on eye wear, but the technology has been successfully applied to architectural glass for many years. When used on surfaces facing the outside of a building, glass that automatically tints when facing sunlight will become clear as the sun shifts in the sky and away from that part of the building. This use of variable transmittance glass provides energy efficiency without the use of window film or window dressings, the latter of which require upkeep from the inside.
Another, somewhat more recent, kind of variable transmittance glazing are glazings the light transmittance of which can be controlled by electricity. These fall into two categories: suspended particle devices (SPDs) and electrochromic glass.
SPDs and electrochromic glasses provide adjustable privacy without the use of drapery or window dressings, which require upkeep and can collect dust and germs, which is a key selling point for using the technology in hospitals or other locations where people with compromised immune systems might live, work or seek treatment.