Two museum guides have helped identify a 17th Century artist whose painting hangs in a California museum after sunlight was reflected onto it.
The name of the artist - and date of when the work was painted - had been a mystery for decades, according to officials at the Hearst Castle museum.
In November a tour guide noticed a beam of light reflecting off a mosaic floor, illuminating the concealed signature.
The light revealed dark-coloured letters against a brown backdrop.
The painting, depicting the Annunciation of the angel Gabriel, has been identified as a 1690 work by Spanish artist Bartolomé Pérez de la Dehesa.
The painting has hung at the elaborate residence of the late newspaper magnate William Randolph Hearst since 1927.
The observant guides - Carson Cargill and Laurel Rodger - returned after-hours to the assembly room of the estates main residence, where they climbed chairs to shine lights on a part of the painting that is not so normally illuminated.
"This is a major new discovery for the oeuvre of Pérez," museum director Mary Levkoff said in a statement to BBC News.
Mrs Levkoff added the painting had undergone two preservation treatments, but the inscription and monogram had never been previously reported.
The guides had each worked at the museum for about three years and had gone through the same training programme together.
According to Hearst records, the painting was purchased in 1927 with another artwork depicting the crucifixion of Jesus Christ from Cannell and Chaffin, a Southern California decorating firm.
Museum historians previously had no other information about the painting, including where it had hung in the past several centuries.
The approximately 8ft (2.5m)-high and 5ft (1.5m)-wide artwork hung in a prominent place beside the fireplace in the estates massive Assembly Room.
Mrs Levkoff contacted other experts in Baroque paintings and compared Perezs signature to other works of art that are known to have been created by him.
She said the artwork is unique for Perez, who was known primarily for painting floral still life paintings before becoming the official painter to King Charles II in 1689.
The museum, which is part of the California state park system, has now installed a new spotlight to illuminate the discovery.