Startup iSpot.tv, which sells a service tracking national TV ads and correlating those with digital responses, has raised $21.9 million in Series B funding.

The investment was led by Insight Venture Partners, with participation from previous investor Madrona Venture Group. The funding round brings Bellevue, Wash.-based iSpot.tv to about $27 million raised to date.

According to iSpot.tv, it will use the new financing to accelerate product development. The company’s service provides real-time tracking of national TV ads, including when and on which network they run, and how each ad subsequently performs on Facebook, Twitter, YouTube and major search portals in terms of views, shares and engagement.

Founded in 2012, iSpot.tv now has more than 60 employees and about 150 clients. The company hosts a library of 100,000-plus TV ads, and its service has more than 5 million monthly visitors.

“We are always looking to invest in technology companies that drive transformation in their industries, and iSpot.tv has proven it can do just that. We look forward to working with the team in the company’s next stage of continued growth,” Insight Venture Partners principal Brad Twohig said.

Added iSpot.tv founder and CEO Sean Muller, “TV is changing rapidly and we are excited to bring on investment partners that share our vision for the future of TV and its measurement.” Prior to founding iSpot.tv, Muller was chief operating officer of MediaNet Digital, a digital music platform company, and before that was chief technology officer at Demand Media.