Tap to launch Settings from your SpringBoard.
Choose General from the the Settings menu.
Select Accessibility from the General menu.
Tap Zoom from the Accessibility menu.
Choose Zoom Region from the Zoom menu and select Full Screen Zoom. Tap the Zoom button at the top left to return to the Zoom menu.
Press the Zoom switch to turn the feature on. Triple tap the screen with three fingers to bring up the zoom dialog.
Drag the zoom slider all the way to the left.
Select Choose Filter from the dialog box and then choose Low Light from the list of filters.
Tap off the dialog box to close it then press the Zoom switch to turn off the feature.
Return to the Accessibility menu by tapping the Accessibility button at the top left of the screen.
Scroll down to the bottom of the menu and tap the Accessibility Shortcut option. Tap to select Zoom from the list of items.
Press the Home button to return to your SpringBoard then triple press the Home button to enable low light mode. You can now move your brightness all the way down to reach dimmer settings than previously possible.
Triple pressing the Home again will restore regular brightness mode.