The companies describe this initiative as a first, one that could be important to advertisers this upfront season as they decide whether to shift some of their TV dollars to Internet video: Nielsen is about to begin measuring ad viewing by people who stream programming on Roku players and TVs.

“Cross-platform audience measurement is critical for advertisers looking to leverage the trend of increasing [online] video consumption,” says Nielsen Global Watch Product Leadership EVP Megan Clarken. “By working with Roku to measure live streaming and on-demand advertising on its platform, we can gain great insights into [Internet] viewing and expand the measurement and delivery of Total Audience Campaign and Content insights for all of our clients.”

The Roku audience data initially will appear in the Nielsen Digital Ad Ratings. Programmers also will be able to order info about viewer demographics so they can offer the equivalent of ratings guarantees — a feature that television networks have long provided.

In addition to this announcement, Roku unveiled other services it will offer advertisers to reassure them they’re getting their money’s worth when they spend on digital video. The company says that about half of its 250 most-watched channels sell ads, including major new ventures such as CBS All Access and CBSN.

“In 2014 we streamed more than 3 billion hours on our platform,” says Roku Advertising VP Scott Rosenberg. “This transition promises huge changes in the way TV advertising is bought and sold.”