A computing device determines whether or not to authorize a network request. In particular, responsive to the computing device receiving the network request from a mobile device, the computing device controls a plurality of light fixtures to each optically transmit a respective challenge code. The computing device approves or rejects the network request based respectively on whether or not a group of the challenge codes is received from the mobile device, each challenge code in the group being optically receivable at a location authorized for approving the network request.