Google has received a patent on its Google Glass technology for a "pay-per-gaze' technology that raises further privacy issues for users.

It puts forth an idea for pay-per-gaze advertising – a way in which people interacting with ads in the real world could be analysed in the digital world.

In the patent, which was filed in May 2011 and granted last week, Google claims that “a head-mounted gaze tracking device” – presumably Google Glass – would send images and the direction the person wearing the device was looking to a server. The system would then identify real-world ads that the person wearing the gadget had seen, allowing Google to then charge the advertiser.

As Google notes in the filing, advertisers can be charged a fee based on whether a person looks directly at an ad in the real world, and the fee can change based on how long they interact with the ad.

Google does not show any advertising in Glass. It goes so far as to forbid app developers from selling apps or ads, too. But there have been suggestions that Google will eventually show ads and the company has consistently said it expects Glass to be profitable.