From the Android Market, search for and install an app called Screen Filter, by haxor industry, and launch it.
The Screen Filter icon will display in the notification bar. Drag the notification bar down and select Screen Filter.
If you don't like the default filter percentage, drag the slider until you find the filter level you want.
It is possible to set the filter so low that you won't be able to read anything on your screen. Anything below 20 percent will be extremely difficult to read, so don't go below it. If you accidentally set it too low, reboot your phone and uninstall the app. You can then reinstall it if you want to try again.