Kate portrait: First official painting revealed 11 January 2013 Last updated at 10:00 The first official painting of the Duchess of Cambridge has been unveiled. It was painted by the artist Paul Emsley and took several months using a technique of building thin layers of oil and glazes on canvas. The duchess sat for the artist twice. Mr Emsley also worked from a series of photographs he took of Catherine. The portrait is due to go on show on Friday afternoon at London's National Portrait Gallery, of which the duchess is patron. Mr Emsley was chosen to paint the portrait by the gallery's director, Sandy Nairne. Catherine, who studied history of art at St Andrews University, was also involved in the selection process. The duchess, 31, whose pregnancy was announced by the Royal Family last month, sat for the artist in May 2012 at his studio, and again in June at Kensington Palace. Light and shadow Mr Emsley said she had expressed a desire to be portrayed as her natural self, rather than her official self. The artist described his work as simple. I don't have lots of things in the background. I do like large faces, I find them strong and contemporary," he said. I'm interested in the landscape of the face, the way in which light and shadow fall across the forms. That's really my subject matter. To have anything else in there is really just an interference. The artist's other subjects have included the former President of South Africa, Nelson Mandela, and the author V S Naipaul. In 2007 he won the BP Portrait Award for his depiction of fellow artist, Michael Simpson. Mr Emsley said it was easier to work from photographs. "I'm always worried about the sitter - are they cold, are they hot, are they comfortable?" he said. Photography today is so accurate and so good that it's really so much easier just to take photographs and work from them. Smiling portrait Originally Mr Emsley had planned to produce an unsmiling portrait. But on meeting the duchess he changed his mind. I think it was the right choice in the end to have her smiling. That's really who she is, I think," he said. He also knew he wanted to make a feature of Catherine's hair. "Everyone, I think, recognises her partly through her lovely hair," he said. I've altered the colour of the eyes slightly to match the colour of the blouse and the blue background. The painting took three-and-a-half months to complete and was presented to the portrait gallery's trustees in November. Mr Nairne said it was an "exciting moment" to display the first commissioned public portrait of the duchess. He added that she was grateful to the duchess for giving up her time for the sittings and to Mr Emsley for creating a "captivating contemporary image." The portrait, called HRH The Duchess of Cambridge, will be displayed as part of the National Portrait Gallery's Contemporary Collections.