I think one other thing you might want to do, after triple tapping the screen with three fingers, (unless you actually want the zooming as well), is to drag the zoom all the way to the left so that no actual zoom happens and only the filter gets applied.Hello world, there is a way to dimming the backlight quickly:
1. Settings -> General -> Accessibility -> Zoom -> Enable Zoom. Tap the screen with three fingers thrice.
Press 'Choose Filter' > 'Low Light', Chose Zoom Region to "Full Screen Zoom".
2. Settings -> General -> Accessibility -> Accessibility Shortcut (It's on the bottom) -> Set to "Zoom"
After that, if you click the home button triple, you can dimm the screen quickly!
It's a nice addition/alternative to using Invert Colors and/or Grayscale mode and/or the Reduce White Point option in Accessibility > Increase Contrast (which has much less of an effect).