Data exfiltration from a device is usually achieved over the network, via hardware implant, or by manipulating the characteristics of an internal electronic component. Optical covert channels transmit data using visible light in a method undetectable to the human eye.
Joe demonstrates using an optical receiver to capture data transmitted through a LED:
joegrand has shared the receiver boards on OSH Park:
digital version using Everlight PLR135/T9 Fiber Optic Receiver
analog version based on Maxim Integrated’s AN1117 application note: