PARIS (AFP) - Scientists are preparing a broadside against British artist David Hockney for suggesting that Renaissance masters projected images onto canvas which they then copied, a British journal says.

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Hockney made the claim in a 2001 book, Secret Knowledge, that ignited huge controversy.

He theorised that 15th- and 16th-century artists were able to achieve heightened realism because they projected the image of their subject onto the canvas and then traced over it.

This projection technique, using a pinhole device called the camera obscura, can be dated as far back as the 5th century BC.

But art historians said it was not used in painting on any significant scale until the 18th century, when better optics became available.

The notion that artists could perform such a feat 300 years earlier, when they lacked even a lens good enough to provide a precise rather than fuzzy image, was strongly contested.

Stanford University physicist David Stork -- a fierce critic of Hockney's idea -- will next week open a new front in the assault on the copying theory, the British weekly New Scientist says.

At a conference on Electronic Imaging in San Jose, California, he will show computer imaging software to analyse Georges de la Tour's 1645 painting "Christ in the Carpenter's Studio," a night-time image in which the only source of light comes from a candle held by Jesus.

Given the type of concave mirrors and lenses available at the time, the brightness in this already dark scene would have been reduced a thousandfold on the canvas if de la Tour had used a camera obscura, according to Stork's calculations.

The projected image would have been so poor it would have been almost impossible to see, let alone trace.

The other possibilities are that de la Tour arranged the scene in bright sunlight, projected it onto the canvas and then traced it, or used dozens of oil lamps or hundreds of candles for illumination.

If so, the artist would still have to completely rework the painting at night in order to make the light look realistic, according to Stork.

In other words, the technology was too limited, even two centuries after the start of the Renaissance, to do what Hockney suggests. De la Tour's work is down to artistic skill, not copying, says Stork.

New Scientist says another attack on Hockney's theory is expected in March, in a study to be published by Thomas Ketelsen, a curator at the Museum of Prints, Drawings and Manuscripts in Dresden, Germany.

Hockney says similarities between a drawing by the 15th-century Jan van Eyck ("Portrait of Niccolo Albergati") and a larger oil painting of the same name can only have been achieved through optical projection.

But Ketelsen, poring over the drawing with a microscope, says he has found previously unseen pinpricks that suggests the copying method was mechanical, not optical -- a clever device called a reducing compass.

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