Digital OOH is an emerging marketing channel with audiences that are both desirable and captive. Here's how to harness opportunities on "the fourth screen."

There's a burgeoning ad opportunity out there that's not too well known -- yet it's right under our noses. You see it every day, even if for just a few minutes at the grocery store or the gas station. Maybe you see it on your way to work or while picking up dinner, maybe at the mall, or on the elevator going to see the boss upstairs.

If you're advertising on digital out-of-home (digital OOH) media already, you're ahead of the curve. If you aren't, now's a good time to take a good, hard look.

Some industry players are referring to digital OOH as "the fourth screen," with television, computers and mobile representing the other three. Digital OOH encompasses everything from elevator news readers to digital billboards to gas station video panels. While the channel has yet to achieve the widespread acceptance of other media, it does represent a fast-growing advertising opportunity.