A company called Soladigm has raised about $20.66 million in equity, though how the company plans to use it is a mystery.

The Milpitas, Calif.-based startup raised the money from Khosla Ventures and Sigma Partners after offering to sell $21.6 million of its shares, according to its Oct. 1 filing with the U.S. Securities and Exchange Commission.

The company was incorporated in 2007, but its website says nothing about its products. Soladigm hints only that it's working on "green building solutions" that involve glass, optical coatings and semiconductors.

"Green building" is a term that means a whole lot of things, of course. Adding solar panels or using building materials that take less energy to produce than conventional components all can qualify.

Soladigm was founded by Paul Nguyen to develop windows embedded with electrochromic devices, according to his LinkedIn page. The company was previously called Echromics and based in Santa Rosa, Calif.

Electrochromic devices can change the colors of the windows and thus controls the amount of sunlight that passes through when receiving an electrical current. The technology could help cool a building.

Soladigm has raised several rounds of money this year. Aside from the $20.66 million equity round, the company also raised about $1 million in the form of equity, debt and warrants/options, according to a SEC filing last month. Back in June, a SEC filing said the company had raised $2 million in debt and warrants/options.

The SEC filings list Rao Mulpuri as a company executive. Mulpuri's LinkedIn profile says he's the CEO. He was previously the president of Novellus Systems Japan, a maker of semiconductor factory equipment.