PARIS (AP).- An intense and controversial restoration of the last great work by Leonardo da Vinci goes before the public Thursday at the Louvre Museum, revealing "The Virgin and Child with Saint Anne" in the full panoply of hues and detail painted by the Renaissance master 500 years ago.

The 18-month-long restoration of the painting that Leonardo labored on for 20 years until his death in 1519 will go a long way to raising "Saint Anne" to its place as one of the most influential Florentine paintings of its time and a step towards the high Renaissance of Michelangelo.

The cleaning has endowed the painting portraying the Virgin Mary with her mother Saint Anne and the infant Jesus with new life and luminosity. Dull, faded hues were transformed into vivid browns and lapis lazuli that had visitors awe-struck.

"It's unbelievable, so beautiful. Now you have that same feeling as when you enter Michelangelo's restored Sistine Chapel. Look at the blue!" one visitor, Odile Celier, 66, said Wednesday.

The exhibit brings together some 130 preparatory drawings and studies by Leonardo and his apprentices — something curator Vincent Delieuvin likened to "a police investigation" — tracing the painting's conception and revealing to experts today the entire development over the last 20 years of Leonardo's life.

Almost like detective work, the impressive display of sketch books and mathematical diagrams hold clues not just to unlocking the art behind the painting, but — for the man who was more famous in his day as an engineer — the years of scientific research that defined his work.

"The exhibit is a science workshop," Delieuvin said. "For Leonardo, art is founded on theoretical knowledge of nature and its functioning."

In one carnet spilling with mathematical sketches, we see how over several years he painstakingly studied light refracting from opaque objects. It decodes the technique that made Leonardo famous. The Saint Anne painting is a glowing example clearly seen in the blue opaque mantle with its almost imperceptible play on light and shadow.

The key to the hazy realism of the tree, too, with the subtle contrast of light in its leaves was cracked by infrared used during the restoration. To get this effect, Leonardo first painted the entire tree structure in full and only afterwards painted the foliage on top.

Another notebook astounds in its detailed analysis of water and air compression that shows the thinking that went into creating the sweeping blue and gray mountains rising up behind Saint Anne and child.

Like the novel "The da Vinci Code," the restoration of the master's last work has been accompanied by high-level intrigue worthy of a political thriller.

Seventeen years ago, the Louvre abandoned an attempt to clean the painting amid fears over how the solvents were affecting the sfumato, a painting technique that Leonardo mastered.

After the cleaning was eventually given the green light in 2009, two of France's top art experts — Jean-Pierre Cuzin and Segolene Bergeon Langle — resigned last year from the Louvre advisory committee responsible for the restoration, amid reports they were outraged that restorers were over-cleaning the work to a brightness Leonardo never intended.

The museum confirmed to The Associated Press last year's resignations but said it could give no further details on the events.

However, on seeing the final product, Bergeon Langle, France's national authority on art restoration, has partly buried the hatchet.

In an interview in the Louvre's in-house magazine, she said she has been reassured on some aspects that bothered her. But she also said she remained unhappy about other points of the restoration. She notably criticized the decision to remove a white patch on the body of the infant Jesus, which she said was painted by Leonardo himself.

Whether it was done by the Renaissance man we will never know, an artist who made only 18 works — all unfinished.

Indeed, mystery still shrouds much of Leonardo's career.

A discovery restorers stumbled across during the cleaning of the Saint Anne painting points to another mystery, this one in Leonardo's hometown of Florence and linked to his missing masterpiece "The Battle of Anghiari" also known as "The Lost Leonardo."

After infrared photography was used to scan the Louvre work, the exhibit shows that two pictures were found that had been secretly hidden in the painting for hundreds of years.

One, drawn by a left-handed artist, is thought to be by Leonardo, who was himself left-handed.

It is a depiction of the hatchings on a horse's head, similar to that in the mural of "The Battle of Anghiari."

Curator Delieuvin would not speculate on the finding — or another more dramatic discovery linked to the lost work revealed earlier this month in Florence.

There, researchers said they may have discovered traces of this lost mural by da Vinci by poking a probe through cracks in a 16th century fresco by Giorgio Vasari painted on the wall of the Palazzo Vecchio, one of the city's most famous buildings.

The research team leader Maurizio Seracini of the University of California said "The Battle of Anghiari" could be hidden behind the fresco done by Vasari years later.

Seracini said that Vasari, an admirer of the Renaissance, would never have destroyed a da Vinci work.

He pointed out a small but possibly telling clue: painted on a tiny flag in Vasari's fresco are the words "Cerca trova" — Italian for "seek and you will find."

The Louvre exhibit, "Saint Anne, Leonardo da Vinci's Ultimate Masterpiece," runs from March 29 to June 25.


Copyright 2012 The Associated Press.