SAN FRANCISCO-Engineers at Stanford University have developed new “smart” windows consisting of conductive glass plates outlined with metal ions that spread out over the surface, blocking light in response to an electrical current.

The dynamic windows can switch from transparent to opaque or back again in about 30 seconds, a significant improvement over dimming windows currently being installed to reduce cooling costs in some buildings, according to the researchers, who reported their work this month in the journal Joule.

Such technology, the authors claim, has the potential to optimize the lighting in rooms or vehicles and save about 20 percent in heating and cooling costs. The prototypes used in the study are about 4 square inches, or nearly 26 square centimeters, in size.

They block light through the movement of a copper solution over a sheet of indium tin oxide modified with platinum nanoparticles. When transparent, the window is clear and allows about 80 percent of surrounding natural light to pass through. When dark, the transmission of light drops to below 5 percent. To test durability, the researchers switched the windows on and off more than 5,000 times and saw no degradation in the transmission of light.

In comparison, “smart” windows already on the market are made of materials such as tungsten oxide, which change color when charged with electricity.

But Michael McGehee, a professor of materials science and engineering at Stanford and senior author of the study, was quoted as saying in a news release that these materials tend to be expensive, have a blue tint, can take more than 20 minutes to dim and become less opaque over time.

The researchers have filed a patent for the new technology.