With the increasing variety of mobile applications, reducing the energy consumption of mobile devices is a major challenge in sustaining multimedia streaming applications. This paper explores backlight scaling, which is deemed a promising technical solution. First, we model the problem as a dynamic backlight scaling optimization problem. The objective is to minimize the energy consumption of the backlight when displaying a video stream without adversely impacting the user's visual perception. Then, we propose a dynamic-programming algorithm to solve the fundamental problem and prove its optimality in terms of energy savings. Finally, based on the algorithm, we consider implementation issues. We have also developed a prototype implementation integrated with existing video streaming services to validate the practicability of the approach. The results of experiments conducted to demonstrate the efficacy of the proposed algorithm are very encouraging.