IOS5.0+中，UIScreen类多了一个属性brightness，可以调节main screen的屏幕亮度，值为0.0f-1.0f之间。点击这里查看UIScreen - brightness属性的解释. 这个做法应该是真正能省点的方法了，屏幕上覆盖黑层的做法，据说是不省电的。

brightness
The brightness level of the screen.

@property(nonatomic) CGFloat brightness
Discussion
This property is only supported on the main screen. The value of this property should be a number between 0.0 and 1.0, inclusive.

Availability
Available in iOS 5.0 and later.
Declared In
UIScreen.h