To track their audience, networks rely on Nielsen ratings and Web content providers tally page view numbers. But creators of digital billboards, posters, and retail store displays have a harder time counting eyeballs.Now, Kingston (Ont.)-based Xuuk is offering a four-inch-square box that it says can do just that. The device, eyebox2, is studded with diodes that flash lights — invisibly — to induce a “red eye” effect in viewers. A camera in the box, which can be discreetly incorporated into a digital poster, tallies all the red-eyes within a 30-foot-range.