Is there a TRUE brightness controlling app? I need to set my brightness below what Android allows me, and thus I'm testing a bunch of "dimming" apps. What they all have in common though is that they put on a filter over other apps so while they protect your eyes they do nothing to get you more battery life.

Is there an app that has low-level access to the screen brightness so that when you go beyond the default minimum brightness it actually gets you more battery life?

RootDim might be fitting, but I need something without root access.
I have been using Lux Lite and it works well for me.

Lux isn't your ordinary brightness app. It intelligently adjusts the brightness of your display based on the environment you're in. If you step into a dimly lit room, Lux will automatically lower the brightness of your display to make it not only comfortable to read, but to also preserve battery power.
Lux is also able to adjust the screen temperature of your display automatically to make it appropriate for night usage. At sunset, you can have Lux automatically warm your display and switch to your Night profile. If you're an astronomer, you can enable astronomer mode to cut out harsh white light. If you have a root enabled phone, you may even be able to apply these enhancements at a low level with Lux's plug-in system.
Perhaps the best thing about Lux is that you can teach it how to behave. If the backlight is too bright or dim for your liking, simply open the provided Dashboard, adjust the brightness slider to the level you desire, then hold the link button. Lux will use this new 'link' as a reference in future.