Turn on Android display when someone approaches the device

Question

Using a bit of computer vision, I'd like to turn on the display of a wall-mounted Android device when a human walks up to (w/in 2 feet of) the device.

I'm an experienced developer, but I know next to nothing about computer vision. Programmatically turning on the display is easy enough. Any guidance, reading suggestions, tutorials, places to start, etc. would be appreciated.

Edit: To clarify after some comments in the answers -- since this is a permanently-ish wall mount situation, the device will have full-time power. Battery life is a non-issue.

Answer 1

Android has Proximity and Light Sensors, but I don't think those are good enough for your need.

If the display has a front camera, you could probable use OpenCV along with some form of Motion detection to detect when someone approaches close to the screen.

EDIT: Here are the docs for Computer Vision using OpenCV.. http://docs.opencv.org/