Video is one of the biggest opportunities on the Web, but it is also one of the biggest black holes. Just because someone hits play on a Web video doesn’t mean they watch it all the way to the end. Boston-based startup Visible Measures is going to try to rectify that with new audience behavior measurement software it will be launching at this week’s DEMO conference. The data-collection software captures every play, pause, rewind, and e-mail-to-a-friend button that viewers click on while watching videos on a given site. Video sites with a higher level of engaged viewers will be able to then take that data to advertisers and try to negotiate for higher ad rates.

Visible Measures is also announcing a Series B financing today of $13 million, led by Mohr Davidow Ventures. General Catalyst, who led the company’s $5 million Series A in March, 2007, participated in this round as well.