Check the battery's voltage also. If you're getting an increase in brightness above idle rpm your charging system may be fine. You should have 13 volts or so when the alternator is charging ok. When you shut off the engine the voltage should drop to about 12.5 volts.
If it's much less than this, your battery may be on its last ions. You can get the battery load tested. I had one checked at an auto parts store and they read 11 volts at 200 amps. The guy asked me "have you been charging this battery"? Within seconds I smelled that hot electronics smell that indicated that my battery was fine. An honest auto parts store might do this test free of charge, and not care whether they sell you a new battery. Good luck.