Google patents ‘pay-per-gaze’ eye-tracking that could measure emotional response to real-world ads. US Patent number US008510166, which Google initially applied for in May 2011, details the use of a head-mounted sensor to determine when you look at advertising and for how long.

Pay per gaze advertising need not be limited to on-line advertisements, but rather can be extended to conventional advertisement media including billboards, magazines, newspapers, and other forms of conventional print media. Thus, the gaze tracking system described herein offers a mechanism to track and bill offline advertisements in the manner similar to popular online advertisement schemes. http://www.theverge.com/2013/8/18/4633558/google-patents-pay-per-gaze-eye-tracking-ads

The Future of Advertising: ‘Pay-Per-Gaze’ Is Just the Beginning: http://mashable.com/2013/08/15/the-future-of-advertising-pay-per-gaze-is-just-the-beginning/