The copy’s background was black, and the painting was covered in a layer of dark varnish, which gave it a yellowish glow and further diminished its vibrancy.

But the Louvre was planning a special exhibition of Leonardo’s work and, because it did not want to move the original Mona Lisa from its protected area, wanted to borrow the Prado painting as a stand-in. A casual comment by one of the Louvre curators, asking whether the painting had ever been studied, got Ms. González thinking.

The next day she took her infrared camera into the gallery and got to work. Just the first pictures were enough for her to conclude that the two paintings had been produced in tandem. After that, it was just a question of watching the evidence pile up.

Perhaps the most exciting discovery was that the painting’s original background had been obscured by a layer of black paint, a practice sometimes used in the 18th century. Luckily, a layer of lacquer protected what was under it. So, once the paint was removed, the same Tuscan background as in Leonardo’s painting appeared, offering a tantalizing preview of what might be seen if Leonardo’s Mona Lisa were restored.

THERE is no doubt, however, that the Prado painting was not a copy made by Leonardo himself. While the corrections are identical, the lines are not. “Like I write an A and you write an A, you can tell it is not the same,” Ms. González said.

Parts of the Prado copy are beautiful, she said, like the hands. But in general, it is not nearly so fine a painting.

Just why it was made remains an open question. It could have been simply for a pupil’s instruction or a double commission.