Android auto-dim on face down?

If you're using Android 5.x/6.x, then under Display setting of Settings app you should be able to find an option named Adaptive Brightness. That should precisely do the job. In preceding Android versions, there is Auto Brightness but I do not know how it functions. If what benjaminS stated is true, then it should work well.

In any case, if the auto/adaptive brightness isn't working out for you then you can consider an automation app to do the job precisely. I tried with Tasker (full 7 day trial; userguide)
on Nexus 6 running stock Android 6.0.1:

1. Profile: State → Sensor → Orientation → Is: Face Down



2. Task: (Action):

   
   
   
   
   

   • Display → Display Brightness:

     
     
     
     
     
     • Level: set the desired minimum
     
     
     • tick Disable Safeguard
     
     
     • tick Ignore Current Level
     
     
   
   

Done. Whenever the device would be facing downwards, the brightness level would change. Whenever the orientation would change, the brightness level would be restored.

If you don't want the brightness to be restored automatically, long press on the profile → tap on settings icon next to a trash can → untick Restore Settings.

Note:

• Auto/Adaptive Brightness might interfere here so you're advised to keep it disabled.


• Display Timeout is not bypassed by this and after the timeout the screen would turn off. You can extend the timeout under Display action group though.