An electric light and external light control system for a space with a dual-mode light sensor is proposed. The dual-mode light sensor (106) measures and computes the amount of external light and electrical light incident on its sensing surface. The individual measured light components (total light, electric light and external light) are transmitted to the window treatment controller (103) and the electric light controller (102). The controllers (102, 103) use this information to optimally control the lighting condition to meet user requirements and reduce energy consumption. Both controllers (102, 103) operate concurrently and independently, but are linked via the dual-mode sensor (106).