what would cause the battery light to come on on a series 2

it happened on sunday, just lit up and hasnt gone out since. i have a voltmeter and all is fne with the battery and alternator. charges fine, car starts each time etc

its just the light on the clockis is on when the engines running like its shorting out or summit?? also strange how it gets brighter when i rev the engine, ie it is dim at idle, but bright when driving at 60 mph


not worried just confused as i have no idea how the battery light works etc

help please guys