iOS 5.0之前的版本并没有开放控制系统亮度的接口，所以app能做到的只是比系统设置的亮度更暗，具体做法就是在整个app的keywindow之上再放一个window，设置黑色背景，通过控制alpha值来达到明暗变化。

- (void)installBrightnessWindow
{
    m_brightnessWindow = [[UIWindow alloc] initWithFrame:self.view.window.frame];
    m_brightnessWindow.windowLevel = UIWindowLevelStatusBar + 1;
    m_brightnessWindow.userInteractionEnabled = NO;
    m_brightnessWindow.backgroundColor = [UIColor blackColor];
    m_brightnessWindow.alpha = 0;
    m_brightnessWindow.hidden = NO;
}

具体方法请查看demo：

http://download.csdn.net/detail/hursing/5183100

5.0以及之后，可以这样调节屏幕亮度，对整个系统生效：

[UIScreen mainScreen].brightness = 0.5;