More generally, this problem is called Optical Camera Communication (OCC). One of the more interesting things you can do is to exploit the rolling shutter read

More generally, this problem is called Optical Camera Communication (OCC). One of the more interesting things you can do is to exploit the rolling shutter readout method of consumer CMOS sensors to recover information with a high temporal resolution. By modulating the backlight of a video display or any other kind of wide-area light, you can transmit information optically to the phone in a way that's invisible to the user. I'm not aware of any commercial applications of such tech, but there are enough keywords in this post to find a few research papers on the topic.