Careful examination of the image has even determined the breed of the dog, which, according to the London veterinary Bruce Fogle, appears to be a Lurcher, (a cross between a Greyhound and a working dog). Little is known about the provenance and history of the portrait.
"I am calling it the Boaden Portrait because I found it in a rare, richly illustrated edition of James Boaden's work of 1824," Hammerschmidt-Hummel said.
Called "An Inquiry into the Authenticity of Various Pictures and Prints, which ...have been offered to the Public as Portraits of Shakespeare," the book displays the portrait as an engraving -- with no caption.
"Nevertheless, my research in close interdisciplinary collaboration with experts from other disciplines shows we are dealing here with an authentic portrait," Hammerschmidt-Hummel said.
"The portrait could only have come about by direct contact of the artist with his model, that is Shakespeare," said Jost Metz, a dermatologist who examined the portraits. Metz specializes in diagnosing signs of disease in Renaissance portraits.
Announced on the 450th anniversary of the poet's birth, the new finding adds to four portraits of the Bard which Hammerschmidt-Hummel authenticated in 2006 amid some controversy.
Before then, only two likenesses of Shakespeare, both posthumous, were accepted as authentic: a bust on his tomb in Stratford's Holy Trinity Church and an engraving shown in the Folio edition of his plays in 1623.
To authenticate the two new portraits, Hammerschmidt-Hummel used four images that had already been thoroughly tested by various experts and found to be genuine, as well as the two accepted likenesses: the Darmstadt Shakespeare death mask, the Davenant bust, the Chandos and the original Flower portraits. She authenticated the four portraits back in 2006.
The experts used forensic imaging technologies, 3D-measurements, laser-scan images and computer montages to compare the pictures.
The tests for authenticity on the new portraits brought to light a series of facial marks and idiosyncrasies that correspond to those found on all the other Shakespeare likenesses. In particular, the two newly found pictures show a growth on the upper left eyelid and swellings in the nasal corner of the left eye, which seem to represent different stages of a disease.
"Renaissance painters faithfully reproduced not only the features of their subjects, but also any signs of disease," Hammerschmidt-Hummel said.
A team of doctors analyzed both paintings and concluded that, in the Boaden portrait, the Mikulicz's syndrome and the additional swelling on the upper left eyelid, interpreted as lymphoma by the ophthalmologist Walter Lerche in 1995, had grown considerably.
The doctors say that the swelling on the left upper eyelid of the Wörlitz picture is just appearing and less noticeable.
With reference to the nasal corner of the left eye of both new portraits, Metz, the dermatologist, stressed that this was a pathological symptom all the authentic images had in common.