Illumination system with automatic adaptation to daylight level

An illumination system and method is disclosed. In one example, the illumination system comprises a light source, a controller controlling the power output of the light source, and a light sensor wirelessly communicating with the controller. In a normal mode, the controller controls the light source such that the light level remains substantially constant. In a change mode, the controller controls the light source such that the light level is gradually changed with a predetermined change rate so as to decrease a deviation from a target level. The controller switches from the normal mode to the change mode on the basis of input signals received from the light sensor. The light sensor measures a light level, and decides whether or not to communicate a signal to the controller. The light sensor refrains from transmitting a signal when receiving the signal does not cause the controller to change its control behavior.