The Mishna in Masechet Shabbat (listen to audio recording for precise citation) establishes that one may not light the Shabbat candles on Friday afternoon at the time when it is uncertain whether or not night has begun. The period of Ben Hashemashot, which begins at sunset and extends for approximately 15 minutes, has not been identified by Halacha as day or night; during this period, we are unsure whether or not the Halachic day has ended. The Mishna therefore rules that if a woman did not light the Shabbat candles before sundown, she may not light them during Ben Hashemashot, since Shabbat may have already begun, in which case it is already forbidden to light candles.
Moreover, if a woman had not lit the Shabbat candles and she looks at her watch and sees that there are only two or three minutes left until sundown, she should not light the Shabbat candles. She must be concerned that her watch may not be precisely accurate, and that the sun as already set. Therefore, although the published time for candle lighting is 18 minutes before sundown, one is not permitted to light candles 16 or 17 minutes after the published time. Once 15 minutes or so after the time have passed, a woman should not light the Shabbat candles, given the possibility that the time on her watch is not precise.
If a woman did not light Shabbat candles on Friday afternoon before the final time, and she also did not leave any lights on in the home, she may ask a gentile to turn on the lights in the home, so long as she is still within the period of Ben Hashemashot. During this period, Halacha allows asking a gentile to perform activity forbidden for a Jew to perform on Shabbat, if this is necessary for the purpose of a Misva. Therefore, within 15 minutes or so after sundown, a woman may ask a gentile to turn on the lights in the home if otherwise the house would be completely dark on Shabbat. If, however, there are lights on but the woman did not light the Shabbat candles, she may not ask a gentile to light the candles for her. Since there is already light in the home, and the Shabbat candles are not needed for light, she may not ask a gentile to light the candles.
Summary: A woman who did not light the Shabbat candles at the preferred time (18 minutes before sundown) may light afterward, until two or three minutes before sundown. If she did not light until that point, she should not light. If there are no lights on in the home, she may ask a gentile to turn on the lights in the home, provided that 15 minutes have not passed since sundown.
