Facebook has named Nielsen, comScore and Integral Ad Science as its independent verification partners for viewability.

Starting in the next few months, the three firms will work to verify viewability and attention metrics for photo and video ads on Facebook.

This announcement follows on from a similar partnership with Moat last September, which covered number and lengths of views for video ads, and is part of an effort by Facebook to improve its transparency for advertisers.

“Providing actionable and verified data to marketers is one of the most important ways we empower them to grow their businesses. As feed-based mobile advertising continues to grow in volume and capability, we are committed to supplying marketers with data they trust,” reads Facebooks statement.

Facebook first introduced viewable impressions in 2014, but notably uses a different definition to the IAB and MRC standards – counting an impression “the moment an ad enters the screen of a desktop browser or mobile app”, rather than requiring at least half of the ad to be in view for one second or more.