Google Glass is an augmented reality head-mounted display, allowing hands-free access to the web. The product resembles normal eyeglasses where the lens is replaced by a small electronic screen. It provides interaction via natural language voice commands, as well as eye-tracking technology. A miniature gyroscope can tell the user's position and orientation at all times. On the side frame is audio output, and a touch control pad, while on top is a button for recording photos and videos with a built-in camera. It is available to developers by 2013 and for the general public by 2014. The design allows for integration of the display into people's day-to-day eyewear. It is light and weighs less than most sunglasses. A prototype unveiled in 2012 received criticism over the potential for Google to insert advertising (its main source of revenue) into the user's field of vision. However, the company denied it would use adverts.