If your partner has doubled an artificial bid made by the opponents, he asks for the lead in that specific suit!
If your partner has doubled the final contract bid by your opponents, it means that he thinks that there are chances that they will go down. If he has the setting tricks in his hand, there will be no problem, but often by doubling he asks his partner for a specific lead - That’s what we call a Lightner double!
It’s your lead!