ALLENTOWN, Pa. — Thanks to modern technology and some expert detective work, a nearly 400-year-old painting that had long been attributed to an unknown artist in Rembrandt’s workshop has now been judged to have been a work of the Dutch master himself.

For decades, the Allentown Art Museum displayed an oil-on-oak panel painting called “Portrait of a Young Woman” and credited it to “Studio of Rembrandt.” Two years ago, the painting was sent to New York University for conservation and cleaning.

There, conservators began removing layers of overpainting and dark, thick varnish that had been added over centuries — and they began to suspect Rembrandt himself was responsible for the original, delicate brushwork underneath.

“Our painting had numerous layers of varnish and that really obscured what you could see of the original brushwork, as well as the original color,” said Elaine Mehalakes, vice president of curatorial affairs at the Allentown Art Museum.

Conservators used a variety of tools, including X-ray, infrared and electron microscopy, to bolster the case that it was the work of one of the most important and revered artists in history.

The scientific analysis “showed brushwork, and a liveliness to that brushwork, that is quite consistent with other works by Rembrandt,” said Shan Kuang, a conservator at New York University’s Institute of Fine Arts who restored “Portrait of a Young Woman.”