SAN FRANCISCO — Determined to sell more television ads, Internet search leader Google is sharpening its focus on the medium with demographic data from the Nielsen Co.

Under an agreement to be announced today, Google will pay Nielsen an undisclosed amount to obtain detailed information about the kinds of people who watch specific TV shows.

The breakdown, drawn from Nielsen’s rating service, typically provides viewers’ ages, gender, marital status and other personal data that help advertisers choose the audience most likely to be interested in their product or service.

Google’s access to the information is significant because it gives the Mountain View-based company more tools to draw upon as it tries to target television ads as effectively as it has done on the Internet.

“This is another indication that Google is very serious about selling more ads in the mainstream media,” said Brad Adgate, director of research for Horizon Media, which helps advertisers buy television ads.

Since May, Google has been delivering television ads for about 14 million subscribers to EchoStar’s Dish network and Astound Broadband cable service.

The set-top boxes used by EchoStar and Astound enabled Google to track when commercials were watched and for how long.