Method and system for providing coupon

[Problem] To utilize the spatial or temporal modulation component imparted to capturable light during the use of a service, and improve the convenience of coupon provision. [Solution] The present invention provides a method for providing a coupon by using a terminal device and at least one server that provides a service to the terminal device, wherein the method for providing a coupon comprises the terminal device capturing light during the use of the service, transmitting key information based on the spatial or temporal modulation component of the light to the server, and receiving coupon information pertaining to the service that is transmitted from the server, and the server receiving the key information from the terminal device and issuing the coupon information in accordance with the key information.