Apparatus, systems and methods employing contact lens sensors are provided. In some aspects, a contact lens includes a substrate that forms at least a portion of the body of the contact lens; an optical communication device disposed on or within the substrate; and a photodetector disposed on or within the substrate, wherein the photodetector harvests light emitted from a device and generates power from the harvested light. In some aspects, an apparatus comprises a tag having a circuit including: an optical communication device; and a photodetector that harvests light received and generates power from the harvested light. The tag can be disposed on or within a contact lens in various aspects.