How to set As Lower Brightness as in Clock application, without rooting
The Clock app is setting flags on its own fullscreen window that override the system default screen brightness, but this only affects that app's window.

See the source for DeskClock.java

 551             winParams.flags |= WindowManager.LayoutParams.FLAG_FULLSCREEN;
 552             winParams.dimAmount = DIM_BEHIND_AMOUNT_DIMMED;
 553             winParams.buttonBrightness = WindowManager.LayoutParams.BRIGHTNESS_OVERRIDE_OFF;


The flags used are documented on the Android Developer site, in particular:


  float BRIGHTNESS_OVERRIDE_OFF Value
  for screenBrightness and
  buttonBrightness indicating that the
  screen or button backlight brightness
  should be set to the lowest value when
  this window is in front.


So, the DeskClock is not using anything special to dim the screen, but it can only affect its own window.