Nielsen is promising to give content companies a glimpse into some viewing metrics of Netflix’s and Amazon’s subscription video-on-demand services. But its measurement of the SVOD universe will be far from comprehensive.

Starting next month, the research company plans to start monitoring usage of Netflix and Amazon Prime Instant Video among participants of its nationwide consumer panel, using audio-recognition technology to determine what they’re watching.

However, Nielsen initially will only let companies see SVOD viewing information for their own content — not the entire scope of what Netflix and Amazon subscribers are watching. That means there still won’t be an industry-wide measure of, say, how many people have tuned in to Netflix’s “House of Cards” relative to other titles (only the show’s producers will be able to see that data from Nielsen). Moreover, Nielsen — for the foreseeable future — won’t be able to measure SVOD viewing on computers or mobile devices, which is a pretty sizable blind spot.

The purpose of the new service, Nielsen says, is to let media companies begin to gauge what effect licensing programming to SVOD services has traditional TV ratings. At some point in the future, Nielsen suggested, clients may be able to access SVOD viewing data for content other than their own; but even then, it’s unclear how complete a picture that will be of overall Netflix and Amazon streaming usage.

Nielsen’s plans to track viewing on SVOD services were first reported by the Wall Street Journal. Netflix and Amazon declined to comment.

Hulu, which is an ad-supported service, is already measured by Nielsen. But Netflix and Amazon, whose SVOD services don’t run advertising, do not provide viewing data to third-party firms (although they do share some metrics with content partners).

As limited as it is, Nielsen’s SVOD-measurement proposition may be the only game in town as an independent measure of viewing on Netflix, Amazon and others. “There is literally no other entity, other than Nielsen, in a position to capture and offer this kind of viewing insight,” Bernstein Research analyst Todd Juenger wrote in a research note. “You cannot capture this with set-top-box data, for instance. You need a panel, and you need sophisticated, special-purpose, on-premises measuring devices (i.e. People Meters).”

According to Nielsen, SVOD services are definitely cutting into TV time. For example, consumers 18-49 who subscribe to an SVOD service spend 20% less time watching TV than non-subscribers do, the company’s research shows.