A light unit includes a light transmitting portion and a body. A detecting member and a light source unit are located in the light transmitting portion. The detecting member detects the environment brightness and the light source unit emits light to compensate light difference between the expected brightness and the environment brightness. A control member is located in the body and electrically connected with the detecting member and the light source unit. The control member includes a signal magnifying unit, a switching unit and a control unit. The signal magnifying unit receives and magnifies the light signal from the detecting member, and the switching unit switches the light signal from analog signal into digital signal and sends the digital signal to the control unit. The control unit controls the light source to compensate the light difference between the expected brightness and the environment brightness.