A kind of variable transmittance glass, suspended particle devices (SPDs) operate with nano-scale particles that are suspended in a liquid. All of this is done in the scale of thin-film, and attached to glass. Controlled by electricity, this kind of switchable glass is opaque when turned “off,” and the particles are in disarray– providing very little light transmittance and a considerable amount of privacy. When electricity (voltage) is applied to the glass, usually via a switch or control panel, the particles line up, light is able to pass through and the glass becomes transparent. The presence of electricity must be constant for SPD to maintain its particle alignment, the same way electricity must continuously flow to a light bulb to keep it lit. Once the electrical flow is interrupted, the particles disperse again and the glass becomes opaque, the way a light bulb goes out. In this way, SPDs differ from electrochromic glass, which works under the same principles but requires only a single burst of electricity to change from one finish to the other.

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