A new type of "smart" window that switches from summer to winter mode has been made by South Korean scientists.
The window darkens when the outside air temperatures soar, and becomes transparent when it gets cold in order to capture free heat from the sun.
Similar windows already exist, but the researchers say their method allows for an almost instantaneous switch from opaque to transparent.
This may help save more energy, the team writes in the journal ACS Nano.
"This type of light control system may provide a new option for saving on heating, cooling and lighting costs through managing the light transmitted into the interior of a house," said the scientists.
"Smart windows can prevent the inside of a building from becoming overheated by reflecting away a large fraction of the incident sunlight in summer.
"Alternatively, they can help keep a room warm by absorbing the sun's heat in winter."
The existing technology uses charged particles called ions sandwiched between panes of glass.
Electric current is then applied to switch the window from opaque to clear and back.
But Ho Sun Lim from Korea Electronics Technology Institute, Jeong Ho Cho and Jooyong Kim from Soongsil University decided on a different approach.
They used a special polymer, a different sort of charged particles known as counterions and solvents such as methanol.
The report states that the result was a glass that was a lot cheaper to manufacture and much less toxic than those currently available on the market.
The window is able to switch from 100% opaque to almost completely clear in a matter of seconds, said the authors.
Although "dimming" windows already exist, it is often necessary to switch them from winter to summer mode and back manually, using additional equipment such as home-automation panels.
"Until now, the numerous technologies developed not only have been chemically unstable, prohibiting their use in long-term switching applications but have been accompanied by the use of expensive special equipment and complicated harsh processing conditions," stated the report.
Dr Stephen Morris from Materials Knowledge Transfer Network that is funded by the UK's Technology Strategy Board said that if the new method allows the window to switch from opaque to transparent and back pretty much instantaneously, then it is going to be a real benefit in terms of energy savings.
"That would mean that you're allowing light in much quicker - and this can reduce the amount of heat loss out of houses or increase the cooling in summer," he said.