A new way to decipher a person's age by looking into the lens of the eye could help forensic scientists identify bodies, Danish researchers said on Tuesday.

Their new technique uses radiocarbon dating to measure special proteins known as lens crystallines that develop around birth and remain unchanged for the rest of our lives. They are the only part of the body apart from teeth that do so.

The researchers correctly identified the ages of 13 people within one-and-a-half years by analyzing a carbon isotope called carbon 14 trapped inside the crystallines, they reported in the journal PLoS One.

"In forensics we are always looking for ways to identify deceased persons," said Niels Lynnerup, a forensic scientist at the University of Copenhagen, who led the study.

"We found with this method you can determine almost to the year, the year of birth."

Scientists have long used radiocarbon to date fossils or bones. More recently, researchers have applied the technique to tooth enamel to tell the age of people who have recently died, Lynnerup said.

The technique employed in the lens analysis is based on the sudden increase in atmospheric carbon 14 beginning in the 1950s until a test ban a few years later when the Soviet Union and the United States began testing nuclear bombs.

These experiments more than doubled the amount of atmospheric carbon 14, which gradually began to decline toward normal levels after the ban, Lynnerup said.

Scientists have recorded these levels annually, giving the Danish team a benchmark to date a person's birth by matching the corresponding year in which the carbon 14 atmospheric content was as high as in the person's eye lens.

The researchers also said their technique could one day help scientists in other fields date proteins and other molecules in the body to determine when cancer tissues or cells develop.

The new method offers certain advantages.

For one, lifting up the eyelid to extract the lens is often much easier then extracting part of the tooth needed for an accurate reading, he said.

The downside is that a lens disappears after a few days as the body decays while a tooth provides a sample scientists can use even after a few years, Lynnerup said.

"Removing the lens is completely identical to a cataract operation," he said in a telephone interview.

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