Next-generation optical communication and optical computing imply an evolution from binary to multinary computing. Light multinary computing encodes data using pulses of light components in higher orders than binary and processes it using truth tables larger than Boolean ones. This results in lesser encoded data that can be processed at faster speeds. We use a general-purpose optical transistor as the building block to develop the main computing units for counting, distributing, storing, and logically operating the arithmetic addition of two bytes of base-10 data. Currently available optical switching technologies can be used to physically implement light multinary computing to achieve ultra-high speed communication and computing.