Over 500 years after it was painted, the Leonardo da Vinci painting Salvator Mundi is still controversial.

Not least, people are still arguing about the identity of the painter. Some art experts insist that the painting, believed to have originally been commissioned by King Louis XII of France, was actually painted by one of Leonardo’s students.

One of the reasons for thinking that it wasn’t the work of the master himself is an orb held in Christ’s left hand in the painting.

It is perfectly transparent, in a way that a real-life crystal ball never could be. The orb breaks the rules of optics and remains a mysterious puzzle for art lovers.

In 2017, Salvator Mundi became the most expensive painting ever sold. In 1958 it went for £45.

The orb seems to contain a few ‘sparkles’ that would only be present in a solid ball of glass. But Christ’s hand and robes are visible, unaltered, through the ball. A real crystal ball would magnify and invert the image of whatever was behind it.

Art historians have been arguing for decades about what the orb was really made of and whether the lack of optical effects is a deliberate mistake by Da Vinci.

Now a new paper about the painting has been published by UC Irvine computer scientist professors Michael Goodrich, Shuang Zhao and doctoral student Marco Liang.

They have recreated conditions under which a hollow glass ball– not solid as was previously believed – would pass light through in a way that exactly matches the Leonardo painting.

They found that a combination of dim light in the room, with a strong ‘spotlight’ overhead could duplicate the Leonardo painting of the orb was a hollow sphere with 0.05 inch-thick walls.

The researchers documented their findings on arXiv .

That makes it much more likely that Salvator Mundi really is all Leonardo’s own work. This would be good news for the un-named Saudi prince who bought the painting in 2017 for a record-breaking $450 million (£346 million.)

It was only after the picture was cleaned that its true value was realised.

In 1958, the painting went for £45. Even allowing for inflation that’s less than a grand in today’s money.

It was only after the painting had been thoroughly cleaned and restored that the master’s handprints could be identified in the paint making up Christ’s shadowed forehead an duh work was reappraised.