Dim Screen but Don't Lock

I'm trying to let unity allow an iphone or android device to dim, but after a while the device will shut off.

Is there a way to allow the device to dim (or, even better: automatically dim the device), and keep the screen from locking after a moment?

I'm currently using

 Screen.sleepTimeout = SleepTime.NeverSleep;

but that doesn't allow the screen to dim. During some parts of the app, I would really like to save some battery power while the user responds to audio cues, but I don't want the device to eventually shut off / go to the lock screen. Does anybody know if something like this is possible?