It is forbidden to make use of the light of the candles, whether for holy or secular purposes, although it is permitted for the additional candle. Some are more stringent, and forbid the additional candle too. According to the secret, if one uses the light, a great rupture is created and the outer forces have access to the supernal light, as R. Isaac Luria pointed out. All of this applies only to the time of the mitzvah.