Medieval lenses exhibit modern performances.
By Oliver Graydon.
Featured in Opto & Laser Europe, Issue 56. November 1998.
Some medieval rock-crystal lenses kept in museums turn out to have
optimized aspheric shapes and an imaging performance comparable to that of
modern designs, according to scientists in Germany.
The lenses were found at a Viking site on the island of Gotland in
Sweden and date from the 11th or 12th century.
"In the light of these results we must rethink our ideas on
optical knowledge in the Middle Ages." said Olaf Schmidt from the Aalen
University of Applied Science. "It seems that elliptical lens design was
invented much earlier than we thought. Then the knowledge was lost for nearly
a millennium."
A Viking lens gives an imaging quality comparable to that of 1950's
aspheric lenses.
Karl-Heinz Wilms, a retired scientist from the Rodenstock firm,
first spotted the impressive imaging of a lens in a Munich museum. He then
approached Schmidt and Brend Lingelbach from the Institute of Ophthalmic
Optics to analyze the lens and others like it that were on display at a museum
in Visby, Sweden, some of which had been used in jewellery.
The lens, with a 50mm diameter and about 30mm thick at the centre,
is shaped from a rock crystal.
"The symmetry of the lenses suggests that they were made on a
turning lathe." said Schmidt. "he flatness on the poles indicates that they
were made in the same way, and have a common origin."
Rock crystal turned on a lathe? The lens was made in 11th-or 12th century
Europe.
He thinks that the lenses may have been manufactured somewhere in
Eastern Europe and that they were used to burn out wounds, light fires and
serve as magnifiers for craftsmen.
The German team measured each lens by projecting it's profile
onto a screen and photographing the image. A three-dimensional fit of the
shape of the lens surface showed that it closely matched and ellipse. The
imaging quality of the medieval lenses were found to be almost as good as
those of modern optics.
"The aspheric design of the lens is unusual considering that the
lens was made about 1000 years ago. In that era scientists had only started
to explore the laws of light refraction." said Schmidt.
Modern optics in medieval times: raytracing shows that the lens has only a
small spherical aberration.
"The optimization was achieved by craftsmen long before
mathematicians could describe the shape and properties of aberration-corrected
lenses. It seems that this knowledge was lost for at least 500 years, until
Descartes calculated the idea shape of a focusing lens but, lacking the
necessary equipment, he could not produce it. Aspheric lenses for spectacles
were not made until the 1950's."