For half a century or so, media advertising has been sold on the basis of “impressions”—that is, the number of people who were theoretically exposed to an advertisement. Even after media started moving online, this practice continued, with many publishers and advertising companies measuring results by traffic or “unique visitors,” or in some cases by clicks. Some media outlets are pushing for better metrics, however, and one of the newest candidates is the amount of time that a visitor is exposed to an advertisement.

The Financial Times is one of the media companies experimenting with this method, which it calls “cost per hour” or CPH advertising—as opposed to the traditional model that charges based on CPM, or cost per thousand impressions. After doing a trial with a number of large brands such as BP and IBM since last fall, the FT says that it is now rolling out the cost-per-hour approach across the company. According to the FT’s ad sales director Dominic Good:

“While CPM values every impression the same, CPH uses time to measure value. The FT has shown through extensive testing that brand familiarity and recollection among readers increases significantly the longer an ad is in view. Adverts seen for five seconds or more on show up to 50% higher brand recall and familiarity than ads that are visible for a shorter period of time.”

Under the new plan, advertisers who work with the FT? will only pay for an advertisement if it is seen by a user for more than five seconds of “active use” time. The FT has been working with Chartbeat, one of a number of companies whose tools allow media companies to track the behavior of their users, to measure this new metric, using what the company calls “active exposure time.” In a nutshell, Chartbeat measures when an active user is actually seeing an ad, rather than just measuring whether a page has loaded or is viewable on screen, by tracking their mouse movements and other interactive data.

Chartbeat co-founder and CEO Tony Haile has been a major proponent of time-based advertising, arguing that pageviews, unique visitors and even clicks are flawed methods for measuring the effectiveness of an ad. Last year, the company became the first analytics providers accredited by the Media Ratings Council to measure advertising based on the amount of time spent. At the time, Haile said:

“We’ve been talking for a while now about the attention web, and lots of people have said they liked it as an idea, but it was just an idea. But now it’s official — so now, there can be an attention economy, in which both publishers and advertisers buy and sell attention minutes or metrics as a measurement.”

Other media companies have also been experimenting with measuring their advertising according to time rather than simple impressions: Upworthy, the viral-content site, has also been a vocal proponent of what it calls “attention minutes,” and Medium—the content platform created by former Twitter CEO Evan Williams—uses what it calls “total time reading.” The Wall Street Journal, the Economist and Bloomberg have also experimented with charging advertisers based in part on time spent looking at an ad.

Why would a traditional media operator like the Financial Times be so eager to move to an untested new measurement for digital advertising? One motivating factor is that the FT doesn’t have a huge amount of traffic compared to some other digital players: just 12 million unique visitors a month or so (The Daily Mail has an estimated 200 million). But time-spent analysis shows that its readers spend more than three times as long on the site as the average reader does at other media outlets, which is a powerful selling point for brands.

As Haile has pointed out, there is no shortage of web pages, but what that glut of information has caused is a shortage of attention. If media companies like the FT can convince advertisers that they are able to measure actual reader attention, that could prove to be a powerful weapon in the battle to keep advertising revenue from sliding down a very slippery slope to oblivion.