By Tomio Geron, Dow Jones Redwood City, Calif. – Turn Inc. has raised $20 million in Series D financing, led by growth investor Greenspring Associates, to build out its demand-side platform for advertising agencies. All existing investors--Focus Ventures, Norwest Venture Partners, Shasta Ventures and Trident Capital--invested at their full pro rata, the company said. Valuation was not disclosed. The Redwood City, Calif.-based company doubled its revenue in 2010 from 2009, said Bill Demas, chief executive of Turn. The company's self-serve platform enables advertising agencies to do all their buying across a range of ad exchanges and ad networks. Turn provides a technology platform that analyzes inventory on exchanges and other networks in real-time to provide the highest return for marketers, Demas said. "What we're able to do is consistently offer superior ROI to digital marketers through our solution and help them reach specific audiences at scale," Demas said. While originally the company operated as something closer to an ad network, the company has moved into focusing on being a "demand-side platform" offering technology for advertising agencies to buy inventory themselves. In the past ad networks offered very little real-time bidding and did not offer the exact type of audience a client wanted but rather a Web page where a certain audience would typically visit, Demas said. These days, Turn offers everything in real-time and much more sophisticated targeting, he said.