A new measurement service, already being used by eight media giants, gives first-ever insights into who is watching streamers — and how those audiences stack up to linear TV.

The curtain is lifting on Netflix's mysterious viewership data, so says Nielsen Media. The godfather of TV ratings recently launched SVOD measurement in earnest and plans to go out with viewership statistics on big-swing originals Mindhunter and Stranger Things 2 in the coming weeks as it courts additional clients to the new service.

"It's about shedding light on a large area of viewership that people have been blind to," says Brian Fuhrer, senior vp product leadership at Nielsen. "We've gone public with some insights already, but the logical thing you'll see next is how all of these big Netflix launches rank against linear TV."

Among those stats floating around are for House of Cards. Now set to end with its sixth season, the show had 4.6 million viewers for its May premiere by standards comparable with the live-plus-7 ratings window, according to Nielsen. Eight media companies, including Disney-ABC, NBCUniversal and Warner Bros., already are paying for those numbers — which, like traditional Nielsen measurements, come from audio codes within programming.

Agreements with Nielsen currently forbid it from going public with the data, but it's considered an inevitability. Ramifications could extend beyond the end of one of Hollywood's biggest mysteries. Originals' ratings data might influence talent contracts, and information on acquired series are expected to impact library deals for studios. But don't expect Netflix to authenticate any numbers it doesn't bring to the negotiating table itself. Chief content officer Ted Sarandos teased his fervent secrecy at the Oct. 26 Stranger Things 2 premiere. (A Netflix spokesperson did not respond to a request for comment.)

Many sellers are said to be equally interested in what's not working on Netflix as what is. One producer cited the streamer's lack of promotion for many of its originals as a source of increasing concern, citing a reluctance to surrendering rights to the Netflix "vortex" without any promise on proof of penetration. "It's hard to believe that this won't change the business," says another TV exec who has seen the data. "But if I were Netflix, I'd be saying that it's not right. Because it's not yet. It's not complete."

What's missing? Mobile, for one. That can account for as much as a third of viewing, and Nielsen's SVOD is only capturing its current sample's streaming habits on TV sets.

Attaining the complete picture, however, will come in time. "Our approach is using the exact same framework and methodology that we do for all of television," says Fuhrer. "You can put these Netflix and broadcast numbers right next to one another and understand the relationship."

A version of this story first appeared in the Nov. 1 issue of The Hollywood Reporter magazine. To receive the magazine, click here to subscribe.