Researchers in Japan have demonstrated mirrored windows that transform from transparent to reflective at the flick of a switch.

The new material comes in the form of a flexible film that can be glued onto existing glass.

Local media reports suggested that publicity-shy celebrities could apply the film to limousine and building windows.

The inventors say it could also be used to make buildings and vehicles more energy efficient.

Although similar technology already exists to make glass opaque or change colour, it is not ideal for environmental control, claim the inventors.

"It is necessary to control light not by means of absorption but by means of reflection," said researchers at the Materials Research Institute for Sustainable Development at Japan's National Institute of Advanced Industrial Science and Technology (AIST).

"The switchable mirror performs this function by switching from a transparent film state to a reflective mirror state.

"When used as window glass, it will block light effectively and reduce the cooling load inside a building or vehicle."

AIST employs 2,500 researchers from a variety of disciplines, and emphasises applied technology over basic research.

The reflective film requires no complex control circuits. A low voltage is passed through the substance to change its state over the course of about 15 seconds, allowing the film to be switched to a partially-reflective state when required.

The polarity of the voltage selects the new state. The film maintains its most recent state even when the power is disconnected.

This change in reflectivity takes place as the electrical current pushes hydrogen ions in the film into a thin layer of a manganese-nickel alloy at its surface.

The alloy itself is reflective, but the hydrogen ions combine with it to form a transparent metal hydride compound. The reaction is reversible and the material has survived at least 4,000 changes from transparent to reflective in testing.

AIST has released images of the transparent and reflective states of the new material.