If the belt is not slipping and this is not its normal behaviour then something is wrong, although it may not be serious or significant especially if you are achieving normal output and charging rates at normal operating revs and in particular if this is a case of the light staying on initially but not coming on again when the revs fall after increasing briefly.
Check all connections if you have an external regulator (unlikely unless you have something fancy fitted)
Check what volts you are getting and compare with the spec for the alternator, if you can, check the charging current but unless you know what you are up to don't!
If you have any doubts about the alternator output get it checked by an auto electrician or at a Lucas depot.
It is possible that it is a poor connection away from the alternator but in the circuits that feed the power to the warning light, but I think that would result in the light glowing dimly rather than coming on at full brightness.
-------------------- Old Chemists never die, they just fail to react